#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.

Notes

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/S¯ which are logically identical, but an open-collector configuration able to sink greater current

Behaviour

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

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.

Construction

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:

Performance

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

• 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