Article : Andy Collinson
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Description
This touch switch uses two sets of contacts for on and off control. Once "on" it remains latched until
the "off" contacts are touched. A relay is used for the output.
Notes
This touch switch uses two sets of contacts for on and off control. Once "on" it remains latched until
the "off" contacts are touched. A relay is used to control lamps, motors or other external circuits.
At switch on Q1 base is tied to ground via R2 and off. Q2 and Q3 are connector asa high gain Darlington pair, and the base potential of Q2 is also grounded via R3 and the relay coil. Q4 is connector to the output of the Darlington at Q3 and Q2 collector and therefore Q4 is also off. The circuit will remain this way indefinitely.
Now if you touch the "On" contacts, a current will flow via R4 into Q2 base and the Darlington pair will switch on. The output current flows via R1 into Q4 base and transistor Q4 switches on and operates the relay. As Q4 is a PNP the collector voltage is positive and will be almost full supply volte less the C-E junction voltage of Q4. This positive voltage is fed via R3 back into Q2 base and the circuit is now "latched" into the on state. You can remove your finger from the contacts, the circuit is still on.
The only way to switch off the circuit is to briefly touch the off contacts. In doing so, Q1 now switches on and as its connected to the junction of R3 and Q2's base, the Q2 and Q3 switch off, as does Q4 and the relay. The circuit is now latched in the off state again.