Education – Introduction to Alternating Current

Hello everyone!

Today we are going to introduce the basics of AC – alternating current. This is necessary in order to understand future articles, and also to explain in layperson’s terms what AC is all about. So let’s go!

AC – Alternating Current. We see those two letters all around us. But what is alternating current? How does current alternate? We know that DC (direct current) is the result of a chemical reaction of some sort – for example in a battery, or from a solar cell. We know that it can travel in either direction, and we have made use of it in our experimenting. DC voltage does not alter (unless we want it to).

Therein lies the basic difference – and why alternating current is what is is – it alternates! This is due to the way AC current is created, usually by a generator of some sort. In simple terms a generator can be thought of as containing a rotating coil of wire between two magnets. When a coil passes a magnet, a current is induced by the magnetic field. So when the coil rotates, a current is induced, and the resulting voltage is relative to the coil’s positioning with the magnets.

For example, consider the diagram below (exploded view, it is normally more compact):

This is a very basic generator. A rotating coil of wire is between two magnets. The spacing of the magnets in real life is much closer. So as the coil rotates, the magnetic fields induce a current through the coil, which is our alternating current. But as the coil rotates around and around, the level of voltage is relative to the distance between the coil and the magnet. The voltage increases from zero, then decreases, then increases… as the coil constantly rotates. If you were to graph the voltage level (y-axis) against time (x-axis), it would look something like below:

That graph is a sine wave… and is a representation of perfect AC current. If you were to graph DC voltage against time, it would be a straight horizontal line. For example, compare the two images below, 2 volts DC and AC, shown on an oscilloscope:

2 volts DC

The following clip is 2 volts AC, as shown on the oscilloscope:

So as you can see, AC is not a negative and positive current like DC, it swings between negative and positive very quickly. So how do you take the voltage measurement? Consider the following:

The zero-axis is the point of reference with regards to voltage. That is, it is the point of zero volts. In the oscilloscope video above, the maximum and minimum was 2 volts. Therefore we would say it was 2 volts peak, or 2Vp. It could also be referred to as 4 volts peak to peak, or 4Vpp – as there is a four volt spread between the maximum and minimum values of the sine wave. There is another measurement in the diagram above – Vrms, or volts root mean squared. The Vrms value is the amount of AC that can do the same amount of work as the equivalent DC voltage. Vrms = 0.707 x Vp; and Vp = 1.41 * Vrms. Voltages of power outlets are rated at Vrms instead of peak as this is relative to calculations. For example, in Australia we have 240 volts:

do NOT do this

Well, close enough. In fact, our electricity distributor says we can have a tolerance of +/- 10%… some rural households can have around 260 volts. Moving on…

The final parameter of AC is the frequency, or how many times per second the voltage changes from zero to each peak then back to zero. That is the time for one complete cycle. The number of times this happens per second is the frequency, and is measured in Hertz (Hz). The most common frequency you will hear about is your domestic supply frequency. Australia is 50 Hz:

perfect frequency

… the US is 60 Hz, etc. In areas that have a frequency of 60 Hz, accurate mains-powered time pieces can be used, as the seconds hand or counter can be driven from the frequency of the AC current.

The higher the frequency, the shorter the period of time taken by one cycle. The frequency and time are inversely proportional, so frequency = 1/time; and time – 1/frequency. For example, if your domestic supply is 50 Hz, the time for each cycle is 1/50 = 0.02 seconds. This change can be demonstrated quite well on an oscilloscope, for example:

[Youtube=http://www.youtube.com/watch?v=qqbXFDRwE-c]

In the video above there is 2 volts AC, and the frequency starts from 100 Hz, then moves around the range of 10 to 200 Hz. As you can see, the amplitude of the sine wave does not change (the height, which indicates the voltage) but the time period does alter, indicating the frequency is changing. And here is the opposite:

This video is a demonstration of changing the voltage, whilst maintaining a fixed frequency.

Thus ends the introduction to alternating current. In the next instalment about AC we will look at how AC works in electronic circuits, and how it is handled by various components.

I hope you understood and can apply what we have discussed today. 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. Or join our Google Group and post your questions there.

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

Announcement – July Competition!

Hello everyone!

