555 timer IC that is working as an astable multivibrator in the circuit. The output sound frequency of the circuit can be adjusted with the 240K variable resistor. The two probes marked "Testing Probes" are the testing point of the circuit. Attache two wires with these two points to check the continuity, for good performance use multimeter wire types. An 8 ohms speaker is connected at the output of the circuit through a 47uF capacitor. A .25 watt 8 ohms speaker is used when the circuit was in test and it was working fine. But speakers with higher watt values will also work here for example 1watt, 2 watt, 4 watt, 5 watt etc. You can also use mini thin flat speakers. Operating voltage of the circuit is 5V to 9V DC. The circuit will produce adequate sound on 5 volts and a little loud on 9 volts.

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The figure below shows a very useful project of a continuity tester using 555 timer IC. It can be used to check faulted wires and circuits. It will provide an audible indication when a connection is ok, shorted or if a circuit becomes open due to faulted components.
The heart of the circuit is a
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Continuity Tester Using 555 Timer IC