The device is actually a simple LED and a cell circuit, where the LED is made to switch by passing the cell voltage to the LED via the conductor in question. If the conductor is not broken, the cell voltage circulates through it and reaches the LED to complete the circuit and in the course illuminates thre LED, providing the relevant information. If the conductor is open internally, the cell voltage is unable to complete the circuit and the LED remains shut OFF, indicating the fault.
The first circuit diagram shows a very simple continuity circuit where only a LED/resistor set up along with a 3 volt source is used.
The prods are connected across the ends of the wires or the conductor which needs to be checked. The results regarding the status of the wire is achieved as explained above.
However this circuit is quite crude and won't be able to check big cable networks where the fed voltage may drop substantially in the path and might fail to illuminate the LED properly.
For checking complex and large wire or cable bundles, rather a much sensitive circuit may be required.
The next circuit shows a configuration which is much rugged and highly sensitive.
Moreover the wire ends may be checked via finger touches, which simply avoids th need of lengthy prods from the continuity tester.
The circuit employs a couple of cheap hi-gain transistors which are coupled together in such a way that the over all gain of the circuit becomes very high.
Even a few milli volts is enough for making the circuit conduct and illuminate the LED.
The connections can be seen in the figure, how through easy finger touch operations, even the staus of big wire bundles may be identified in seconds.
If the wire bundle is without breaks, the LED lights up brightly, and in case the wire is open somewhere, keeps the LED completely shut OFF.
This sensitive circuit can also be used as a line tester, the 3volt point is held with hand, and the 1M end is touched to the point where the LINE presence needs to be tested.
The presence of phase, lights up the LED and vice versa.