Education – Capacitors in parallel

Update: This article has been replaced with a more detailed article about capacitors (04/06/2010)

 

Education – Dividing voltage with resistors

[update - 31st May 2010 - new, improved article about resistors is here, please read after this post ]

Today I found myself needing to interface an external voltage with an analogue to digital pin of a PICAXE microcontroller. The plan was to use the PICAXE as a voltage meter, just for fun. However it can only accept signals up to the chip supply voltage which was 5V. So the maximum input voltage of 20v needed to be reduced proportionally to a maximum input voltage of 5V. This can be easily done with two resistors, using a voltage-division circuit. It is very easy, using the example below:

Now the fun part – mathematics! First – the formula:

So first, we know Vout is 5, Vin is 20, just need to find values for R1 and R2. Note that the sum of the voltages potentials across R1 and R2 need to equal Vin, and I want to reduce the voltage by around 75% (that is, from 20 to 5). Plus, I wanted to be able to calibrate Vout precisely, so will use a 500 ohm trimpot as R2. So now I need to calculate R1. Which leaves us with 5=20(500/(R1+500)). I am going to assume you can do the algebra, so we have R1 = 1500, or 1.5k ohms.

If you have made it this far, I’m going to give you the solutions for each variable, so you have a ready-reckoner for future use:

So there you have it – voltage division can be easy and fun. But like most things electronic, reading about it and doing it are two different things. So let’s have a look at a real-life example:

[no audio in clip] First R1 and R2 are measured, then Vin, and the voltage drops over the two resistors.

I hope you learned something today, and would like your feedback. Don’t forget to subscribe (top right) for more updates!

bbboost chapter two – which regulator?

[3 July 2010 - this project has been retired, but the posts left for reference]

In the journey to create the bbboost, first we need to start at the core – that is, the voltate regulator itself. Searching for one that meets our specification was easier than expected, I just searched for “adjustable linear voltage regulator IC” in the Farnell website and listed the results by price. The likely candidate was the National Semiconductor LM317T. Hopefully most of you would realise that this was not a surprise, the LM317 is very popular. Limor over at adafruit industries uses a Micrel MIC2941, which is also an excellent regulator, due to the low dropout voltage, which means you can create 3.3V from 3.7v (for example).

However it is just too expensive, at $1.51 each for lots of 50. The LM317T is available individually for ~78 cents, or 58 cents in amounts greater than 100. Furthermore, the LM317 can provide up to 1.5 amps of current, greater than our intial spec for the bbboost. However to keep costs down, we will stick with the assumption of one amp, unless you choose to find a 1.5A plugpack. It also has short-circuit protection on the output, and thermal shutdown. This means if it overheats, it will turn off instead of becoming damaged. However, the maximum current available will decrease if the regulator becomes hot. Now there’s an interesting experiment!

Lots of interesting information can be found on the data sheet below:

LM317T data sheet

One of the good things about data sheets are the example circuits, of which we can make use of for the basis of our bbboost. So thanks to National Semiconductor, here is the hand-drawn base for our bbboost, with one difference – there will be two voltage adjustment potentiometers (variable resistors). The 5k (R1) will control the voltage, however R2 will be used as a fine adjustment control. Handy if you really need 8.45V and not 8.49V…

(Sorry for the hand-written schematic. I’m still working on using the software I have. Next time…)

We will decide on a value for R2 later on, after experimenting with the voltage display. So far, our list of materials is:

• C1 – 0.1 uF 50V greencap capacitor
• C2 – 1.0 uF 50V electrolytic capacitor
• IC1 – LM317T linear voltage regulator
• R1 – 5k potentiometer
• R2 – very low value potentiometer
• R3 – 240 ohm 1/2 watt resistor

C1 is used to smooth any ripples in the input voltage that can be created during the AC to DC conversion in the plug pack. C2 is used to improve the transient response (i.e. keep the output voltage nice and smooth).

So at this point we will put the project to one side – I’m waiting for the parts to arrive! Be sure to subscribe for updates (see the top-right) and explore the other posts on the blog. Bye for now!

The better world of books

Learning about anything can be an expensive exercise. Textbooks, materials, lessons, time, patience, and the time and patience of those around you. Naturally it is desirable to save some expenditure. My greatest expense has usually been books, as I love to read. Some people will say that the e-book is taking over – true, but very slowly. The kindle, ipad, ereaders, joojoos, etc are coming. But frankly there is nothing like the smell, feel and ease of use that only a real book can offer.

