Kit Review – SC/Jaycar USB Power Monitor

Introduction

Every month Australian electronics magazine Silicon Chip publishes a variety of projects, and in December 2012 they published the USB Power Monitor by Nicholas Vinen. Jaycar picked it up and now offers a kit, the subject of our review. This small device plugs inline between a USB port and another device, and can display the current drawn, power and voltage at the USB port with a large LCD module. This is useful when you’re experimenting with USB-powered devices such as Arduino projects or curious how external USB devices can affect your notebook computer’s battery drain.

Assembly

The kit arrives in typical Jaycar fashion:

… everything necessary is included with the kit:

The instructions arrive as an updated reprint of the original magazine article, plus the usual notes from Jaycar about warranty and their component ID sheet which is useful for beginners. The PCB is quite small, and designed to be around the same size as the LCD module:

As you can see below, most of the work is already done due to the almost exclusive use of SMD components:

That’s a good thing if you’re in a hurry (or not the best with surface-mount work). Therefore the small amount of work requires is simply to solder in the USB sockets, the button and the LCD:

It took less than ten minutes to solder together. However – take careful, careful note of the LCD. There isn’t a pin 1 indicator on the module – so instead hold the LCD up to the light and determine which side of the screen has the decimal points – and line it up matching the silk-screening on the PCB. Once finished you can add the clear heatshrink to protect the meter, but remember to cut a small window at the back if you want access to the ICSP pins for the PIC microcontroller:

How it works

The USB current is passed through a 50 mΩ shunt resistor, with the voltage drop being measured by an INA282 current shunt monitor IC. The signal from there is amplified by an op amp and then fed to the ADC of a PIC18F45K80 microcontroller, which does the calculations and drives the LCD. For complete details purchase the kit or a copy of the December 2012 edition of Silicon Chip.

Operation

First you need to calibrate the unit – when first used the meter defaults to calibration mode. You simply insert it into a USB port. then measure the USB DC voltage brought out to two pads on the meter. By pressing the button you can match the measured voltage against the display as shown below – then you’re done.

Then you simply plug it in between your USB device and the socket. Press the button to change the measurement. The meter can measure the following ranges:

For an operational example. consider the next three images are from charging my phone – with the power, current and voltage being shown:

“P” for power…

current in mA

“b” for bus voltage

If you want to use the USB ports on the right-hand side of your computer, just press the button while inserting the meter – and it flips around:

Finally – here’s a quick video of the meter at work, whilst copying a file to an external USB hard drive:

Conclusion

I really like this – it’s simple and it works. Kudos to Nicholas for his project. You can purchase it from Jaycar and their resellers, or read more about it in the December 2012 edition of Silicon Chip. Full-sized images available on flickr. This kit was purchased without notifying the supplier.

In the meanwhile have fun and keep checking into tronixstuff.com. Why not follow things on twitter, Google+, subscribe  for email updates or RSS using the links on the right-hand column? And join our friendly Google Group – dedicated to the projects and related items on this website. Sign up – it’s free, helpful to each other –  and we can all learn something.

Welcome to “Silicon Chip” Magazine readers

Hello readers

Published in the January 2012 issue of “Silicon Chip” magazine is an eight page feature article by Jonathan Oxer introducing the Arduino system and how the hardware and software work together to allow anyone to turn their technological ideas into reality. If you have read Jon’s article and were directed here – thanks for visiting! We have much more content than just Arduino tutorials, however to get started with them please click here or select from one of the chapters listed in the “Arduino Tutorials” section on the right-hand side of this web page.  Our site is a work in progress and if you have any feedback or questions please email john at tronixstuff dot com, or visit our moderated Google Group.

Be sure to take advantage of the discount code on page seventeen made available by Little Bird Electronics - Australia’s largest Arduino and related electronics distributor; and also check out the range of Arduino-compatible equipment at Freetronics.

For those not familiar with the magazine, here is the cover for the January 2012 issue:

So what is “Silicon Chip” magazine all about? It is Australia’s window to the wide world of electronics, backed by a team of engineers and enthusiasts with decades of experience and knowledge. Each month you can read about in-house projects by the team and also submitted by readers – covering basic circuits right through to digital and computer systems, quality hi-fi and audio projects, news, reviews, readers’ letters, the humorous columns and a wide variety of kits to assemble. There is also a wide range of advertising from related businesses that helps you find new products and suppliers that you may not have known about.

Silicon Chip is the only Australian electronics magazine and one of the few left in the world with a broad appeal to the beginner and expert alike, and the projects described are always good value and not priced or designed out of most peoples’ reach. I unashamedly recommend you pick up a copy from the newsagent or take out a subscription if possible, it’s a great read and there’s always lots to learn and laugh about.

