digiLED - for testing digital circuits

for testing digital circuits This design is a somewhat unusual test aid for digital circuits. Most such circuits are usually tested in a static state to provide time to look at the logic levels at various points by mak- ing d.c. measurements. This is often not good enough, however, consider- ing the various clock signals, resets, and trigger pulses that make a circuit "tick". What this really demands is a tester that can also detect single pulses. As the circuit diagram shows, either and the supply is taken from the cir- cuit under test. The tester indicates clearly by means of a pair of LEDs (D5 and D6) whether the logic level tested is "0" or "1". The input is fed into the non-inverting input of a schmitt trigger comparator (IC1). The inverting input of this 3130 is fed from one of two fixed reference levels depending on whether CMOS or TTL is selected. The hysteresis in- herent in a schmitt trigger ensures that a "dead zone" must first be passed before the circuit reacts to the...