Which brings me to the point of this post (“finally” you think…) I have found a source of cheap, interesting used (and new) books!

BetterWorldBooks.com – “The online bookstore with a soul”

Their operation revolves aruond gathering library discards, retired books and so on, rounds them all up, indexes them into a warehouse and sells them very cheaply. For those living in the US, postage is free, and those further out – postage is only US$3.97 per book.

From their site’s about page:

Better World Books collects and sells books online to fund literacy initiatives worldwide. With more than six million new and used titles in stock, we’re a self-sustaining, triple-bottom-line company that creates social, economic and environmental value for all our stakeholders.

We were founded in 2002 by three friends from the University of Notre Dame who started selling textbooks online to earn some money, and ended up forming a pioneering social enterprise — a business with a mission to promote literacy.

We’re not a traditional company with an add-on “cause” component. Social and environmental responsibility is at thecore of our business. You could say it’s in our DNA.

We’re breaking new ground in online bookselling. We believe that education and access to books are basic human rights. That’s why books sold on BetterWorldBooks.com help fund high-impact literacy projects in the United States and around the world.

All books are available with free shipping to any location within the United States (or $3.97 worldwide). And in case you’re concerned about your eco-footprint, every order is shipped carbon neutral with offsets from Carbonfund.org.

Here’s the best part: In addition to selling new titles, Better World Books supports book drives and collects used books and textbooks through a network of over 1,800 college campuses and partnerships with over 2,000 libraries nationwide. So far, the company has converted more than 25 million donated books into $7.3 million in funding for literacy and education. In the process, we’ve also diverted more than 13,000 tons of books from landfills.

Because we believe that most every book has lasting value and the potential to help change the world, we see our job as helping to find new homes for unwanted books. Thus far, we’ve donated 1.5 million books to partner programs around the world. Our five primary literacy partners are Books for Africa, Room to Read, Worldfund, the National Center for Family Literacy, and Invisible Children. Good company, no doubt.

Every book purchased from Better World Books contributes to individual literacy throughout the world and the promise of a better life. Clearly, we can’t do this work without our customers. That’s why we’re so passionate about trying to offer the best price, selection, customer service, and overall shopping experience.

Although that does sound all ‘warm and fuzzy’ – they deliver on their promise. My last order was for “Digital Electronics – A Practical Approach” by Kleitz, 5th edition. US$10 delivered to Australia. I even found a copy of Don Lancaster’s “CMOS Cookbook” [a classic] for US$4, or “Calculus” by Anton for the same price.  The selection below in total was less than US$70 – a bargain.

So before you hit Amazon or eBay, give this operation a try, I think you will be pleasantly surprised!

[Note - this article was initiated by myself personally without the aid or knowledge of the company in mention]

breadboards and batteries… bbboost

[3 July 2010 - this project has been retired, but the posts left for reference]

During my life in the field of electronics study, research, and daydreaming many ideas (good, bad and dangerous) and projects have been constructed using the typical solderless breadboard that everyone has used at one stage or another. There is nothing wrong with this approach, except for the power supply situation. You could either buy an expensive desktop power supply ($40 upwards), use a fixed plugpack (if you have the right voltage) then build some power smoothing into your circuit, or even use a 7805 or similar regulator to get your +5 volts. Failing that, it’s back to batteries.

You might as well just throw money into the garbage if you keep using batteries and have AC power nearby. I have had enough of worrying about all of this and have decided to conceive a desktop power supply that meets the following criteria:

• cheap to construct
• safe to use (not exposed to mains voltage)
• can accept any voltage DC plugpack and offer a variable, smooth DC output of up to 1 amp
• can be mounted on a small PC board with spacers to save money, or enclosed in a housing for a professional look
• have a digital output voltage meter – so that it looks cool and is convenient. No more guessing with analogue meters and dealing with parallax error
• increases the constructor’s knowledge of electronics!

Let us call it the bbboost – the bread board booster!

So over the next month or two we will do just that. If you would like to put forward ideas, suggestions or criticisms, please do so. Otherwise, get ready to say goodbye to breadboard batteries…

My calculator was afraid of the dark

For me calculators are like pens… many have passed through my hands, but my home is like an airport to them, they arrive, spend some time, then vanish during the middle of the night to destinations unknown. Especially that beautiful Hewlett-Packard 48G… come back! Yesterday after rummaging around for some graph paper my old accounting calculator appeared, blinking his/her display as it was exposed to the light for the first time in who knows how long.

