Achieve troubleshooting ecstacy with this simple TTL tool!

When building and troubleshooting TTL circuits, I've used this thing more than just about any other tool. I needed something that could detect whether a pulse was occurring on a signal line, and just as importantly, whether more than one pulse occurred or not.

This is a pair of independent pulse traps. It uses LEDs to show the states of two lines, whether pulses have occurred on them since it was last reset, and to indicate a count of pulses received. The count is valid up to 7, beyond which an LED lights up to mean "greater than 7". A 74LS74 flip-flop is used to guarantee that an LED will wind up being lit, even if a multiple of 16 pulses occurred. The count limit could be easily increased by cascading a few more counter chips, and the Truly Fanatical could use 7-segment displays instead of LEDs.

The signals to be monitored are first run through a 74LS04 inverter. The outputs of those are inverted again and used to drive "current state" LEDs. The 74LS04, then, provides buffered output of a line being monitored, as well as the inverse of a line being monitored. I use physical jumpers to choose between counting the pulses of the inverted line (if active low) or a doubly-inverted line (if active high). The outputs of the 74LS04 are then fed, through the jumpers, to the inputs of a pair of 74LS93 4-bit counters. The low 3 bits of the counters drive the count LEDs, and the high bits are used to clock 1s into 74LS74 flip-flops, whose Q outputs drive the "greater than 7" LEDs. This insures that once a "greater than 7" LED goes on, it stays on until the reset button is hit. For a reset signal, I just discharge a 50uf capacitor with a momentary contact switch, and then let the capacitor raise the reset line to 5v through a 1K resistor.

Back to Hardware/Projects page