Simple light-emitting display logic pen circuit

This luminescent display logic test pen is composed of only a NE555 time base integrated block and a small number of peripheral components. This kind of test pen can be used to judge the high and low signal levels and the presence or absence of pulse signals in digital circuits.

The circuit is shown in the figure below, which shows a simple light-emitting display logic pen circuit composed of a 555 time base circuit. The 555 time base integrated circuit is connected as a multivibrator. When probe A is floating, 3DG6 has no base current and is cut off. Pin 5 of the integrated circuit is at +E potential. The multivibrator circuit is suppressed and does not vibrate. Pin 3 outputs high potential and the LED is on, that is, there is light and no sound. When probe A is connected to logic "0", pin 4 of the reset terminal of the 555 circuit is forced to reset, pin 3 outputs low potential, and the LED goes out, that is, there is no light or sound. When probe A is connected to logic "1", the voltage drop at both ends of R2 causes pin 5 of the 555 circuit to deviate from the +E potential, the multivibrator is controlled to start, and pin 3 outputs an audio signal with a frequency of 1kHz, HTD The transducer emits sound and the LED lights up, that is, there is light and sound.