It was alive! Finally something useful has returned home to the world of the useful. Well sort of… in the dark it shut down. *sigh* But in the spirit of repairing instead of replacing, this wouldn’t beat me. Armed with some miniature screwdrivers and a small box to hold all those screws in, I leapt in and started removing every screw in sight.

First I removed the rubber feet, as they are well-known hiding spots for crucial screws and rivets… but nothing there.

At this stage the screws looked like heading for the departure gates, so they went into the screw zoo…

Finally the cover came off, revealing some nice PCB work. Curiosity got the better of me and I kept unscrewing..

But at this stage, perhaps my enthusiasm had taken me too far… this looks easy to break and difficult to put back correctly!

Whoops… looks like things have gone a little out of hand. If this doesn’t go back together properly, the whole exercise and calculator has been a waste.

But there is something that intrigues me, two diodes, and something that looks like a cell or capacitor.

I was hoping it would be a capacitor that was charged by the solar cell, that would give me an excuse to use the SMD 0.2 Farad capacitors that I ordered by accident from Farnell last month. (Never order components at three in the morning)

But no, it looked like a typical button cell. Time to give it a health check:

(Yes, I know. Cheap multimeter) So the cell measures 0.66 volts DC. But what should it be? Only one way to find out…

Sigh. So much trouble caused by such a small thing. (Note to Casio: I know you want to sell more calcualtors to lazy people who cannot dissasemble them to replace the battery, but really, you couldn’t have done a better job in hiding this one). The cell is a GR927, and not listed in the Panasonic Australia or US web site. Argh. It is probably an OEM product, as it isn’t listed in Farnell or RS. So back to Google. Aha! It’s a “399 silver oxide battery” – 1.55V @ 60 mAh. Off to the centre of retail madness…

Ok, I’m back! That was quick.

And the saga is over. The calculator can now return to my desktop, adding up those material costs and how much money that isn’t around. I keep the cell in my dead battery box, one day we will have a disposal centre for them…

So there you have it people – don’t throw something out for a simple fault. Spend a few moments to investigate and repair. You might have lost half an hour, but you saved money, a tiny portion of the environment, and gained the satisfaction of doing it YOURSELF!

Kit Review – Seeedstudio Capacitance Meter

This is the first of many (hopefully) reviews of electronics kits. In the past I have often wondered what a kit would be like, as they aren’t something you can look at in a store, apart from a box of components or a magazine review. Especially products that need to be imported from abroad. So now I’m going to do some legwork for you! Let’s begin…

Recently several retailers had been offering a capacitance meter kit which seemed too cheap, however looking at the specifications it was too good to pass up. I ordered mine from a retailer in China, however it is available from much more local sources such as Little Bird Electronics, Sparkfun, and so on.

But on with the review. From placing the order on the web (paying with Paypal) to receiving the package took eleven working days. It was sent via registered airmail. Your time will vary wildly, depending on the time of year. For example, during Chinese New Year, nothing happens! Another benefit of using local retailers, no delay. Anyhow, thankfully the kit was packaged well with bubble wrap in a sturdy cardboard box.

After opening the box and attacking the bubble wrap I opened the package to check the parts against the list, and they were all there. The kit I received was a version 2.0, which would explain the part outlines and holes on the PCB but not in the list. There are also pads for external leads, and and holes for header pins if you wish to reprogram the microcontroller (but the pins were not included). The resistors were metal film 1% values, and the board was silk-screened and solder masked. However, an IC socket was not included… I feel this should have been – for the price this kit will attract many beginners who may overheat the microcontroller IC.

Also note that there is a plastic layer over the LED display, this took me by surprise as have not seen this happen on other displays in the past.

So it was time to get started. Being colourblind I measure all the resistors and place them in numerical order (from R1…Rx) on a breadboard to avoid mistakes. Then before soldering the overhead light, fume extractor and helping hands are moved into place to make life healthier and easier.

And using a magnifying glass is also very useful in spotting soldering mistakes and generally helping poor eyesight…

The layout of the components is screened on top of the PCB, so you can merrily go forth and solder. However, the polarity of the electrolytic capacitors is not shown clearly or mentioned in the instructions. After some detective work it turns out the positive pin of the electrolytics goes into the square pad. First I soldered in the hardware (switch, push button, DC socket), then the resistors, then the capacitors…

Then the crystal. semiconductors, ending with the microcontoller. As mentioned earlier, an IC socket should have been included to save a lot of people a lot of worry. Not everyone has steady hands or a good sense of timing! The extra ten cents wouldn’t have hurt the retail price. Anyhow…

I plug in my 9V DC plugpack, turn it on … and it worked first time! Woohoo. Note that due to the use of an LM78L05 voltage regulator, the meter runs on around 8 to 16 volts DC, using less than 100mA current. Watching the display was almost mesmerising, there’s nothing like that feeling of assembling something and seeing it work.

