#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..

Notes

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.

COSMOS?

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

Construction

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..

A MOSTAP Test Jig

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

Performance

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

• MOS TAP - original circuit and article from Elektor magazine February 1975
• LEAP#285 The Tap - Boldport Club project for the original BJT design
• MOSTAP - Boldport product information
• MOSTAP - in the Boldport shop
• MOSTAP - OSH files on GitHub
• MOSTAP - club community site, packed with resources for the project
• CD4011 datasheet
• 2N7000 datasheet
• 1N4148 Datasheet
• 4000-series logic
• 7400-series logic
• ..as mentioned on my blog