This week, Kai and I built an electronic game of skill out of a wire coat hanger.

You’ve probably seen this one before, the idea is to move a hoop along a bent wire without touching the wire. If you touch the wire, a buzzer sounds and you have to start again.

We built ours out of an old plastic box which once contained chocolate. A wire coat hanger was the main component. I cut off a few inches of the coat hanger to bend into a loop and used the rest to make the squiggly wire. The ends of the wire were bent into tight loops through which I put small bolts to secure it to the box.

The electronics and battery holder were simply glued to the inside of the box using hot-glue which is really an incredibly useful substance.

After I soldered a flexible wire to the loop, Kai put some bicycle handlebar tape around it to make the grip. For strain relief on the wire, I used a cable-tie.

The end of the wire is a yellow “safe zone” which was made simply by wrapping electrical tape around the wire to prevent electrical contact.

The noise maker

I didn’t have a buzzer handy but I did have a 555 timer chip so I used it to build an oscillator and connected that to a small piezo speaker in keeping with my principle of using whatever parts I’ve already got wherever possible.

Much has been written about the 555 timer IC. I have wired mine in the classic astable mode. The values of the components are not at all critical, the formula below shows how to compute the output frequency from the component values. The 100µF capacitor was added to provide a little sustain so that the sound is less “scratchy” when the wire is touched lightly.

Of course, if you are not a complete nerd, you would probably just get a buzzer.

Formula to calculate the output frequency

The result

I was going to paint the box but my wife (who is an artist) said “Noooo! it looks so post-modern in clear plastic with the chocolate label still on it” - so it has stayed un-painted.

This has cost me $0. It is built completely from parts I have found around the house. I like the challenge of trying to use what I have and this project completely achieves that goal. Here are some more photos:

I guess normal people don’t spend their time doing this kind of thing but you know I had an old desktop case which was for an AT motherboard and I wanted to make myself a new web server and a tower case would be too big to fit into my server cupboard. So the old desktop case went under the knife - a power jigsaw that is, with a metal cutting blade.

After butchering the back panel, I neatened it up a bit with a hand-held hacksaw blade and a metal file. After that, I vacuumed the case out thoroughly to make sure there were no metal shavings inside.

As you can see, the bigger motherboard fits now. Removing a large chunk of the back panel allows the connectors for the keyboard and other ports to be accessible.

Unfortunately, the CPU cooler sticks up too high so I could not fit in the PSU. However, I am a computer nerd and always have plenty of odd spare parts lying about such as a Micro-ATX PSU which fitted in just nicely after I drilled a few new mounting holes.

Once I put in the hard drive, CD-ROM, PCI cards and everything, the box is getting rather full but it does all fit.

And here it is in my server room - uh, I mean server cupboard. It’s running a PIII 667 overclocked to 750MHz with 384MB of RAM and a 250GB hard disk. Debian Linux of course (if you have been reading my blog, you must realise by now its my favourite OS). And it is all working great. After all, it has just served this web page to you!

Now all I need to do is enter the new machine into the Linux Counter.

Kai has been loving the Kids Electronics Lab we made last weekend. Bea tells me he played with it all day on monday while I was at work. I’m happy that he likes it.

I have been pondering the best way to add some over-current protection to prevent damage to the batteries in the event of a short. A colleague suggested I try a Polyswitch - he even donated one for this project (thanks Ian!) thus saving me about $2.

This device is also known as a PTC fuse and is commonly used to protect loudspeakers. When a certain current is reached, it will trip and go into a high-resistance state until the current is removed. You can see a photo here of the device dropping the current to 120mA despite a dead short across the battery. The trip current is well over an amp so it does not affect normal operation of the board. Neat.

Today with the help of my four year old son Kai, we built a little experimenter’s board because he has been wanting to “learn about ‘lectricty” for a while now.

First I found an old piece of MDF from my shed and cut off a bit about 10″ by 6″. Then I bought some screw eyes from the hardware shop. I already had this bunch of colourful clip leads.

To build it, we drilled some 1mm pilot holes and then screwed in the eyelets with a small washer under each one. Kai loved having a go of a power tool even if it was only a little battery-powered Dremel. The wire from each part gets clamped under the washer as the screw eye is tightened. I used some pliers to screw the eyes in nice and hard so the wire will stay clamped.

I have soldered a current limiting resistor in series with each part. I used 100Ω for each of the LEDs and a 15Ω ½ watt resistor for the motor. These values were arrived at by guesswork, experimentation and whatever I could find in my parts box using the time-honoured principle of “Whatever works man”. I used a little hot-glue to hold the parts in place.

I attached the battery holder to the board with some velcro so it can be easily lifted up to change the batteries.

And here’s the finished product. I still do not have any kind of over-current protection for the batteries. I guess it won’t be long before the child tries to connect the battery terminals together and kills them. I’ll see what I can come up with in the future but for now we are having too much fun to bother about it.

I spent about $2 all up to buy a packet of screw eyes. Everything else came out of my junk box. That is my driving philsophy here. Just use what I’ve got, otherwise, I may as well have spent $50 on a store-bought electronics kit.

The board contains 4xAA batteries, 2 LEDs, a momentary push button and a DC motor. Kai can make up circuits by clipping the clip leads onto the eyelets. I have left some room on the board so we can add other bits on in the future. Its simple and fun enough for a four year old.

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Kid Type

UPDATE - Adding current protection

On a lazy Sunday afternoon, some time back now. I decided to build a synthesizer.

Analogue electronics is not really my thing, I’m much more a digital person but when I found a design online for The #3 Standard WoggleBug by Grant Richter, I just had to build one.

