A logic probe is a great addition to any electronics tool kit, especially if you don’t have access to an oscilloscope. Using a logic probe ensures a quick and easy means of determining the logic state of a digital output which can be a huge benefit when testing and fault finding.
The design shown here is simple and easy to build which is intended for hobby / student use. In operation the logic probe contact is touched against the particular signal point of interest and it’s logic state is indicated by the illumination of logic high (Logic 1) or low (logic 0) indicating LEDs. If the monitored signal is pulsing between high and low logic states a third LED illuminates to indicate this condition. The logic probe circuit is designed to draw very little current (approx 20mA) and can be connected to the supply of the circuit under test (+5V) via crocodile clips.
Parts list
The circuit diagram
The logic low / high LEDs are driven by transistor switches designated TR1 and TR2. In order to detect fast pulses, IC1 is used as a monostable delay circuit that briefly illuminates LED 3 for approximately 0.1uS. This is sufficient to be seen visually, and ensures that even very short duration pulses do not go by unnoticed.To prevent any damage that might occur by incorrect reverse power connection , diode D1 guards against wrong polarity.
