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.
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.
Parts and Unboxing
|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|
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
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