Digital logic touch switch circuit

Many circuits for TAPs (Touch Activated Pro-gramme switches) have previously been pub-lished. However, all of these required the use of two pairs of touch contacts, one to set the TAPto the ‘on’ position and one to reset it to the ‘offposition. The novel feature of this circuit is that it requires only one touch contact. Touching the contact once sets the TAP; touching it a secondtime resets the TAP.

N1 and N2 form a flip-flop (bistable multivibrator). Assume that initially the output of N2 islow. The inputs of NI are also pulled low via R2, so the output of Nl is high. The inputs of N2arethus high, which satisfies the criterion for theoutput to be low, which was the original assump-tion. CI is charged to logic high through R3 from the high output of N1. 

touch switch circuit
Digital logic touch switch circuit

If the touch contacts are now bridged by a fi nger, the logic highon CI will be applied to the inputs of N1 through R1 and the skin resistance. The output of N1 will go low, so the output of N2 will go high, holding the inputs of N1 high even if the finger is removed. The TAP is now set.

Once the finger is removed, CI will discharge through R3 into the low output of N1, If the touch contacts are subsequently bridged, the inputs of N1 will be pulled low by C1 (since it isnow discharged). The output of N1 will thus gohigh and the output of N2 low, which will holdthe inputs of N1 low even after the finger hasbeen removed. The TAP is now reset to its orig-inal state. C1 will charge to logic high through R3 from the output of N1, ready for the contact to be touched again.

The only constraint on the operation of the circuit is that the interval between successive operations of the switch must be at least half a second to allow C1 time to charge and discharge.