Finally, that’s it for 2011. A big thank you to all of our readers for your visits, feedback, compliments, criticism, donations, and the crazy emails received through the year. And of course to all the great suppliers who help out with promotional considerations and sponsor our monthly competitions. Keeping this site together has been interesting, educational and a whole lot of fun, and I hope you think so too. There is a lot coming up for 2012 – so stay tuned via twitter, Google+, or subscribe by email or RSS on the right-hand side of this page.

Have fun and Happy New Year

John Boxall.

Kit review – High Accuracy LC Meter

Hello readers

Time for another kit review. Lately one of my goals has been to make life easier and in doing so having some decent test equipment. One challenge of meeting that goal is (naturally) keeping the cost of things down to a reasonable level. Unfortunately my eyesight is not the best so I cannot read small capacitor markings – which makes a capacitance meter necessary. Although I have that function within my multimeter, it is often required to read resistors in the same work session.

Thus the reason for this kit review. A day trip to Altronics saw me return with (amongst other things) their High Precision LC Meter kit. The details were originally published in the May 2008 issue of Australia’s Silicon Chip magazine. The meter specifications are:

• Capacitance – 0.1pF to over 800 nF with four-digit resolution;
• Inductance – 10 nH to over 70 mH with four-digit resolution;
• Accuracy of better than +/- 1% of the reading;
• Automatic range selection, however only non-polarised capacitors can be measured.

The power drain is quite low,  between 8 (measurement) and 17 milliamps (calibration). Using a fresh 9V alkaline battery you should realise around fifty to sixty hours of continuous use. At this point some of you may be wondering if it is cheaper to purchase an LC meter or make your own. A quick search found the BK Precision 875B LCR meter with the same C range and a worse L range for over twice the price of the kit. Although we don’t have resistance measurement in our kit, if you are building this you already have a multimeter. So not bad value at all. And you can say you built it

Speaking of building, assembly time was just under two hours, and the kit itself is very well produced. The packaging was the typical retail bag:

The first thing that grabs your attention is the housing. It is a genuine, made in the US Hammond enclosure – and has all the required holes and LCD area punched out, so you don’t need to do any drilling at all:

The enclosure has nice non-slip rubberised edging (the grey area) and also allows for a 9V battery to be housed securely. The team at Altronics have done a great job in redesigning the kit for this enclosure, much more attractive than the magazine version. The PCB is solder-masked and silk-screened to fine standard:

There are two small boards to cut and file off from the main PCB. We will examine them later in the article. All required parts for completion were included, and it is good to see 1% resistors and an IC socket for the microcontroller:

At first I was a little disappointed to not have a backlit LCD module, however considering the meter is to be battery operated (however there is a DC socket for a plugpack) and you wouldn’t really be using this in the dark, a backlight wouldn’t be necessary. Construction was easy enough, the layout on the PCB is well labelled, and plenty of space between pins. Lately I have started using a lead-former, and can highly recommend the use of one:

Assembly was quite simple, just start with the lower profile components:

… then mount the LCD and the larger components:

… the switches and others – and we’re done:

The only problem at this point was the PCB holes for the selector switch, one hole was around 1mm from where it needed to be. Instead of drilling out the hole, it was easier to just bend up the legs of the switch and keep going:

At this stage one has to cut out two supports from the enclosure, which can be done easily. Then insert the PCB and solder to the sockets and power (9V battery snap). Initial testing was successful (after adjusting the LCD contrast…):

If you look at the area of PCB between the battery and the left-hand screw there are eight pins – these are four pairs of inputs used to help calibrate and check operation of the meter. For example, by placing a jumper over a pair you can display the oscillator frequency at various stages:

Furthermore, those links can also be used to fine-tune the meter. For example one can increase or decrease the scaling factor and the settings are then stored in the EEPROM within the microcontroller. However my example seemed ok from the start, so it was time to seal up the enclosure and get testing. Starting with a ceramic capacitor, the lowest value in stock:

Spot-on. That was a good start, however trying to bend the leads to match the binding posts was somewhat inconvenient, so I cut up some leads and fitted crocodile clips on the end. The meter’s zero button allows you to reset the measurement back to zero after attaching the leads, so stray capacitance can be taken into account.

Next, time to check the measurement with something more accurate, a 1% tolerance silvered-mica 100 picofarad capacitor:

Again, the meter came through right on specification. My apologies to those looking for inductor tests – I don’t have any in stock to try out. If you are really curious I could be persuaded to order some in, however as the capacitance measurement has been successful I am confident the inductance measurement would also fall within the meter’s specifications.

