Built around a 555 timer IC, this pulse generator can be built into a probe for logic
troubleshooting. Rll is frequency controlled and gives a range of about 5 to 200 Hz. C2 can be reduced for higher frequencies. S2 selects one shot or pulse operators. Ql and Q2 provide a fast rise-time pulse, which acts as a clipper amplifier. Q4 acts as a regulator. The supply can be anything from about 4.5 to 18 Vdc. LED1 is an indicator that shows circuit operation.