During June there was a small competition which was quite fun, so from July and onwards we shall do it again – but on a larger scale. All you have to do for a chance to win is the following:

1. Read the blog posts and articles in July, as there will be five questions you will need to answer placed randomly amongst the posts. To keep track, subscribe using one of the methods on the right hand side of this page
2. When you have answers for all five questions, email them to competition@tronixstuff.com
3. If you follow me on twitter (@tronixstuff) and retweet one post in July, you will receive two entries, so put your twitter address in your email.
4. On August 1st, all the email addresses will be placed in a random draw and one selected. If the entry drawn has all five questions correct, they will win the major prize!
5. If the first entry drawn does not have five correct answers, they will win the minor prize, and the major prize will carry over until August, to be combined with the new major prize.

The prizes!

Major prize

One assembled, used JYE Tech Digital Storage Oscilloscope – from the kit review.

Minor prize

One assembled, used JYE Tech Capacitance Meter – from the kit review

Hopefully everyone can have some fun reading about electronics and learning along the way. As with any competition, there are a few rules:

1. If you have won a previous competition, you cannot enter
2. If you know me personally, you cannot enter
3. The prizes carry no warranty, we accept no liability for anything at all that they may cause
4. Prizes only include what is in the photograph, and will be sent via standard airmail free of charge
5. My decision is final
6. You can witness the draw in person with prior arrangement
7. The time used is Australian Eastern Standard Time (GMT: +10)

If you cannot wait for a chance to win, the DSO and capacitance meter kits are available from our friends at Little Bird Electronics.

So keep your eyes peeled and have fun!

Electronic components – the Resistor (Part Three)

Hello readers

Today we conclude the series of articles on the resistor. You may also enjoy part one and two.

With regards to this article, it is only concerned with direct current (DC) circuits.

Pull up and pull down resistors

When working with digital electronics circuits, you will most likely be working with CMOS integrated circuits, such as the 4541 programmable timer we reviewed in the past. These sorts of ICs may have one or more inputs, that can read a high state (like a switch being on) or a low state (or like a switch being off). In fact you would use a switch in some cases to control these inputs. Consider the following hypothetical situation with a hypothetical CMOS IC in part of a circuit from a hypothetical designer:

The IC in this example has two inputs, A and B. The IC sets D high if input A is high (5V), and low if A is low (0V). The designer has placed a button (SW1) to act as the control of input A. Also, the IC sets C high if input B is low (0V) or low if it is high (5V). So again, the designer has placed another button (SW2) to act as the control of input B, when SW2 is pressed, B will be low.

However when the designer breadboarded the circuit, the IC was behaving strangely. When they pressed a button, the correct outputs were set, but when they didn’t press the buttons, the IC didn’t behave at all. What was going on? After a cup of tea and a think, the designer realised – “Ah, for input A, high is 5V via the button, but what voltage does the IC receive when A is low? … and vice-versa for input B”. As the inputs were not connected to anything when the buttons were open, they were susceptible to all sorts of interference, with random results.

So our designer found the data sheet for the IC, and looked up the specification for low and high voltages:

“Aha … with a supply voltage of 5V, a low input cannot be greater than 1.5V, and a high input must be greater than 3.5V. I can fix that easily!”. Here was the designer’s fix:

On paper, it looked good. Input A would be perfectly low (0V) when the SW1 was not being pressed, and input B would be perfectly high (connected to 5V) when SW2 was not pressed. The designer was in a hurry, so they breadboarded the circuit and tested the resulting C and D outputs when SW1 and SW2 were pressed. Luckily, only for about 30 seconds, until their supervisor walked by and pointed out something very simple, yet very critical: when either button was pressed in, there would be a direct short from supply to ground! Crikey… that could have been a bother. The supervisor held their position for a reason, and made the following changes to our designer’s circuit:

Instead of shorting the inputs straight to supply or earth, they placed the resistors R1 and R2 into the circuit, both 10k ohm value. Why? Looking at SW1 and input A, when SW1 is open, input A is connected to ground via the 10k resistor R1. This will definitely set input A to zero volts when SW1 is open – perfect. However when SW1 is closed, input A is connected directly to 5V (great!) making it high. Some current will also flow through the resistor, which dissipates it as heat, and therefore not shorting out the circuit (even better). You can use Ohm’s law to calculate the current through the resistor:

I (current) = 5 (volts) / 10000 (ohms) = 0.0005 A, or half a milliamp.

As power dissipated (watts) = voltage x current, power equals 0.0025 watts, easily handled by a common 1/4 watt resistor. Our resistor R1 is called a pull-down resistor as it pulls the voltage at input A down to zero volts.

