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From the Q&A

With TJ Byers

Experimenter’s TTL Clock Generator


I experiment a lot with novel logic circuits, and I am always scrambling to breadboard a clock circuit at the last minute to test my new brain child. This is getting old. Do you have a schematic of a simple clock/pulse generator that will fit in a small plastic box — something with a clock slow enough that I can monitor the circuit's progress using LED indicators?

Robert Martinez
via Internet


Yes, I can throw something together using parts you probably already have in your junk box. The circuit below will provide clock frequencies of 1 Hz, 10 Hz, and 100 Hz, and a companion 500 uS pulse at double the clock frequency.

A 555 timer provides the basic clock and pulse signal. For this application, the traditional astable configuration has been modified so that Rb (2.2k) doesn't affect the timing resistors, R1 through R3. By keeping the discharge path (pin 7) separate from the charge path, we can vary the pulse width output without affecting the clock frequency, and vice versa. The 555 output drives a J-K flip-flop that squares up the clock waveform. The 4001 NOR gates are used to sharpen the rise and fall times, and invert the output pulse. The generator has been designed to operate from a nine-volt battery for portability, but there's no reason you can't tap into your five-volt power supply and eliminate the battery and 78L05 regulator circuit.