Continuity tester for pcb

Posted on Aug 15, 2012

The continuity tester is for tracing wiring on printed circuit boards. It only consumes any appreciable power when the test leads are shorted, so no On/Off switch is used or required. The applied voltage at the test terminals is insufficient to turn on diodes or other semiconductors. Resistors below 50 ohms act as short circuit; above 100 ohms as open circuit. The circuit is a simple multivibrator—Tl and T2, which are switched on by transistor T3. The components in the base of T3 are Dl, Rl, R2, and the test resistance.

Continuity tester for pcb
Click here to download the full size of the above Circuit.

With a 1.5 volt supply, there is insufficient voltage to turn on a semiconductor connected to the test terminals. The phone is a telephone earpiece but a 30 ohm speaker would work equally as well.

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