Quick On-Board Junction Test

Circuit operation:


Both inputs of IC1A are connected together by two equal value resistors (R4 and R5) and to half the voltage supply obtained by means of the voltage divider R2 and R3. So, the same voltage should be present at both input pins.

In practice, half the voltage supply (i.e. about 4.5V) will be present at the inverting input (pin #2) of IC1A, but the constant voltage generator formed by R6 and D1, feeding the non-inverting input (pin #3) of IC1A by means of the voltage divider R7 and R8, clamps this pin to about 4.1 - 4.3V: this will cause the output of the op-amp to stay low.

If the circuit input (R2 to R3 junction) is shorted to negative ground (a condition equivalent to a shorted transistor junction) pin #2 of the op-amp will go to 0V and the voltage at pin #3 will decrease to about 0.3 - 0.35V (caused by the constant voltage generator mentioned above): the op-amp output will go high, activating the piezoelectric sounder.

When a real transistor or diode junction is connected to the input of the circuit instead of shorting the input probes directly, the piezo sounder will emit only a short single beep just as the probes will come in contact with a good junction, due to the time delay provided by the discharge of C2 when the voltage at pin #3 is falling from about 4.1V to 0.3V.

To provide a better signaling system, Fet Q1, IC1B and related components were added. This op-amp is wired as a 1Hz square wave generator and Q1 acts as a solid-state switch, going on and off one time per second having the Gate driven by the op-amp output. In this way, the junction of the device under test is connected and disconnected to the voltage sensitive circuit built around IC1A one time per second and the result will be a clearly audible train of short beeps signaling the good condition of the junction or track under test.





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