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 complementS/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 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.
The RB and CB pins are intended to allow chaining of mulitple sensor units with a common reset/contact bus.
| 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 |
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
- Tap Sensor - original circuit and article from Elektor magazine December 1974
- The Tap - Boldport product information
- The Tap - in the Boldport shop
- The Tap - OSH files on GitHub
- The Tap - club community site, packed with resources for the project
- SN7400N info and datasheet - parts.io
- PN2222ATA info and datasheet - parts.io
- BC556BTA info and datasheet - parts.io
- LEAP#130 DarlingtonTouchSwitch
- ..as mentioned on my blog