And with R2, when SW2 is open, input B is connected directly to 5V via R2. However. as the IC inputs are high impedance, the voltage at input B will still be 5V (perfect). When SW2 is closed, input B will be set to zero volts, via the direct connection to ground. Again, some current will flow through the resistor R2, in the same way as R1. However, in this situation, we call R2 a pull-up resistor, as it pulls the voltage at input B up to 5V.

Generally 10k ohm resistors are the norm with CMOS digital circuits like the ones above, so you should always have a good stock of them.

If you are using TTL ICs, inputs should still not be left floating, use a pull-up resistor of 10k ohm as well.

Pull-up resistors can also be used in other situations, such as maintaining voltages on data bus lines, such as the I2C bus (as used in our Arduino clock tutorials).

So that is the resistor. I hope you understood and can apply what we have discussed. If you feel something is missing, or would like further explanations, please ask.

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. Or join our new Google Group.

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

Thank you!

Welcome to t r o n i x s t u f f

Welcome to the tronixstuff website. Regular articles are published about many things, including analogue and digital electronics, electronic kits, part reviews – and you can also follow our popular series of Arduino tutorials. The latest posts are below, and at your top-right are indexes to previous articles. On the right are ways to subscribe for new post notifications,  as well as links to our Google Group for discussion. Look around, learn something. And remember – there is so much more than just Arduino! (“Arduino” is a trademark of Arduino Team).

Before following any tutorials, please read and understand the boring stuff. You follow anything here at your own risk.

Getting Started with Arduino – Chapter Eleven

This is part of a series titled “Getting Started with Arduino!” – A tutorial on the Arduino universe. The first chapter is here, the index is here.

Welcome back

In this instalment we will start to investigate radio data transmission; then introduce rotary encoders.

As technology has marched along and generally improved on the past, things have been getting small and relatively cheaper. What was once jaw-droppingly amazing is now just meh. But as you are reading this, you know differently. We can now take control of this technology for our own devices. What has been in the past quite unapproachable is now relatively – the concept of wireless data transmission. But no just sending a signal, like a remote-control garage door opener – but sending actual,  useful data – numbers and characters and so on.

How is it so? With this pair of tiny devices:

Quite small indeed – the pins are spaced 2.54mm apart, and the graph paper is 5mm square. The transmitter is on the right. This pair operates at 315 kHz, over up to a theoretical 150 metres. The data transmission speed is 2400 bps (bits per second). These units are serial passthrough, that is they can replace a link of wire between the serial TX (pin 1) from one Arduino, and the RX of another Arduino (pin 0). They don’t need aerials for very short distances, but the use of one will extend the range towards the maximum. And finally, the transmitter needs between 2 and 10 volts; the receiver 5. Here is the data sheet for these modules: 315MHz.pdf. Normally they are sold individually, for example the transmitter and receiver. You can also find faster (data speed) modules, however we will use these ones today.

In preparation to use these modules, another library needs to be installed – the VirtualWire library. Download the latest revision from the following location: http://www.open.com.au/mikem/arduino/.

There is also a guide to using the library in .pdf format as well. Please note the library author’s instructions with regards to licensing on the last page of the guide. For a refresher on how to install a library, please head back to chapter two. This library will save us a lot of time, it takes care of checking for errors, and only allows complete, correct data (i.e. what is received matches what is sent) to be used in the receiver’s sketch.

However, as wireless is not 100% reliable, you need to take into account that transmissions may not be received, or erroneous ones will be ignored by the receiver’s sketch. You can reduce the data speed to improve reliability and range. Furthermore, you cannot use PWM on D9 and D10 if you are using VirtualWire.

Therefore if you are using this for some important communications, have the transmitter repeatedly sent the message. Later on in this series we will investigate more powerful solutions. Anyhow, moving along …

Example 11.1

First of all, we will demonstrate the use of these modules with a basic sketch. It sends some text from one Arduino to another. The receiving Arduino sends the data to the serial monitor box. Of course you could always use an LCD module instead. In my own inimitable style the sketches are very simple, yet allow you to use their contents in your own work. Here is the sketch for the transmitter – tx.pdf and the receiver – rx.pdf.

When working with two sketches at the same time, you can have two Arduinos connected to your PC simultaneously,  just remember to select the correct USB port for the correct Arduino. Do this with the tools > serial port menu option in the IDE. Otherwise you will become very frustrated if you upload the rx sketch to the tx Arduino.

