A simple unit for straightforward testing of an audio, TTL, or CMOS circuit is always useful.
A simple oscillator, based on IC1a, generates a frequency that can be preset between about 500 Hz and 1.5 kHz with P1. Before the signal is ready for use, it is buffered by IC1b.
Switch S2 serves to match the signal to the equipment on the test. With the switch in the upper position, the tester can be used with audio circuits; in the lowest position, with TTL circuits (only HC or HCT types), not standard TTL logos or LS2 types). If testing of LS2 type circuits is required, the value of R2 must be reduced to 1 KΩ. The 4093 cannot provide sufficient current for testing standard TTL logic circuits.
The output of the probe that is used to check various points in the equipment on a test is applied to the input of IC1c. If there is a signal present, the LED lights, and the buzzer sound.
The tester battery may be powered by a 9-V battery or a 9-V mains adaptor. It may also to possible to obtain the power from the equipment on the test. The tester draws a current of about 11 mA.