Capacitor, frequency, transistor online detector composed of 555

As shown in the figure, this is an online detection circuit for capacitors, frequencies, and transistors. This detector can switch between capacitors, frequencies, and transistors by adjusting the preset switch K1. Putting K1 in II can detect capacitors. 555, R8~R12, and the capacitor Cx to be tested form a monostable circuit. The larger the capacity of Cx, the wider the pulse, that is, td=1.1(R8~R12). The longer Cx is, the larger the average current value is, and the larger the indication of the corresponding meter is. In the circuit, BG1 is a single junction transistor, which forms a relaxation oscillator with R1 and C3. The oscillation pulse signal it outputs is used as the trigger pulse of IC. Putting K1 in III can measure the frequency. At this time, the monostable circuit IC outputs a signal with a fixed width of td'=1.1(R8~R12)CB. The higher the frequency of the input signal. The more times the IC is triggered, the larger the average current value is, and the larger the indication of the corresponding meter is. Putting K1 in I can perform online transistor testing. This principle is similar to detecting capacitors. Transistor BG1 in the circuit completes the conversion of positive and negative power supplies. As the IC outputs high and low levels alternately, if the light-emitting diodes LED1 and LED2 light up in sequence, it means that the transistor is good, otherwise the transistor is bad.