Touch-Sensitive Electronics

By Gus Donnachaidh

Originally published in EUG #24

Recently, I needed a touch-sensitive switch which would work with the slightest touch on the contacts. In addition, it needed two-stage action with a quick touch momentarily turning on a relay; with a sustained touch starting a second relay until the power to the circuit was disconnected.

The current into the base of TR1 is limited by a large value resistor R1. TR1 is a PNP which means that the current is negative going. TR1 turns on TR2 which is a high current transistor which turns on RLA1.

TR1 also charges up capacitor C1 through VR1. VR1 can be set to delay the charging of C1. When C1 is sufficiently charged, it will turn on TR3 which is another PNP transistor, this turns on SCR1. The resistors between the collector and emitter of TR3 limit the current into the gate of SCR1 which is important if SCR1 is not to disintegrate. SCR1 turns on RLA2 and keeps it on. In my case, the contacts of RLA2 operate the same circuit as RLA1.

I would like to adapt the second stage of the circuit so that SCR will switch off and the whole circuit will return to its 'ready' state after a fixed period variable between say 5 and 15 minutes.

Hopefully, some other members who are interested in electronics will send in some circuits using the CAD+ program (EUG #23). If any members have an interest in electronics but may like to learn more then write in!

Also, remember with CAD+ that it is possible to design almost any icons to suit any application. If you have a special interest which can be illustrated with diagrams then design these and send in some information.

Gus Donnachaidh, EUG #24