As shown earlier, there were two smaller PCBs included:

The top PCB is a shorting bar used to help zero the inductance reading, and the lower PCB is used to help measure smaller capacitors and also SMD units. A nice finishing touch that adds value to the meter. The only optional extra to consider would be a set of short leads with clips or probes to make measurement physically easier.

When reading this kit review it may appear to be somewhat positive and not critical at all. However it really is a  good instrument, considering the accuracy, price, and enjoyment from doing it yourself. It was interesting, easy to build, and will be very useful now and in the future. So if you are in the market for an LC meter, and don’t mind some work – you should add this kit to your checklist for consideration. It is available from Altronics stores and resellers.

In the meanwhile have fun and keep checking into tronixstuff.com. Why not follow things on twitter, Google+, subscribe  for email updates or RSS using the links on the right-hand column? And join our friendly Google Group – dedicated to the projects and related items on this website. Sign up – it’s free, helpful to each other –  and we can all learn something.

Kit review – Current Clamp Meter Adaptor

Hello readers

Time for another kit review. Over the last few days I have been enjoying assembling a useful piece of test-equipment – a Current Clamp Meter adaptor. This kit was originally described in the September 2003 issue of Silicon Chip magazine. The purpose of this adaptor is to allow the measurement of AC current up to around 600 amps and DC current up to 900 amps. A clamp meter is a safe method of measuring such high currents (which can end you life very quickly) as they do not require a direct connection to the wire in question. As you would realise even a more expensive type of multimeter can only safely measure around ten amps of current, so a clamp meter becomes necessary.

To purchase a clamp meter can be expensive, starting from around $150. Therein lies the reason for this kit – under $30 and a few hours of time, as well as a multimeter that can measure millivolts DC/AC.

How the adaptor works is quite simple. It uses a hall-effect sensor to measure magenetic flux which is generated by the current flowing through the wire being measured. The sensor returns a voltage which is proportional to the amount of magnetic flux. This voltage is processed via an op-amp into something that can be measured using the millivolts AC/DC range of a multimeter. As the copyright for the kit is held by Silicon Chip magazine, I cannot give too much away about the design.

John’s soapbox: People may ask “hey, can you send me the schematic? I don’t want to pay for the reprinted article or buy the kit”. My answer will be no. The hobby electronics industry in this country is shrinking every day, so please support Leo and the gang at Silicon Chip by paying for a reprint or Altronics by buying the kit (it’s out of production at Jaycar). The less kits they sell, the less-inclined they will be to produce new kits.

You can purchase a complete kit from Altronics, or build one yourself by following the article in the magazine.  The hall effect sensor UGN3503 is now out of production, but according to the data sheet (.pdf), the Allegro A1302 is a drop-in replacement.

Now, time to get started. To make life easier I forked out for the whole kit, which arrived as below:

Upon opening the bag up, one is presented with the following parts:

It is great to see everything required included with a kit. And the extra battery-clamp is a nice bonus. As usual an IC socket was not included, however these can be had for less than five cents each… so I have recently solved that problem by importing a few hundred myself. The hall effect sensor is very small; considering the graph paper below is 5mm square:

The PCB was very well done – to a degree. The solder-mask and silk-screening was up to standard:

… however a few holes needed some adjustment. Doing a component test-fit before soldering really paid off, as none of the holes for the PCB pins were large enough to accept the pins, and one of the sensor socket holes needed some modification:

A small hand-held drill is always a handy thing to have around. Once those errors were taken care of, actually soldering the components to the PCB was simple and took less than ten minutes. The potentiometer VR3 needed to be elevated by 3.5mm so it would fit through the enclosure panel in line with the power switch. As I couldn’t use PCB pins, a few link offcuts from the resistors worked just as well. When soldering the components, start with the low-profile items such as resistors, and finish with the switch and potentiometer:

Now it was time to make the clamp. First up was to cut the iron-powdered toroidal core in half. All I had to do this with was a small hacksaw, so I hacked away at it for about half an hour. This process will make a mess, filings will go everywhere. So you will need some pointless rubbish to catch the filings with:

Each half of the core is placed inside the clamp. Until I am completely happy with the clamp they will be held in with blutac. A lead also needs to be constructed, with the sensor at one end and the 3.5mm stereo plug at the other. Some heatshrink is provided to cover the ribbon cable, but I recommend placing some over the solder joints where the sensor meets the ribbon cable, as such:

Next, the sensor needs to be placed between the two halves of the core – however a piece of plastic slightly thicker needs to sit next to the sensor, to stop the clamp damaging the sensor by closing down on it. Then, using the continuity function of a multimeter, check that there aren’t any shorts in the lead. Feed the newly-constructed lead through the battery clamp in order to keep things relatively neat and tidy, and you should result with something like this:

As you can see I have had a few attempts at cutting the core. The next step was to drill the holes for the enclosure, and then solder the wires that run from the PCB, run them through the hole in the side of the enclosure, and fasten the banana plugs to plug into the multimeter.