Furthermore, you will need to remove the wire from digital 0 to the data pin on the receiving units before uploading the sketch. And finally, remember to set the serial monitor window at 9600 baud.

Here are my two boards in action:

Although having both boards connected to the one computer is only useful for demonstration purposes, in real life this is obviously useless. Remember that once you upload your sketch the Arduino doesn’t need a computer, only a power supply. You can feed yours between 7 and 12 volts DC through the socket. A nice switchmode power pack will do nicely, or if you are a cheapskate like me, a PP3 battery and clip soldered to a DC plug:

You may find that when you use a battery powered Arduino that it basically does not work. Arduino genius Jon Oxer (co-author of Practical Arduino) has found a solution for this issue – place a 10k resistor between GND and digital 0 (RX), or between digital pins 0 and 1. The next thing to consider it improving the reception range. This can be done using two methods – the first by connecting an external antenna, either a length of wire, or perhaps a purpose-built aerial. The second method is to increase the supply voltage of the transmitter up to 12 volts.

Now it is your time to do some work:

Exercise 11.1

You now are able to send characters using the radio link from one Arduino to another. Now it is time to control things remotely. For the purpose of the exercise, we will just control three LEDs, turning them on and off. You already know how to control other things with digital output pins, so we just need to focus on getting the switching on and off. Hint – you can send characters via the wireless link, so create your own codes.

You will need:

• Two standard Arduino setups (computer, cable, Uno or compatible)
• two breadboards and some connecting wire
• One transmitter and one receiver unit
• three LEDs
• 3 x 560 ohm 0.25 W resistors. They are to reduce the current to protect the LEDs

Here is the schematic of my interpretation:

… the transmitter:

… the receiver:

and the video:

So how did you go? Hopefully this exercise was easier than you had first expected. If not, here are the example sketches: exercise 11.1 tx and exercise 11.1 rx. A basic transmit/receive system like this would also be handy for testing the range that wireless modules can operate over, or testing a particular site to see if you could implement such wireless modules. It’s always better to test before agreeing to make something for someone.

That concludes our work with radio wireless links – for now.

Next on the agenda is the rotary encoder. Recall how we used a potentiometer in the previous chapters as a dial, to select menu options using the readdial() function. It was simple, cheap and it worked, but some may say it was a kludge. There must be a better way! And there is, with the rotary encoder. A rotary encoder looks like a potentiometer, but it is a knob that can be rotated in either direction infinitely. Furthermore, the knob is also a normally-open button. The encoder we will be using in this chapter is a 12-step encoder, in that you can feel it physically resist rotation slightly twelve times over the 360 degrees of rotation.

Here is our example:

On one side there are three pins, and two on the opposing side. On the perpendicular sides are legs for strength, that is they are meant to be soldered into a PCB to lock it in nicely. The problem for us is that those legs interfere when trying to use the encoder in a breadboard, so I have bent them up and cut them off:

The pins are easy to understand. The two pins on one side are the button contacts, just like any simple button. The other side with the three pins – the centre goes to ground, and the outside pins are the forwards and backwards output pins. The data sheet for our encoder is here. After fooling about with this all afternoon, the quickest way to get a feel for how it works is with a simple demonstration. So first we will test it out, then see how we can use it in our user-interfaces.

Example 11.2

This example is very easy to assemble. You only need an encoder, and the usual Arduino setup. Here is the sketch, and the schematic:

and in real life:

and finally a snapshot of the output. Don’t forget to set the speed in your serial monitor box to 115200 baud:

So as you can see, this is a much better solution that then potentiometer that we used in the past. Plus having the button integrated in the encoder is very convenient, you can really create a user interface with only one control. In the next instalment of this series we will implement the encoder in an existing design. So on to Chapter Twelve.

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.

Kit review – adafruit industries Game of Life

[Updated 17/01/2013]

In this review we examine the Game of Life kit from adafruit industries. This kit is simple to construct, yet interesting to watch in operation, almost mesmerising. If you love blinking LEDs, this is the kit for you. Furthermore, it is very easy to construct which makes it a great kit for someone who is learning to solder. But before we run through putting it together, what is the Game of Life?

In 1970, a mathematician by the name of John Conway created the concept of the Game of Life, which is a example of a cellular automaton. Imagine a grid of cells, and each cell can either be dead or alive. Each cell interacts with the cells around it, and these neighbouring cells determine the life of the cell that they are neighbours to. There are a few simple rules to this:

• a live cell with less than two neighbours will die, due to under-population;
• a live cell with more than three neighbours will die, due to overcrowding;
• a live cell with two or three neighbours lives on;
• a dead cell with three neighbours will come to life, due to regeneration.

