continuity checker

100 continuity checker The question often arises — is it a high resistance or is there an open circuit somewhere? The purpose of this tester is to check whether or not there is a conductive path with a resistance of less than 5 M12 between two points. A higher resistance than this is indicated as an open circuit. The results are indicated by two LEDs. As the circuit diagram shows, the drain of FET T1 is directly con- nected to the positive supply line (consisting of two 1.5 volt cells) and the source is connected to the negative rail via resistors R2 and R3. The circuit untder test is connected between the gate and the negative rail. Since the FET only conducts with a gate voltage (as opposed to current), no distinction is made between large and small resistance values (provided these are lower than 5 Ma). When open circuit, the voltage on the gate is +3 V with respect to S1 ground and T1 will conduct thereby causing the voltage at the source to just about reach the supply volta...