But did it really work? Let’s see… where are my capacitors?

The specifications state it can measure between 1 picofarad and 500 microfarads. The manual states that for better accuracy with measuring small values to enclose the meter in a metal box and attached the ground to the box. No time for that! Made do with four 20mm spacers to raise it from the desk. The specs state it is accurate to less than 2%. The user also needs to take note that the capacitor tolerance levels can vary, especially with electrolytics. Always try and check the manufacturer’s data sheet if possible. Supplier websites such as Farnell, Digikey and Mouser can be useful for that purpose.

So, first of all I tried a 0.1uF greencap, and it measured 99.3 nanofarads. Not a bad start. Always remeber to press the ‘zero’ button before each measurement.

Next a 0.01 uF greencap, returning 10.2~10.3 nF. Fair enough.

Then a 330 picofarad ceramic. Just to note at this point, one should clean the component leads before measuring, dirty leads will affect the value measured. Furthermore, short the capactor by crossing the leads over to discharge it completely. Anyway, that 330 pF returned 319 pF

How low can we go? Let’s try a 1.5 pF…

At this level, the metal shielding would be a good idea. The meter returned a floating reading 1.4~2.1 pF.

Finally, an electrolytic. 330 uF.

Which returned 343 uF. Not bad considering the tolerance of electrolytics can vary, at the minimum they can be +/-10%.

Now let’s see it action! The first capacitor tested is a 4.7uF electrolytic, the second a 1.5 pF ceramic. There is no audio in this clip.

So there you have it. For less than twenty US dollars you can have a decent capacitor meter that is easy to construct, quite sturdy, and very useful for the electronics enthusiast. This kit is available from Little Bird Electronics.

Thank you for reading and I look forward to your comments and so on.

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

DIY replacement cordless phone battery

For a few years now I have been using a Uniden cordless telephone system at home. Nothing special, a base unit with three handsets. However three years have passed and the batteries on the three handsets no longer hold any charge. The local excuses for electronics stores wanted almost $33 for a new battery pack (which consisted of three nickel-metal-hydride AAA cells of 800mAh capacity and a JST connecter). Hmm… three phones need three batteries…almost $99 for three batteries. Ouch.

Upon examination the battery packs were quite simple to reproduce, and there was plenty of length from the plug connectors to reuse them.
So let’s have a look. Here’s the original pack:

Here I have used three AAA NiMH 1.2V 900mAh (greater capacity cells!) with solder tabs. Thankfully there were not wider than the original cells.

So I tape them together and do a test-fit…

… solder the tabs together, keeping mind of positive and negative terminals. Two cells need to be aligned in the same direction, so some 28AWG wire connects between the + and -, covered with some electrical tape. Some narrow heatshrink would have been better, but that’s expensive as I don’t have any at home…

And here we have the finished pack in the phone. I placed the phone handset in the charge unit and kept an eye on it for three hours, it charged perfectly and the standby time is back to around seven days.

Money saved as well as the environment. Currently you can buy a whole new set including three handsets, one base unit and two charging stations for $99, but that would generate demand for much more materials, transport and so on. By replacing the packs with the cheaper cells the lesser of two environmental evils has been chosen.

Hello, world

Hello there. This is the blog of John Boxall. Who am I? For over seventeen years my profession has varied, roughly orbiting in the worlds of electrical, electronics, mobile communications, information technology, business and humanities. However now in the mid-life I have come full circle and returned to my favourite things – electronics design and having fun.

The world of open source hardware (and software!) has really highlighted a new beginning in the electronics and maker world – enabling people to start taking over by creating their and modifying their own things for the benefit of themselves or others. Well, that’s my take on it. Hopefully my writings and projects are of interest to you.

In the middle of nowhere

Please comment with your thoughts, criticisms and anything else as well.  Over time, I would like to share with you some electronic and other projects with you. Of course they will be open source, and licensed accordingly with Creative Commons.

Oh, there is a ‘subscribe’ box to your right at the top, why not enter your email address, then every time something new appears on the blog you will know as soon as possible. And a warning – I am colour blind, so my apologies in advance if anything looks odd design-wise. Please let me know if something is difficult for you to read so we can change it. My email address is john (at) tronixstuff (dot) com.

Anyhow, take it easy, be good to the world, and make something!