For example, consider the following situations:

1 – death; 2 – life; 3 – death; 4 – life; 5 – rebirth; 6 – death. This kit displays a simulation of the Game of Life process using a 4 x 4 grid of LEDs. Once you start watching the kit in operation, you often try to predict what will happen next. So, let’s assemble it and see what happens.

As usual, adafruit ship their kits in reusable anti-static bags:

Upon opening it up and turfing out the contents, we are presented with the following:

Everything is included to make the kit operational – no surprises. I scored an extra green LED – thanks! The kit can operate from between 3 and 5 volts, hence the 2 x AA cell holder included. The PCB is of excellent quality, with strong solder masking and a very descriptive silk screen:

This really is a simple kit to assemble. All the resistors are identical, so you can insert them into the board and solder them all in once hit. Time to fire up the iron and the fume extractor.

Careful when clipping off the excess leads, they can fly all over the place!

Next for the IC socket. Good to see a socket was provided.

At this point I would like to mention that all the documentation for the kit, instructions, schematic, code for the microcontroller – everything – is available freely, as this is an open source kit. If you intent to make your own, or modify the original design, you must respect the terms of the original Creative Commons licence as detailed in the documentation. Moving on, time for the capacitor and the link. The original design used an LM7805 regulator to control the incoming power supply, however this version (1.3) can operate from 3 to 5V, so an LDO isn’t needed. Therefore a link is placed between pins 1 and 3 of the regulator’s spot on the PCB.

Also note that there are three spaces for capacitors, but only one is necessary – solder it into the space for C3. I put it into C2 by accident, but luckily this is acceptable for the design, and I had some spares in the stock here. Now it is time for the LEDs. The kit ships with green LEDs, which look fine. My original plan was to solder in snap-off pin sockets so I could change the LEDs over at a whim, but none in stock. So on with the green! For visual appeal they look good flush with the PCB, as such:

and in with the rest:

Almost finished, time to solder in the power/reset button and we’re done:

at the top-left

Hooray – the main work is done. The six holes on the left of the IC are for in circuit programming, but I’m an arduidan at the moment, so will leave that alone. The IC is already programmed before it leaves for the outside world, so you don’t have to worry about it. Next was to test the board and make sure it worked. I loosely connected 5V and hit the power button:

Looking good. Now for the power supply. Although it can run from 2 x AA cells, mine will just sit on the desk. Last month I bought a few USB extension cables for $1 each, so I can just chop one up and use it to power the GOL from my PC. The first thing to do in this case is separate the wires in the cable, and determine which is which:

Luckily for me this cable had the power lines appropriately colour coded. However, one should always check, so I plugged it into the PC and set the meter on the black and red wires:

5.04 volts DC – close enough for me. I soldered in the lead, and also screwed in some spacers to act as support legs so the kit will stand up on its own. And as a long-term temporary measure, a great wad of blutac to hold the wire and keep the pressure off the joints:

Hey, it works for me. Anyhow, the assembly is finished. Time to clean the desk off, put the soldering iron somewhere safe to cool off, and wash my hands.

The whole lot took just under one hour, including checking the news website every now and then. It has a place just next to my PC:

home sweet home

To operate the GOL is very simple, once power is applied, hold down the button to turn it on and off. Then you can reset the cells with a quick press if you are bored with the pattern. Here is a video of it in action:

So there you have it – another successful kit build. This was a lot of fun, I enjoyed learning about John Conway and his theories, and enjoy watching the display. If you are feeling adventurous you can actually connect these kits together to form larger, blinkier games of life. Details of this and other things is available in the kit’s documentation pages. So get one, have fun with it, or give it to someone else to get them interested in electronics.

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

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.

Part review – ScrewShield for Arduino

Hello interested readers

Today we are going to examine a part that makes connecting external wires to an Arduino Duemilanove or 100% compatible board easier than trying to electrocute yourself – the Wingshield Industries ScrewShield. Is is such a simple and useful thing I am almost angry at myself for not getting one earlier. Better late than never!

The ScrewShield allows you to connect wires to all of your Arduino I/O pins via PCB-mounted terminal blocks. And it is also designed as a shield, so you can stack more shields on top like any other. Now to save costs it comes unassembled, but that isn’t a problem. Here is the contents of the bag upon arrival:

The quality of the PCBs are very good:

And no instructions were necessary – so time to fire up the soldering iron and fume extractor (hi Kortuk).