Now it was time to start calibration. There are two stages to this, and both are explained well in the instructions. This involves adjusting the trimpots which control the output voltage in millivolts, which can be affected by charge in the human body. Therefore it is recommended to use a plastic screwdriver/trimming tool to make the adjustments:

They are generally available in a set or pack for a reasonable price. The second stage of calibration involves creating a dummy DC current load using a 12v power supply, 5 metres of enamelled copper wire and a 18 ohm 5 watt resistor:

By putting 100 turns of the copper wire around one side of the clamp, putting the resistor in series and looping it into 12 volts, the current drawn will be 0.667 amps. (Ohm’s law – voltage/resistance = current). Then it is a simple task to set the multimeter to millivolts DC and adjust potentiometer VR1 until it displays 66.7 mA:

So there you have it – 66.7 millivolts on the multimeter represents 660 milliamps of current. So 1 amp of current will be 100 millivolts on your multimeter. Excellent – it works! The whole mess was inserted into the enclosure, and I was left with something that looked not terribly unprofessional (time to invest in a label-maker):

It turns out that the thick OFC cable and the battery wouldn’t be able to coexist in the enclosure, so the battery is external.

The current clamp meter kit was an interesting and satisfying kit to assemble. Originally I assumed it would be simple, but it required plenty of drilling, cutting the darn toroid in half, tricky soldering for the clamp lead, and some patience with lining up the holes for the enclosure. Not a kit for the raw beginner, but ideal for teaching with a beginner to improve their assembly skills, or anyone with some experience. Plus it really does work, so money has been saved by not having to buy a clamp meter or adaptor.

As always, thank you for reading and I look forward to your comments and so on. Furthermore, don’t be shy in pointing out errors or places that could use improvement. Please subscribe using one of the methods at the top-right of this web page to receive updates on new posts, follow me on twitter or facebook, or join our Google Group for further discussion.

High resolution images are available on flickr.

[Note - The kit was purchased by myself personally and reviewed without notifying the manufacturer or retailer]

Otherwise, have fun, be good to each other – and make something!

Project – Simple RFID access system

In this tutorial you can make an RFID access system. It’s very simple and can be used with a wide variety of end-uses.

Updated 18/03/2013

The purpose of this project is to prototype a basic RFID access system. Although it is not that complicated, this article is my response to a kit reviewed in the Australian “Silicon Chip” (November 2010) electronics magazine. Their article describes the kit in detail – operation, schematic, use and installation. However the code for the microcontroller (PIC16F628A)  is not published due to the kit manufacturer holding copyright over the design. This is a shame, as many organisations have been quite successful selling open-source kits. So instead of moaning about it, I have created my own design that matches the operation of the original, instead using the ATmega328 MCU with Arduino bootloader. Consider this a basic framework that you can modify for your own access system, or the start of something more involved.

original article

There are pros and cons with the original vs. my version. The biggest pro is that you can buy the whole kit for around Au$40 including a nice PCB, solder it together, and it works. However if you want to do it yourself, you can modify it to no end, and have some fun learning and experimenting along the way. So let’s go!

The feature requirements are few. The system must be able to learn and remember up to eight RFID access tags/cards, etc – which must be able to be altered by a non-technical user. Upon reading a card, the system will activate a relay for a period of time (say 1 second) to allow operation of a door strike or electric lock. Finally, the RFID tag serial numbers are to be stored in an EEPROM in case of a power outage. When a tag is read, a matching LED (1~8) will show which tag was read. There are also two LEDs, called “Go” and “Stop” which show the activation status. The original kit has some more LEDs, which I have made superfluous by blinking existing LEDs.

This is a simple thing to make, and the transition from a solderless breadboard to strip board will be easy for those who decide to make a permanent example. But for now, you can follow with the prototype. First is the parts list:

• Atmel ATmega328 with Arduino bootloader;
• 16 MHz resonator (X1 in schematic);
• ten LEDs of your choice;
• two normally-open push buttons;
• two 560 ohm resistors (all resistors 1/4 watt);
• one 1k ohm resistor;
• three 10k ohm resistors;
• one BC548 transistor;
• three 0.01 uF monolithic capacitors;
• one 100 uF electrolytic capacitor;
• one 1N4004 diode;
• Microchip 24LC256 EEPROM;
• 125 kHZ RFID module;
• 125 kHz RFID tags/cards;
• connecting wire;
• large solderless breadboard;
• LM7805 power regulator;
• relay of your choice with 5V coil (example).

