A revival of a forty
year old kit
At a time when powerful processors are so cheap and much of their functionality left unused in a typical application, it’s delightful to look back to hobbyist electronics projects from forty years ago, and admire the optimised design and the skill of those who created them.
In the first edition of Elektor magazine in English, December 1974, appeared a lovely touch-activated circuit. We've re-created it for the enjoyment of those who may appreciate the elegance of the design today.
The functionality of the circuit is comprehensively and beautifully explained in the original article. Here it is as a PDF, and it's really worth reading. Here is the schematics I re-created for the board.
We haven’t made any changes to the circuit, and remarkably all of the same components are still available today, including the original 7400 IC!
Before you do anything, note which side the components are mounted. Look for the symbols described here for an indication.
The 'legend' — the component symbols — on the back-/solder-side of the board is meant to help you orient the components from the top. Also note that there are two wire jumpers, and these need to be bridged with a wire or a clipping from other components.
Also note that T8 is a PNP transistor, whereas the rest are NPN transistors. We suggest that you place it first so to avoid confusion.
Here are the components included in the kit:
- x13 bipolar (BJT) single transistor NPN, Fairchild PN2222ATA
- x2 NAND gate 4 gate, 2 input, TI SN7400
- x3 Small signal diode 1V Multicomp 1N4148
- x1 bipolar (BJT) single transistor PNP Fairchild BC556BTA
- x4 10MΩ resistor, Multicomp MCF 0.25W 10M
- x7 27KΩ resistor, Multicomp MCF 0.25W 27K
- x3 100KΩ resistor, Multicomp MCF 0.25W 100K
- x1 1KΩ resistor, Multicomp MCF 0.25W 1K
- x2 220pF capacitor, Vishay K221J15C0GF53L2
- x1 47nF capacitor, Multicomp MC0805B472K500A5.08MM
- x2 14-pin IC DIP socket , TE Connectivity 1-2199298-3
- x1 20-contacts 2.54 mm header, Multicomp MC34739
To maintain the original look of the circuit (it was exposed copped), we've used transparent solder mask (instead of the standard green) to protect the copper. The touch pads and solder pads are exposed, and may oxidise (this is normally prevented by a 'finish', which isn't applied here); is that's the case, apply some alcohol to clean the pads up.
When assembly is complete you can connect an LED (and current limiting resistor) to the outputs. You could easily use the Cordwood Puzzle for that as it has three LEDs.
The TAP is an open source design, as is most of our work. You can find the design files for the hardware and packaging at our GitHub repository. You can edit the files using our own source software, PCBmodE.