Using NE555 to make a digital logic test pen

This circuit uses the set and reset characteristics of the trigger terminal 2 and the threshold terminal 6 of the 555 circuit to form a test pen to detect whether the digital logic state is normal. If the probe IN input is low level "0", LED2 lights up, and when the input is high level "1", LED1 lights up. The R1, C2 and R2, C3 networks are acceleration networks. This logic pen is suitable for testing digital circuits such as TTL and CMOS. When measuring, this circuit should be grounded with the circuit under test.