A Veritable Almanac!
This circuit appeared in ETI electronics as a three stage version that illuminated up to three incandescent lamps. This version has four stages that drive low current LEDS instead and uses pressure pads rather than copper hand held tubes as in the original circuit.
R1, R3, R4, R5, R6 1K
R7, R8, R9, R10 3K3
D1 -------- D10 1N4148
TR1 ------- TR5 BC548
LED1 --- LED4 GREEN
PP3 BATTERY CONNECTOR
PRINTED CIRCUIT BOARD
HOW IT WORKS
The skin resistance of the person completes the circuit between R1 and VR1 when applying pressure to the copper pads on the double sided pcb board. This produces a voltage at the base of TR1. This voltage is coupled by TR1 to R2.
The circuits containing TR2, TR3, TR4 and TR6 measure the voltage from the output at R2. If the voltage is higher than that set for each circuit the corresponding LED turns on.
Each measuring circuit has its trigger voltage set by 0.6V required by the transistor and the number of diodes, for each diode in the circuit the voltage increases by 0.6V.
The first LED will turn on if the voltage output across R2 is greater than 1.8V because it has two diodes plus the transistor in the circuit and so on.
VR1 will adjust the sensitivity of the circuit.