Using NE555 to make diode and triode detector

Using NE555 to make diode and triode detector

This circuit can detect the PN junction type (PNP, NPN) to determine whether the tube is good or not. The detection is convenient, simple and practical. The 555 integrated circuit and R1, R2, C1, etc. form an astable multivibrator, and its oscillation frequency is f=1.44/(R1+2R2). C1 inserts the corresponding poles of the crystal triode to be tested into the corresponding e, b, and c pole holes of the tube holder (if it is a diode, insert it into the two holes of e and c). If the tube to be tested is a PNP type and is good, it can only provide a conduction path for the PNP tube when the 555 outputs an oscillating square wave at a low level, that is, it conducts when the square wave is at a low level, and the LED2 light-emitting tube connected in series with it is powered and emits light; when the oscillating square wave is at a high level, the PNP tube is cut off, and neither LED1 nor LED2 will emit light.

If the transistor under test is an NPN type, its operation and conduction are just the opposite of the PNP tube mentioned above. If it is a good tube, UED1 lights up and LED2 does not light up. Therefore, the light emission of LED1 or LED2 can be used to determine whether it is NPN or PNP and whether it is good or bad. For a damaged and open-circuited transistor, neither LED1 nor LED2 will emit light; while for a transistor with a broken c or e pole, it will light up in turn at the high and low levels of the oscillating square wave, but due to the retention effect of human vision, both appear to be on. Therefore, when testing transistors, if both LED1 and LED2 are on or not, it means that the transistor is damaged.

For diodes, including silicon tubes or germanium tubes, the method of judging whether they are good or bad is the same as that of triodes, but when inserting the tube pins, only the e and c holes need to be inserted.