An electric continuity tester for checking the electrical connections, cables can be designed using this circuit diagram. This electric continuity tester produces a beep when there is a short and does not emit any sound when the circuit is interrupted or when the resistance along the circuit exceeds 1ohm. To prevent failure of sensible components, and for a long battery life, tester injects a weak signal in the circuit is checked.
When testing the connections, there are few opportunities to be involved in resistance measurement, semiconductor and other components. Furthermore, it is possible that some parts do not withstand the current and / or voltage testers inject it. For this reason a good tester will notify the PN junctions with low impedance (diodes, transistors) and resistant. Furthermore, the device must be sensitive enough to work with a weak signal for testing.
Due to the large amplification of the operational amplifier 741, current and voltage for the test signal may be limited to 200 uA and 2 mV. The difference in voltage between the inverting input (pin 2) and noninverting (pin 3) of the 741 is amplified considerably. Voltage falls out on R2 ensure that operational amplifier become negative once the inverting input has a greater potential than the noninverting input. Noninverting potential entry can be increased by adjusting P1, so that the input is more positive than the inverting.
Oscillator built with N1 will generate when the buzzer sound. Voltage falls to R2 is the result of a good contact between terminals tester.
Frequency and volume can be adjusted with the potentiometer P2.
A value less than 1 ohm or a good contact point or a short circuit. To calibrate the continuity tester to follow the following procedure:
Put a resistance of 1 ohm (5 or 10%) between terminals P1 and adjust tester so that the buzzer to be at the entry into operation. Remove 1 ohm resistance and creates a short circuit between terminals tester, at which volume can be adjusted using potentiometer P2. When the tester terminals are opened (short circuit is removed), buzzer should stop.