three-stat TTL logic probe - a small, but useful, TTL test aid

a small, but useful, TTL test aid Regular readers of Elektor will know that we often publish various items of test gear. The design shown here is nothing really unusual but it is none the less worth considering because it is very handy. The end product is about the size of a thick felt-tip marker but this marker comes with built-in "intelligence". One of the three LEDs in the circuit will light depending on the voltage red yellow ov 0V8 2V4 green measured at the test point (TP). This voltage is first of all fed to two com- parators (Al and A2). A reference voltage is fed to the other input of each comparator from voltage divider R4/R5/R6. The values chosen give thresholds at 0.8 and 2.4 V as the range betwee"n these two levels is a "forbidden area" for TTL. If the voltage at TP is lower than 0.8 V the output of A2 goes low and causes the red LED (D6) to light. If the measured voltage is higher than 2.4 V the output of Al will be low so the green LED (D5) will light. Sometimes, of ...