When selecting a relay, make sure it can handle the required load current and voltage – and that the coil current is less than 100mA.

If attempting to switch mains voltage/current – contact a licensed electrician. Your life is worth more than the money saved by not consulting an expert.

And here is the schematic:

Here is the prototype on the solderless breadboard. For demonstration purposes an LED has been substituted for the transistor/relay section of the circuit, the power regulator circuitry has not been shown, and there are superfluous 4.7k resistors on the I2C bus. To program the software (Arduino sketch) the easiest way is by inserting the target IC into an Arduino-compatible board, or via a 5V FTDI cable and a basic circuit as described here.

The Arduino sketch is also quite simple. The main loop calls the procedure readTags() to process any RFID tag read attempts, and then monitors button A – if pressed, the function learnTags() is called to allow memorisation of new RFID tags. Each tag serial number consists of 14 decimal numbers, and these are stored in the EEPROM sequentially. That is, the first tag’s serial number occupies memory positions 0~13, the second tag’s serial number occupies memory position 14~28, and so on. Two functions are used to read and write tag serial numbers to the EEPROM – readEEPROMtag() and writeEEPROMtag(). The EEPROM is controlled via the I2C bus. For a tutorial about Arduino, I2C bus and the EEPROM please read this article. For a tutorial about Arduino and RFID, please read this article. The rest of the sketch is pretty self-explanatory. Just follow it along and you can see how it works. You can download the sketch from here.

 
And finally, a quick video demonstration:

So there you have it. I hope you enjoyed reading about this small project and perhaps gained some use for it of your own or sparked some other ideas in your imagination that you can turn into reality.

In the meanwhile have fun and keep checking into tronixstuff.com. Why not follow things on twitter, Google+, subscribe  for email updates or RSS using the links on the right-hand column? And join our friendly Google Group – dedicated to the projects and related items on this website. Sign up – it’s free, helpful to each other –  and we can all learn something.

Kit Review – Silicon Chip Low Capacitance Meter adaptor for DMMs

Hello readers

Time again for another kit review. In the spirit of promoting all things electronic and Australian, today we are going to look at a kit that was published in our electronics magazine Silicon Chip (March 2010) – their Low-capacitance meter adaptor for DMMs. Simply put, it converts capacitance (from a theoretical 1 picofarad) to millivolts, which you can then read with almost any digital multimeter. This is useful as even more expensive multimeters (such as my Fluke 233) only measure down to 1 nanofarad (1000 picofarads). Although this kit is available on the Australian market, the retailers will export to those abroad. If you are outside Australia and having trouble sourcing one, send me an email. Moving on…

Here is our unassuming finished product:

Please note that this is not an open-source product, so you need to either purchase the kit of parts, or a back-issue of Silicon Chip magazine, March 2010 for the schematic and instructions. Now it is time to get started. But before that, how does it work?

Without giving too much away, a very rough explanation would be that a square wave signal is formed, then cleaned up through a Schmitt trigger-inverter. This square wave is then split into two, one signal passing through the capacitor under test and some resistors, and the other signal passing through a calibration variable capacitor and the same value resistors – thereby both signals pass through two different RC circuits. Finally the two signals are fed through a XOR gate, which creates a series of positive pulses that are a function of the capacitor under test.

Kit assembly was not that difficult, like anything just take your time, read the instructions carefully, and don’t rush things. If you are happy with your through-hole soldering skills, and have a power drill, this kit will be easy for you to work with. Unusually for some kits, this one comes with almost everything you need:

The quality of the included housing is very good, there are metal threaded inserts for the screws; and even through the ICs are simple 74xx-series, sockets have been included. Resistors are metal film, the trimpots are enclosed multiturns – all very nice. I am a little disappointed with the housing/adhesive label combination however, in the past various kits from Jaycar would have a box with a nice silk-screened, hole-punched front panel. Such is life. The PCB is solder-masked and silk-screened, however a little less denser than PCBs from other kit suppliers:

And thus brings a slight issue with the housing and the PCB – either the PCB is too wide, or the box is too narrow. A quick clip of the PCB with some cutters will fix that:

The instructions are quite good – they are a reprint of the magazine article, and slightly modified by the kit production company. Furthermore, the silk-screening on the PCB makes things a breeze. The simple passives were easy to install, however take care not to overheat the variable capacitor, their casings can melt rather quickly:

Following that, the ICs were inserted, and the rotary switch. From experience, one should trim the shaft down to about a 25mm length before soldering it into the board. Take very good care when placing the rotary switch, there is a lump on the switch which matches the small circle at 8 o’clock on the PCB diagram. Finally, don’t forget to alter the switch so it only has four selections. Soldering it in can look difficult, but is not. Just push it into the PCB, checking it is flush, even and all the way in. Then bend a couple of the pins over, invert the PCB and solder away – as such:

Now it is time to start on the enclosure. Each end has two banana-type sockets, the left are the full binding-post, and the right are just sockets. Carefully mark where you want to start the holes – the positions are vertically half-way, and horizontally 15mm in from the edge, however double-check yourself. Always check the fit of the socket while drilling, as it is easy to go too far and make the holes too large – at which point you’ll have to buy another enclosure.