The first thing to do was jig up the socket pins with the PCBs using my favourite method, a lump of blutac:

Then it was a simple matter to turn it over and solder away; then repeat the process for the other wing. Time for a quick break to see how they look:

Once the sockets have been soldered in, the next step was to connect the terminal blocks together for each appropriate line:

And then time for another soldering session:

And we’re done. Looks kind of like a Lego spaceship from my childhood:

You can never have too many Arduino shields:

Another use for the ScrewShield is to make it easy to connect multi-core wires to a breadboard. Using PCB terminal blocks is usually difficult as the pins are a fraction too large for the holes in the average breadboard. However you can only use the analogue shield to do this, as a reader has pointed out, the pin spacing for the digital side is a little off:

Nice one. It’s always great to have a product with more than one use.

So there you have it. Another inexpensive, interesting and very useful part for the Arduino fans out there. If you use an Arduino – you really should get one of these. They are available from the usual retail outlets, and I purchased mine from Little Bird Electronics here in Australia.

If you have any questions at all please leave a comment (below). We also have a Google Group dedicated to the projects and related items on the website – please sign up, it’s free and we can all learn something. High resolution photos are available from flickr.

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

[Note - these parts were purchased by myself personally and reviewed without notifying the manufacturer or retailer]

Part review – 4541 CMOS programmable timer

Hello readers!

Today we are going to examine the 4541 CMOS programmable timer IC. The main function of this chip is to act as a monostable timer. You are probably thinking one of two things – “what is a monostable timer?” or “why didn’t he use a 555 timer instead?”. A monostable timer is a timer that once activated sets an output high for a specified period of time, then stops waiting to be told to start again.  If you are not up to speed on the 555, have a look at my extensive review.

Although the 555 is cheap, easy to use and makes a popular timer, I have found that trying to get an exact time interval out of it somewhat difficult due to capacitor tolerance, so after some poking around found this IC and thought “Hmm – what have we here?”. So as always, let’s say hello:

As you can see this is a 14-pin package by Texas Instruments. It is also available in various surface-mount options. It is also currently available from Fairchild, NXP, ON Semi, and ST Micro. Note that this is a CMOS semiconductor, and that you should practice good anti-static precautions when handling it. Futhermore, when designing it into your circuit, don’t leave any pins floating – that is not connected to +5V or ground; unless specified by the data sheet. Here is the data sheet from ON Semiconductor.

This IC is interesting in that it contains a timer that can count to one of four values: 2^8, 2^10, 2^13, and 2^16. That is: 256, 1024, 8192 and 65536. With wiring you select which value to count to, and also the action to take whilst counting and once finished. This is quite easy, by connecting various pins to either GND or +5V. The following table from the data sheet details this:

And here are the pinouts:

The speed of the counting (the frequency) is determined by a simple RC circuit. For more information on RC circuits, please visit this post. You can calculate the frequency using the following formula:

There are two external resistors used in the circuit – Rtc and Rs. Rs needs to be as close as possible to twice the value of Rtc. Try and use 1% tolerance metal-film resistors for accuracy, and a small value capacitor. Also remember to take note of the restrictions printed next to the formula above.

Before examining a demonstration circuit, I would like to show you how to calculate your timing duration. As you can see from the formula above, calculating the frequency is easy enough. Once you have a value for f, (the number of counts per second) divide this into the count value less one power you have wired the chip. That is, if you have wired the chip up for 2^16, divide your frequency into 2^15.

For example, my demonstration circuit has Rtc as 10k ohm, Ctc as 10 nF, and Rs as 20k ohm; and the chip is wired for 2^16 count. Remember to convert your values back to base units. So resistance in ohms, and capacitance in farads. Remember that 1 microfarad is 1×10-6 farads. So my frequency is:

So my timing duration will be 2^15 divided by 4347.826 Hz (result from above) which is  7.536 seconds give or take a fraction of a second. To make these calculations easier, there is a spreadsheet you can download here. For example:

Here is my demonstration monstable circuit. Once the power has been turned on the counter starts, and once finished the LED is lit. Or if the circuit already has power, the reset button SW1 is pressed to start counting. You can see that pins 12 and 13 are high to enable counting to 2^16; pin 6 is low unless the button is pressed; and pin 9 is low which keeps the LED off while counting.

