Continuity Tester

A continuity tester is a useful bit of kit for any electronics enthusiast but many of the off-the-shelf models you can buy suffer from one serious disadvantage: they can end up destroying the component under test!Most silicon semiconductor junctions start to conduct when the forward voltage across the junction exceeds about 0.6 V, but other devices such as Schottky diodes conduct with a much lower forward voltage. If insufficient care has been taken in the design of a continuity tester to reduce the measuring current flow then the component probed can become electrically stressed and may fail. The design presented here produces a measuring voltage of just 5 mV into a test resistance of 10 ?. In theory the measuring voltage can rise to a maximum of approximately 0.8 V (the battery voltage, VB, less Vbe, the voltage drop across the base-emitter junction of one transistor) but the measuring current will always be limited to safe value of 0.6 mA to avoid damage to the component under test.

Resistors:
R1 = 33kOhm 
R2 = 4kOhm 7
R3 = 1kOhm 8
Capacitors:
C1,C2 = 10µ F 16V radial
Semiconductors:
T1 = BC557
T2,T3,T4 = BC337 or BC347
Miscellaneous:
BT = AAA 1.5V battery with holder
Bz1 = DC buzzer for 5-6 V
Length of PVC electrical wiring pipe (used as case)
PCB, order code 020002-1