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Project Notes

#285 The Tap

Harking back to a 7400-series logic circuit published in a 1974 Elektor article, this retro Boldport Club project is a classic.



The Touch Activated Programmer (TAP) Sensor from Elektor magazine of December 1974 was a very early example of a capacitive touch circuit.

And Boldport Club members were lucky enough to get a “modern” rendering of the same circuit. The PCB design is as psychadelic as you would expect from a 70’s vintage design. What is amazing is that the parts are still quite readily available.

The PCB is well up to Boldport standards. Altough this can create some assembly confusion - you must pay close attention to which side of the board the parts are inserted. It looks just as good both ways.

Electrically, touch pads A, B and C are connected to an NPN transistor pair in a Darlington configuration. I’ve covered the operation of this kind of switch in LEAP#130 DarlingtonTouchSwitch.

All manner of outputs are provided. For each touchpad there are four:

  • Q/Q1¯ - the logic-level output and its complement
  • S/ which are logically identical, but an open-collector configuration able to sink greater current


Action Q1 Q1¯ Q2 Q2¯ Q3 Q3¯
At power-on 1 0 1 0 1 0
Touch A 0 1 1 0 1 0
Touch B 1 0 0 1 1 0
Touch C 1 0 1 0 0 1
Touch RESET 1 0 1 0 1 0

S/S¯ and Q/Q¯ Outputs

S/S¯ and Q/Q¯ outputs follow the same logic behaviour.

It appears the only difference is that Q/Q¯ are mostly suited to digital integration as they are limited in the current they can sink (16mA).

  • Q and Q¯ source current, logic 1 : 0.4mA
  • Q and Q¯ sink current, logic 0 : -16mA

The S/S¯ outputs are controlled with low-side NPN switches and can sink up to 1A - the absolute maximum collector current for the PN2222ATA. Higher currents would need closer attention to heatsinking and wire/connector ratings.

RB (Reset Bar) and CB (Contact Bar)

The RB and CB pins are intended to allow chaining of mulitple sensor units with a common reset/contact bus.

Parts and Unboxing

Ref Item Qty
T1-7, 9-14 bipolar (BJT) single transistor NPN, Fairchild PN2222ATA x13
T8 bipolar (BJT) single transistor PNP Fairchild BC556BTA x1
IC1,2 NAND gate 4 gate, 2 input, TI SN7400 x2
D1-3 Small signal diode 1V Multicomp 1N4148 x3
R1-3 100KΩ resistor, Multicomp MCF 0.25W 100K x3
R4-7 10MΩ resistor, Multicomp MCF 0.25W 10M x4
R8 1KΩ resistor, Multicomp MCF 0.25W 1K x1
R9-15 27KΩ resistor, Multicomp MCF 0.25W 27K x7
C1,2 220pF capacitor, Vishay K221J15C0GF53L2 x2
C3 47nF capacitor, Multicomp MC0805B472K500A5.08MM x1
  14-pin IC DIP socket , TE Connectivity 1-2199298-3 x2
  20-contacts 2.54 mm header, Multicomp MC34739 x1

kit_unboxing1 kit_unboxing2 kit_parts kit_pcb_rear kit_pcb_front

The transistor markings on the silkscreen threw me at first. They are special! I was confused because they don’t replicate the standard NPN/PNP schematic exactly. Just focus on the arrow flow: NPN points out from the emitter; PNP points in to the emitter.



The Boldport and Elektor resources already provide thorough details of the circuit. But as is my way, I just had to redraw the circuit. This time with Fritzing:





Running some initial current tests. It never peaks beyond 23mA.


Hooked up for monitoring under test (analog scopea and logic analyzer)


The final build:



The first trace I took was from a transition to “A” on (from RESET state). CH1 shows Q1 output, CH2 shows Q1¯ output. As we can see, there’s some pre-emptive voltage reduction prior to the switch and some jitter and one significant bounce. From this we can see that using the TAP as an input for a sensitive instrument would still need some debouncing.


Here is the same switch with alogic analyser. There’s quite a delay (2-3µs) between the analog transition and the digital - but I think that is more the scope than the circuit. The important point is that we still see one bounce at logic levels, and the other outputs are as expected.

  • CH1 - Q1 output (trigger)
  • D0 - Q1 output
  • D1 - Q1¯ output
  • D2 - Q2 output
  • D3 - Q2¯ output
  • D4 - Q3 output
  • D5 - Q3¯ output
  • D6 - unused
  • D7 - unused


Credits and References

Project Source on GitHub Project Gallery Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.