 

And my demonstration laid out (I really do make everything I write about):

Easily done. Although this IC has been around for a long time, and many other products have superseded it, the 4541 can still be quite useful. For example, an Arduino system might need to trigger a motor, light, or something to runfor a period of time whilst doing something else. Unfortunately (thankfully?) Arduino cannot multi-task sketches, so this is where the 4541 can be useful. You only need to use a digitalWrite() to send a pulse to pin 6 of your timer circuit, and then the sketch can carry on, while the timer does its job and turns something on or off for a specified period of time.

Well I hope you found this part review interesting, and helped you think of something new to make.

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. Or join our new Google Group. High resolution photos are available on flickr.

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

Notes: In writing this post, I used information from NXP and On Semiconductor. Thank you.

Electronic components – the Resistor (Part Two)

Hello readers (and those from Portland State University)

Today we continue with the series of articles on basic electronics with this continuation of the article about the resistor. Part one can be found here.

With regards to this article, it is only concerned with direct current (DC) circuits.

In this chapter we will examine how two or more resistors alter the flow of current in various ways. First of all, let’s recap what we learned in the previous chapter.

Ohm’s Law – the relationship between voltage, current and resistance:

Resistors in series:

Resistors in parallel:

Dividing voltage with resistors:

However the fun doesn’t stop there. As there is a relationship between voltage, current and resistance, we can also divide current with resistors. For now we will see how this works with two resistors. Please consider the following:

There is a balance between the two resistors with regards to the amount of current each can handle. The sum of the current through both resistors is the total current flowing through the circuit (It). The greater the resistance the less current will flow, and vice versa. That is, they are inversely proportional. And if R1 = R2, I1 = I2. Therefore, I1/I2=R2/R1 – or you can re-arrange the formula to find the other variables.

Here is an example of doing just that:

Our problem here – there is 6 volts DC at half an amp running from left to right, and we want to use an indicator LED in line with the current. However the LED only needs 2 volts at 20mA. What value should the resistors be?

First of all, let’s look at R1. It needs to change 6V to 2V, and only allow 20 mA to pass. R=E/A or R= 4 volts /0.2 amps = 200 ohms.

So R1 is 200 ohms. I1 is .02 A. Now we know that the total current is equal to I1+I2, so I2 will be 0.48A. That leaves us with the known unknown R2  We can re-arrange the formula R2/R1=I1/I2 to get R2 = (R1 x I1)/I2 – which gives us R2 of 8.3 ohms. Naturally this is a hypothetical, but I hope you now understand the relationship between the current through the resistors, and their actual resistance.

What we have just demonstrated in the problem above is an example of Kirchhoff’s current law (KCL). Gustav Kirchhoff was another amazing German physicist who worked on the understandings of electrical circuits amongst other things. More on GK here. His current law states that the amount of current entering a junction in a circuit must be equal to the sum of the currents leaving that junction. And not-coincidentally, there is also Kirchhoff’s voltage law (KVL) – the amount of voltage supplied to a circuit must equal the sum of the voltage drops in the circuit. These two laws also confirm one of the basic rules of physics – energy can not be created nor destroyed, only changed into different forms.

Here is a final way of wrapping up both KCL and KVL in one example:

The current through R3 is equal to I1 + I2

Therefore, using Ohm’s law, V1 = R1I1 + (R3 x (I1+I2)) and V2 = R2I2 + (R3 x (I1+I2))

So with some basic algebra you can determine various unknowns. If algebra is your unknown, here is a page of links to free mathematics books, or have a poke around BetterWorldBooks.

There is also another way of finding the currents and voltages in a circuit with two or more sources of supply – the Superposition Theorem.

This involves removing all the sources of power (except for one) at a time, then using the rules of series and parallel resistors to calculate the current and voltage drops across the other components in the circuit. Then once you have all the values calculated with respect to each power source, you superimpose them (by adding them together algebraically) to find the voltages and currents when all the power sources are active. It sounds complex, but when you follow this example below, you will find it is quite simple. And a lot easier the th.. fourth time.  Just be methodical and take care with your notes and calculations. So let’s go!

Consider this circuit:

With the Superposition theorem we can determine the current flowing through the resistors, the voltage drops across them, and the direction in which the current flows. With our example circuit, the first thing to do is replace the 7V power source with a link:

Next, we can determine the current values. We can use Ohm’s law for this. What we have is one power source, and R1 which is in series with R2/R3 (two parallel resistors). The total current in the circuit runs through R1, so calculate this first. It may help to think of the resistors in this way:

Then the formula for Rt is simple (above), and Rt is And now that we have a value for Rt, and the voltage (28V) the current is simple:

Which gives us a value of 6 amps for It. This current flows through R1, so the current for R1 is also 6 amps.

