inverter circuit

If transistor is just like a switch, it shouldn't affect the led since it forms a closed loop with the battery. So why does the led turn on and off?

I am new to digital electronics(and electronics in general) any help is appreciated.

Thanks in advance.

  • 2
    \$\begingroup\$ The transistor, when ON, diverts current away from the LED thus turning the LED OFF. The transistor, when OFF, doesn't divert current away and so the LED is ON. The jargon for this is that the BJT shunts the current away from the LED, when the BJT is active. \$\endgroup\$
    – jonk
    Sep 20, 2019 at 15:22

1 Answer 1



simulate this circuit – Schematic created using CircuitLab

Figure 1. The equivalent circuit using a relay.

  • When the relay coil is un-powered the LED is lit.
  • When the relay coil is powered the contact closes and short circuits the LED. It turns off.

Note that the circuit is a little inefficient. Not only does it draw power when the LED is off but it draws more power in that condition as the full voltage is across R1.


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