Once you have the sockets fitted – on the left:

and on the right:

… you will need to solder the socket rear to the PCB pins (left) and a small link to the PCB pins (right). It is important to get a good, solid connection – as these sockets may come under a lot of use later on. Next it is time to start on the housing. If you can, photocopy the label so you have a drilling template:

You will notice in the above photo one of my favourite tools, a tapered reamer. Using that, you can carefully turn a small hole into a larger hole, without risking making a mess with a drill. Again, cut the rotary switch’s shaft before soldering:

And as punishment for using twitter at the same time, I had ended up drilling the back instead of the front. D’oh. However cosmetic appearance is secondary to functionality, so all is well. Next was to install the PP3 battery snap. The battery will be a tight fit, so a length of heatshrink has been supplied in order to avoid the battery case shorting with the PCB pin:

And finally we have finished soldering:

Now it is time for calibration. And for me to get a little cranky, which is quite rare as I am somewhat easygoing. Calibration requires three 1% tolerance capacitors, 100 pF, 1000 pF and 10000 pF. And they are not included with the kit. And can not be purchased from any of the kit retailers. So they had to be ordered from Farn… element-14 at a reasonable expense. Considering the kit production company also imports, wholesales and retails electronic components, they could have bought a volume of these special capacitors and added a few dollars to the price of the kit. Such is life. So here are the little buggers:

From top to bottom:

• Silvered-mica 100 picofarad 1% tolerance, element-14 # 1264880, RS # 495745;
• Polystyrene 1000 picofarad 1% tolerance, element-14 # 9520651, RS # 495868 (silvered mica) and
• Polystyrene 10000 picofarad 1% tolerance, element-14 # 3358951, RS # 495953 (silvered mica)

However it is worth the effort to chase them down. There is no point using this kit if you calibrate with normal capacitors; their tolerance can be as much as 20 percent either way. Thankfully the calibration process is quite simple. You will need a small, plastic flat-blade screwdriver to make the adjustments, as your body has stray energy which can alter the capacitance measurements.

Before starting, connect your multimeter to the output sockets and set the range to millivolts – then adjust the variable capacitor until you have the meter display as close to zero as possible. This is used to ‘null out’ stray capacitance. Next, set the dial to A, connect the 100 pF capacitor to the input posts, and adjust VR3 until the meter displays one volt DC – this represents 100.0 picofarads:

I could not for the life of me get this to 1 volt. After fitting the case at the end, I tried again with the case on with the same result. It is very important to get the capacitor as close as possible to the binding posts, with such small values stray capacitance can affect the result. However in my line of work, one-tenth of a picofarad is not relevant. For now. Next, set the dial to B, connect the 1000 pF capacitor, and adjust VR2 until the meter displays 1 volt – this represents 1000 picofarads:

Excellent – spot on. Unfortunately the leads on my 10000 pF capacitor were not long enough to attach into the binding posts, so that step had to be passed. I will have to re-order the correct part next week and calibrate then. However the other two setting are basically working perfectly, which is a good indication for the general performance of the kit. Kudos to Jim Rowe from Silicon Chip magazine for this design.

Before closing up the enclosure, I decided to wrap the battery with some paper, as having it  rub up against other parts is not a good idea:

Now for a test run – time to measure the smallest capacitors I have in stock, first a 4.7 picofarad ceramic:

and next, a 12 picofarad ceramic:

Excellent, we can call these readings a success. I was also quite amazed that the tolerance of the cheap ceramic capacitors was so low. Note that in real-life, you may not be able to have the capacitor under test directly connected to the binding posts. In these cases you will need a short set of heavy-gauge leads to the test capacitor. If you do this, you will need to adjust the variable capacitor to reset the display to account for stray capacitance in the leads.

In conclusion, this kit has proved very successful, with regards to assembly, the quality of components and instructions, and of course the final result. I made a few errrors with regards to the housing, but that didn’t affect the final result. And for less than fifty Australian dollars, I have a very low value capacitance meter. However in due course I would consider the purchase of a full LCR meter for greater accuracy and ease of frequent use (some can measure down to 0.1 picofarad). But for the time being, this has been an excellent, educational  and affordable solution.

