Using electronic diagram below can be made a very simple circuit that activate a relay at the touch of a sensor with finger.
Sensor contact can be made from an RCA connector or some other audio jack, which has high resistance insulation .
Inverting input (-) of IC1 is powered at half of the supply voltage with help of R4-R5, while the non-inverting input is connected to ground potential through R3. When the sensor is touched AO output switch and relay state H (9 V or 12 V) is enabled. This time relay operates device connected thought it .
Resistor R1 and capacitor C1 prevents accidental triggering.
Circuit absorbs a current (standby) of only 5 mA, if is used a operational amplifier 741, and only 0.5 mA, if is used TLC271 AO. When the relay is actuated, the current increases to 30 mA.