I was searching for a way to use diodes to make a NOT gate.

I found a circuit online and simulated it in an application.

The configuration has a battery (1),where the negative terminal is connected to the cathode of a diode and the positive was connected near the ground (1) of the circuit. The ground (1) was also connected to the cathode of the diode.

The cathode of the diode was connected to the anode of an LED and the cathode of that LED was connected to the positive of the second power source and goes to the second ground (2). The negative terminal of that second power source was connected to the anode of the diode I mentioned earlier which goes down to the ground (1).

I tried to analyze what was going on.

I turned on the switch on the left, and the LED turned off. I turned the switch off, the LED turned on. I removed the ground and the outcome changed. The circuit does not act like a NOT gate anymore. What is happening?

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1 Answer 1


That circuit has several big design faults.

  1. D2 short-circuits E3 to ground. A high current will flow which may destroy the battery, D2 or both.
  2. Both ends of D3 are connected to ground. It will never light.
  3. E1 is backwards.
  4. When S4 is closed it will short-circuit E1 to ground. A very high current will flow but there will be no voltage available to power anything.

If you want the switch to turn off the LED when the switch is closed then try this:


simulate this circuit – Schematic created using CircuitLab

Figure 1. Closing SW1 causes the LED to switch off.

  • With SW1 open current flows through R1 and D1. D1 lights.
  • With SW1 closed current flows through R1 but D1 is short-circuited, there is 0 V across it and it turns off.

We wouldn't call this a NOT gate as it is not a true logic circuit.


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