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Project Notes

#345 mostap

Upgrading the tap to modern CMOS and MOSFET technology, circa 1975 - another Boldport Club retro classic. The mostap is a touch sensor using NAND-gate flip-flops and FET output drivers.

Here’s a quick demo of it in action..



The MOS TAP circuit was originally published in Elektor magazine in Feb 1975. It was an update or the original TTL Tap circuit to use “state-of-the-art” COSMOS (CMOS) components.

In practical terms, that means 4000-series logic instead of 7400-series logic.

The Boldport Club version of this venerable circuit makes an additional improvement by replacing the output stage BJT drivers with 2N7000 n-channel MOSFETs so that it really does live up to it’s name.


I love the name! The Elektor article is the first time I’ve come across it. As far as I can figure CMOS was initially marketed with “COS/MOS” labeling by RCA .. so it’s a trade name that was displaced by CMOS as an industry standard term.

Interestingly, the name appears to have re-emerged in recent times with the DARPA COSMOS program .. geared towards high speed SoC-type applications. See also “Beyond CMOS” from 2013.

Parts and Unboxing

Ref Item Qty
R1,3,5,7,9,12 10MΩ resistor Multicomp MCF 0.25W 10M x6
R2,4,6,8,10,11 1MΩ resistor Multicomp MCF 0.25W 1M x6
R13 2.7MΩ resistor Multicomp MCF 0.25W 2M7 x1
C1 47nF capacitor Suntan TS15001H473MSBUB0R x1
C2 470pF capacitor Suntan TS15001H471KSBPA0R x1
IC1-3 Quad 2-input NAND gate TI CD4011BE x3
T1-5 MOSFET n-channel transistor Fairchild 2N7000_D26Z x5
D1-5 Small signal diode Diotec 1N4148 x5
  IC DIP 14 contact socket TruConnect DS1009-14 x3
  20-pin 2.54mm header Multicomp MC34739 x1
  Lovely lovely copper PCB x1

kit_unbox kit_parts kit_pcb_front kit_pcb_rear


The MOSTAP is designed to be chainable, but I only have one so I am not using the modifications for chaining (dash-dot lines in the schematic). In the stand-alone configuration:

  • 5 sensors with flip-flops and output drivers (touch pad A, B, C, D, E)
  • the final flip-flop (X) is configured as a monostable reset
  • touch-pad X resets the flip-flops




Initial Breadboard Test

Works OK on a breadboard, but it is a bit cumbersome..



I decided to whip together a test jig mounted on the Boldport box. It features:

  • power connector and power LED
  • female header for all output connectors
  • for each open-drain output:
    • LEDs and 470Ω current limiting resistors on each open-drain output
    • 12kΩ pull-ups (so logic-level output is available)
  • reset bus push-button (momentarily pulls-down the reset bus)
  • common bus push-button (momentarily pulls-up the common bus)

Here’s a sketch of the layout I used:


And the final build:


Operating Mode Modification

The wire link connecting touch pad X to the reset bus is responsible for the “one-shot” behaviour. IF removed, then all outputs remain latched until explicitly reset.

Replacing the link with a switch allows the operating mode to be selected:

  • open - latched mode. Outputs remain on until pad X touched/reset bus pulled low
  • close - one-shot mode. Outputs remain on until another output is activated or all reset



The mostap performs excellently as a touch switch - with none of the bouncing and transients I’d seen in the original tap.

Here’s a capture of a set of tap outputs when activated:

  • CH1 (yellow) - A output
  • CH2 (blue) - A’ output

The mostap was mounted in my test jig for this, and the supply voltage set at 5V.


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.