Next, the current through R2:

Using Kirchhoff’s Current Law, the current flowing through R2 and R3 will equal It. So, this is 4 amps.

At this point, note down what we know so far:

For source voltage 28V, Ir1 = 6A, Ir2 = 2A and Ir3 = 4A; R1=4 ohms, R2 = 2 ohms, R3 = 1 ohm.

Now – repeat the process by removing the 28V source and returning the 7V source, that is:

The total resistance Rt:

Gives us Rt = 2.3333 ohms (or 2 1/3);

Total current It will be 7 volts/Rt = 3 amps, so Ir3 = 3;

So Ir2 = 2A – therefore using KCL Ir1 = 3-2 = 1A.

So, with 7V source: Ir1 = 1A, Ir2 = 2A and Ir3 = 3A.

Next, we calculate the voltage drop across each resistor, again by using only one voltage source at a time. Using Ohm’s law, voltage = current x resistance.

For 28V:

Vr1 = 4 x 6 = 24V; Vr2 = 2 x 2 = 4V; Vr3 = 4 x 1 = 4V. Recall that R2 and R3 are in parallel, so the total voltage drop (24 + 4V) = 28 V which is the supply voltage.

Now, for 7V:

Vr1 = 4V, Vr2 = 4V, Vr3 = 3V.

Phew – almost there. Now time to superimpose all the data onto the schematic to map out the current flow and voltage drops when both power sources are in use:

Finally, we combine the voltage values together, and the current values together. If the arrow is on the left, it is positive; on the right – negative. So:

Current – Ir1 = 6 – 1 = 5A; Ir2 = 2 +2 = 4A; Ir3 = 4-3 = 1A;
Voltage – Vr1 = 24 – 4 = 20V; Vr2 = 4 + 4 = 8V; Vr3 = 4 – 3 = 1V.

And with a deep breath we can proudly show the results of the investigation:

So that is how you use the Superposition theorem. However, there are some things you must take note of:

• the theorem only works for circuits that can be reduced to series and parallel combinations for each of the power sources
• only works when the equations are linear (i.e. straight line results, no powers, complex numbers, etc)
• will not work when resistance changes with temperature, current and so on
• all components must behave the same way regardless to polarity
• you cannot calculate power (watts) with this theorem, as it is non-linear.

Well that is enough for today. I hope you understood and can apply what we have discussed today. The final chapter on resistors can be found here.

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. Or join our new Google Group.

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

Notes: In writing this post, I used information from allaboutcircuits.com, plus information  from various books by Forrest Mims III and “Practical Electronics Handbook” 4th ed., Ian Sinclair. And used a lot of paper working out the theorem for myself. Thank you!

Seriously – don’t buy a cheap plugpack…

Hello readers

Instead of a normal day involving fun and learning with electronics, I got the scare of my life and a very sore back. You’re probably thinking it was something to do with the bedroom, but (un)fortunately no. It was revenge of the cheap plug pack. (In Australia we call wall warts plug packs).

In the recent past I wrote about a couple of cheap plug packs from eBay – here. Foolishly I kept using the other working plug pack. Not any more!

Consider this photo:

Notice how there is the adaptor with the Australia pins – this slides on and off relatively easily. Today I went to unplug the whole thing, by gripping the small adaptor which would pull the lot out at once. However my grip was not strong enough and my fingers slipped, pushed down and pulled at the plugpack itself – just enough to leave a gap and the pins exposed. (see below) At which point my fingers slipped and grabbed the live pins.

Although I consider myself to be a large physical specimen (185cm tall, 120kg) the shock was amazing (in a bad way). I fell arse over and ended up flat on the floor, and some strange feelings in my chest. After a few moments I sat up and had a walk around. Luckily my doctor is only ten minutes walk away so she gave me a once-over and told me to relax for the rest of the day.

So – the morals of today’s story:

One – don’t cut corners on safety by using substandard equipment

Two – no matter how familiar you are with electronics or electrical work – ELECTRICITY CAN KILL YOU!

Three – always see a doctor, even for the slightest shock.

If you have a tale of woe to share, please leave a comment below or in our Google Group.

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.

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