This is a very simple circuit to be used to test TTL circuits. The circuit has 2 NPN bipolar transistors (2N2222), 1 LED, and 2 resistors (220 ohms and 2 megohms)When a high logic level is applied to the input terminal, the two transistors go into saturation state and the LED turns on.
When a low logic level is applied to the input terminal, the two transistors go into cut off state and the LED turns off.
High logic level = 5 volts
Low logic level = 0 volts
The 5 volts is obtained from the circuit under test.
Related links:
Logic Circuit
Logic Levels
Darlington Transistor
How to test Diodes and Bipolar Transistor
Semiconductor Diode
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Logic Probe
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