You can purchase the kit directly from Jaycar.

So have fun and keep checking into tronixstuff.com. Why not follow things on twitter, Google+, subscribe  for email updates or RSS using the links on the right-hand column, or join our Google Group – dedicated to the projects and related items on this website. Sign up – it’s free, helpful to each other –  and we can all learn something.

High resolution images are available on flickr.

[Note - The kit was purchased by myself personally and reviewed without notifying the manufacturer or retailer]

Otherwise, have fun, be good to each other – and make something!

Electronic components – the Diode

Hello readers

Although my posts have generally been about microcontrollers, kits and related items, I have been rather lax in writing about electronics in general, and that magical world of wonder known as analogue electronics… i.e “Before Arduino” So let’s go back to some of the basics. Starting with the diode

What is a diode? It is an electronic component that allows current to only flow in one direction. Before the advent of semiconductors, vacuum tube diodes were used. Thankfully no more…

A diode is comprised of two types of semiconductor crystal (usually made from silicon or germanium) that are highly refined then doped with an impurity. Depending on the impurity, the crystal can either be called an “N-type” or “P-type”. When you put an N-doped region next to a P-doped region, a diode or PN junction is formed. In our diodes, the P-region is called the anode, and the N-region is called the cathode. As you can imagine, these properties are useful, allowing current to flow only in one direction.

The basic symbol for a diode in a circuit diagram or schematic is this:

So in a circuit, the current only flows in one direction, for example:

When a diode is connected in this way, it is said to be forward-biased, that is the anode is connected to a higher voltage than the cathode. If the diode was reversed, with the cathode connected to the higher voltage, it would not allow current to flow, and therefore would break the circuit. A forward-biased diode is considered to be a closed switch, as the voltage does not drop as the current passes through the diode. However that is assuming the diode is perfect. And like many other things in life, it is not perfect.

All diodes are not perfect, and have what is called a forward voltage drop, this is the amount by which the voltage decreases as the current passes through the diode from anode to cathode. For silicon diodes, this is ~0.7 volts; for germanium diodes ~0.3 volts.

Diodes are also manufactured to handle a certain amount of power. Recall that:

power (watts) = current (amps) x voltage (volts)

As the voltage drop with our normal diode is 0.7V, the power dissipated by the diode can be calculated by simply multiplying the current by 0.7.

For example, if we have a 1 watt diode, how much current can it handle?

1 = current x 0.7; current = 1/0.7

Current = 1.42

So the 1 watt diode can theoretically handle 1.42 amps of current.

What happens if you use a diode the other way, that is attempt to allow current to flow from the cathode through to the anode. Ideally nothing will happen – to a point. Diodes have a breakdown voltage, when a reverse-biased (backwards) diode starts to allow current to flow through it. The breakdown voltage of each type of diode is different, it depends on the manufacturer. The best way to find out what the breakdown voltage of your diode is to check the data sheet. For example, a popular diode is the 1N4001. From page two of the data sheet (pdf), comes the following table:

So for the 1N4001 diode, the breakdown voltage is 50V. Peak repetitive means that the diode can sustain doing this more than once. Excessive voltage will not usually destroy a diode. Excessive current will destroy a diode. This is interesting, as you can use a diode as a voltage regulator, provided that you don’t exceed the maximum current it can handle. Refresh your memory about voltage division with resistors. The disadvantage of using two resistors is that it can be difficult to purchase precise values.

So let’s use a zener diode instead. They are manufactured with a much more precise (and lower) voltage; and handle less power. Zener diodes have a slightly different symbol:

Zener diodes will usually (hopefully) have their breakdown voltage within their part number. For example, an NXP 4.7V zener diode’s part number is: BZX79-B4V7. The 4V7 is the breakdown voltage, with a V for the decimal point. It can handle 500 mW, but this is not obvious – once again, you will need the data sheet (pdf). Below is a photo of a typical zener diode. It is very small, the grid paper beneath it is 5mm square. The ring or dark band around one end of the diode always indicates the cathode end.

And now for an example. We have a tiny Zilog ePIR that requires a nice smooth 3.3v DC, and only draws 10mA, however the power rail on our prototype is 5V. This is a job for a 3.3V zener diode. Here is our schematic:

We need to calculate the appropriate resistance to limit the current through our zener diode. We are using a Fairchild BZX55C3v3 (data sheet pdf). Maximum power is 500mW or 1 watt. To calculate the value of the resistor, we will need the maximum current for the diode, calculated by

current = power / voltage

current = 0.5 watts / 3.3 volts

current = 0.150 A or 150 mA.

