Logic Probe with Seven Segment Display

Logic probe is circuit that indicates  the logic state of the node of any TTL logic circuit. It will work if we must supply the probe with the same power of the circuit that we want to analyze, in other words same Vcc and same GND. The “Test” wire of the probe must be connected to the desired node of the circuit that we want to check.

When the level is low,the green LED will be light and the probe will display a “zero” (0) and when  the level is High, the red LED will be lighted and the probe will display a “one” (1). The probe will display nothing and the bot of LED will not light. Logic level is determined by the voltage of the “Test” wire connection with something that want to be tested. here is the logic level and their voltage :

• “Low”        : 0V-2V
• “Impedance”    : 2V-3V
• “High”        : 3V-5V

[Source: electronics-lab.com]