Strength Tester

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Electronic Strength Tester

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.

RESISTORS


R1, R3, R4, R5, R6                1K

R7, R8, R9, R10                     3K3

R2                                           470R


DIODES


D1 -------- D10                         1N4148


TRANSISTORS


TR1 ------- TR5                         BC548


LEDS


LED1 --- LED4                         GREEN


PRESET RESISTOR


VR1                                           200K


MISCELLANEOUS


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.


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