Using Ohm’s law, resistance = voltage /current

resistance = 1.7 volts / .15 A

resistance = 11.333333 = 12 ohms

So we would use a nice metal film 1% tolerance 12 ohm resistor, rated at 500 mW. Easy, 1.2 cents from RS or element-14.

Another type of diode is the signal diode. They handle much less current, usually around 100 mA, but are more suited for high-frequency signals, or semiconductor protection.Signal diodes can have a high breakdown voltage, but low power handling ability. A very popular signal diode used is the 1N4148 (data sheet), an example of which is below:

For example, a signal diode may be places across the coil of a relay that is being controlled by a transistor – as it allows the current produced by the change in magnetic field when the coil is deactivated to head through the coil instead of the transistor. For example, when using an Arduino to control a relay coil:

Our next diode type is the germanium diode. They have a very small voltage drop of 0.2V, and are mostly used in crystal radio sets. They are very fragile, but are ideal for putting across a radio wave signal to convert it from AC to DC, which can then be amplified. If you are interested, here are some guides to making a crystal radio.

Another type of diode is the Schottky diode (named after the German physicist Walter Schottky). The symbol for a schottky diode is this:

There are two main differences between a schottky diode and a normal diode. One – a schottky diode does not have a discernible recovery time between conducting and not conducting a current. For example, a normal diode may take around a few hundred nanoseconds; whereas a schottky does not. This makes them useful in situations that involve very very high speed switching of current (for example, DC-DC converters such as Limor Fried’s mintyboost). Two – a schottky diode has a smaller forward voltage, a typical example (data sheet) is 0.55v.

Finally we come to rectifier diodes. Their main feature is the ability to handle large amounts of current, from 1 amp upwards; and higher breakdown voltages. For example the 1N4001 (data sheet) diode is 50V at 1 amp; the 1N5401 (data sheet) is 100V at 3 amps. The main purpose of these diodes is to protect against incorrect polarity from power supplies, and to convert AC to DC. For example, if you were designing a childrens’ toy that used a 9V battery, you would use reverse-bias a rectifier diode between 9V and GND in case the child forced the battery in the wrong way.

But how can rectifier diodes convert AC to DC power? Very easily – through the use of a bridge rectifier. A bridge rectifier is basically four rectifier diodes connected together, for example:

When the AC power is between 0 and maximum wave, the positive DC rail is fed by the path: 1,2,3,4; the negative DC rail is 8,7,6,5. When the AC power is between 0 and minimum wave, the positive DC rail is fed by the path: 5,6,3,4; the negative DC rail is: 8,7,2,1.

Bridge rectifiers come in various shapes and sizes, for example DIP packaging for 1A 100V models:

right through to 300A 1600V models…

Last but not least is the light emitting diode (LED). An LED is a special kind of diode, when it is forward-biased and a current applied, it releases energy in the form of light instead of heat. Here is the common schematic symbol for an LED:

When using an LED it is critical to ensure you have the correct voltage, otherwise your LED will overheat, burn your fingers when you touch it then eventually break. Always consult your data sheet. Calculating the correct voltage is quite simple. Using a bog-standard 5mm RED LED as an example (data sheet), you can use the following formula:

R = (Vs-Vled) / A

where:

• R = value of resistor to use in ohms
• Vs is your supply voltage in volts DS
• Vled is the forward voltage of the LED at the recommended current
• A is the recommended operation current of the LED

So for our example, we will use a 9V battery, and the LED from the data sheet above, Vled is 2V and A is 20 mA or 0.02 A

That gives us R = (9-2)/0.02 = 7/0.02 = 350 ohms.

Therefore, place a 350 ohm resistor between the positive of the battery and the anode of the LED. The most popular value of resistor to use would be a 390 ohm, 1/4 watt.

You can find LEDs in many different colours, and also units with two or more LEDs in the one housing, example red, green and blue. Some LEDs also create light in non-visible wavelengths, such as infra-red – these are used in remote-control applications and night-vision equipment. However if you are reading this, you would know by now where to find LEDs.

Well that wraps up my introduction to diodes. As always, thank you for reading and I look forward to your comments and so on. Please subscribe using one of the methods at the top-right of this web page to receive updates on new posts. Or join our new Google Group.

Otherwise, have fun and make something!

Some information for this post came from various books by Forrest Mims III; ”All-new Electronics Self-Teaching Guide” by Harry Kybett and Earl Boysen; and data sheets by Fairchild, NXP, ON Semiconductor and Vishay; images of bridge rectifiers from Farnell Australia, LED and schottky diode symbols from electricalwhat.com.