Probe tester circuit diagram is based on a clock generator

This probe tester circuit diagram is based on a clock generator, IC1a-1c, which provides a square wave with a frequency of 3 Hz. Made using a few number of components, the probe can perform three test functions selected by switch S1.
With S1 in position A, T1 clock pulses applied to the probe, this means that the probe can inject 3 Hz signal in digital circuit once, bypassing the logical states imposed by output of digital integrated circuits.
With S1 in position B, the tester can detect logic levels. When the probe is not connected, tact is present at entry IC1d, uses and so D1 will flash in the rate of 3 Hz.
LED remains off if the detected signal is logic 0 and lights continuously if logic 1 is detected.
Tester can operate with input signals of frequencies up to 30 to 40 MHz.

With S1 in position C, the input signal is applied to the clock input of the shift register IC2. After eight pulses received at the probe entry level "1", which is always present on the inputs A and B of IC2, reach out QH, so D2 LED lights. LED flashes as long as there is a clock signal probe entry, provided that its frequency is not greater than 25 MHz.

Circuit Diagram: