The illuminating display type logic test pen is composed of only one NE555 time base integrated block and a small number of peripheral components. Such a test pen can be used to distinguish between high and low signal levels and pulse signals in a digital circuit.
The circuit is shown in the figure below. The figure shows a simple light-emitting display type logic pen circuit composed of 555 time base circuit. The 555 time base integrated circuit is connected to a multivibrator. When the probe A is suspended, the 3DG6 has no base current and is turned off. The integrated circuit 5 is at the +E potential, the multi-harmonic circuit is suppressed, and the vibration is not generated. The 3-pin output is high, and the LED is bright, that is, there is no sound. When the probe A is connected to logic "0", the 555 circuit reset terminal 4 pin is reset, the 3 pin output is low, and the LED is off, that is, no light and no sound. When the probe A is connected to the logic "1", the 3DG6 track children, the voltage drop across the R2 causes the 555 circuit 5 pin to deviate from the +E potential, the multivibrator is controlled to start, and the 3-pin output frequency is 1 kHz audio signal, HTD The transducer sounds and the LED is bright, that is, there is light and sound. 
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