First I built the circuit on some veroboard. This thing is controlled by a bunch of potentiometers, there is no keyboard and certainly no MIDI!

Then I was wondering what to make for an enclosure. My friend Jasper found an empty CD spindle and you know, it was just crazy enough to work.

To create the control panel, I got a CD-R and messed it up a bit with a blowtorch. I mounted the knobs on this and glued it to the central shaft of the CD spindle. A little more glue and the whole thing was assembled inside the spindle.

Here is the finished product. As you can see, the cover of the CD spindle makes a handy travel case.

If you want to hear what it sounds like, Click here for an MP3 (1MB). Okay, this is not a pure recording, I have added some drums and a couple of effects because this thing sounds really rough and is almost impossible to control. You’ll have to imagine me twisting the knobs frantically while you listen.

Anyway, I had fun building it and that’s what makes me happy.

I have now finished the code. Tested it and after a few bug fixes, everything works just great. The final software is 930 lines of assembler. You can download it here if you want to.

I also learnt how to use ProTel so I could draw up the final circuit properly. Here’s the result. Notice that I’ve got it down to just one chip now, the little PIC is doing everything. It can directly drive the LED and the MIDI output. I’ve used multiplexing to enable it to drive the 7 segment display and read all the inputs. The PIC’s built in timer keeps everything running accurately, all in all, I’m very pleased with the result.

I threw it into a box and its done! You can see in this photo that I also added a second MIDI output.

Sorry there have been no updates in months. Things have been busy lately, that’s the problem with having a life. I’m only a part-time geek after all.

Since I last did any work on this project, I have upgraded my PC to a Duron 900 and have installed Windows 2000. I am most impressed with the stability of Win2K, I rarely have more than two crashes per week as opposed to about three times a day under Win98.

However, Windows 2000 will not run the NOPPP software which I have been using to download code into the PIC. I suspect the way ports are accessed in Win2K is not backward compatible with Win95.

To cut a long story short, I have found the Linux version of NOPPP works just fine so I’m using that instead. gpasm is a free compiler for the PIC which runs on Linux so I’m all set to continue development without using Windows.

I’ve redesigned the thing again and got rid of the buffer chip. I’m driving the MIDI output directly from the PIC and it works fine. I’ve used a 7 bit bus arrangement to allow all the inputs to share the same I/O pins as the LED display.

One little hitch was the discovery that the big red illuminated push button needs 12 volts to light up. But I’m running everything off 5 volts. So I’ve made a little adapter out of a small piece of veroboard so I can substitute a LED. I tellya these modern LEDs are so bright! I nearly blinded myself messing about with it.

Well, It’s great to have something work. I finally got it to send out MIDI clock messages and my sequencer accepted them. Yay!

One thing I had a lot of trouble with was understanding the MIDI signals. I couldn’t seem to find much documentation on them. I had to do a lot of trial-and-error and also use my CRO to analyse the signals coming out of some MIDI equipment. Here’s what I found:

MIDI works by current, not voltage. The receiving end has a 1k input impedance and the sender is expected to put a 5V voltage differential across this which produces a current of 5mA. Current flow is indicative of a logic 0. No current flow is a logic 1. There is no current flow when the line is idle.

A fairly standard way to implement a MIDI output is to tie pin 4 of the DIN plug to 5V and pin 5 to your digital output. This forms a hardware inverter so you can use normal logic levels in your circuit.

The data rate is 31250 bps +/- 1%

Another confusing thing is that nowhere is it described how the serial data is sent. The literature will tell you that there is 1 start bit, 8 data bits and one stop bit. Great, just like RS-232 I thought. Well no, it’s not anything like RS-232. The start bit in MIDI is a 0, the stop bit is a 1 and the data bits are sent in reverse order. All perfectly fine but I had to discover this myself.

The firmware is now over 1000 lines and is almost finished. I’ve just got to figure out how to store the BPM setting in the internal EEPROM and it’s done.

I’m steadily building the firmware for this thing. Something that sounds simple is turning out to be a little more complex than I thought, still, that’s pretty normal in the software world. My program is around 600 lines and I’ve still got a lot of stuff left to do.

Another problem I’ve come across is the Potentiometer voltage ramp arrangement has turned out to be not very accurate, the reading varies wildly depending on voltage, temperature, current and just about any other variable you can think of. After wasting a lot of time with it, I’ve decided it’s just not a suitable method for setting the speed.

So instead, I’ve got myself a rotary encoder. This looks like a potentiometer but it’s actually a binary counter which increments as you turn it. It has detents so it won’t drift once you turn it to a specific position and is generally much more suitable for this role.

Of course, I now have to modify the software to read it.

Now it’s time to move back into the software domain. Microchip supply a development environment called MPLAB. You can download it for free off their website. Only problem is, it doesn’t work, it kept giving me these errors “Could not initialise dialog box” and would refuse to compile anything. I suspect that there is a DLL missing in the installer, but I have little time to debug other people’s software.

Fortunately, there is another little utility that comes with MPLAB called MPASM for Windows. This program is a little irritating since it dies after every compile but having to reload it every time is a minor inconvenience compared to MPLAB.

So, finally I managed to compile some of the example code. The next step was to learn the PIC’s programming language and write some stuff of my own.

I haven’t done any microcontroller programming for six years, and even then it was on a Motorola 6805, a totally different architecture; so I expected a bit of a learning curve here. But to my surprise, I managed to learn the new instruction set and write some code that worked within an hour. Writing firmware must be like riding a bicycle.

The time had come to do some more electronics. I put together my LED display and wired it up to the PIC. A little bit of soldering, a little bit of coding and in hardly any time, I had a fully functional 3 digit LED counter tumbling away through the numbers.