A Simple LED Logic Probe

Here is a simple LED logic probe I made in dark times when there was no multimeter to be found. It has back-to-back light emitting diodes for bidirectional current indication and one current limiting resistor. The logic probe is compatible with 3.3 V and 5 V logic levels.

Figure 1. The simple LED logic probe

The drawback of the bidirectional feature is that the probe is usually referenced with ground or the power rail, so the "direction" is really just an indicator of the reference voltage used. In actual use, only one of the LEDs light up.

I built the probe to debug 7400 series logic student projects on breadboards. The probe is only really useful when you can control the clock to examine the system tick by tick.

Figure 2. A schematic of the logic probe

A Simple LED Logic Probe

Notes