Switching circuit
Électronique / Electronic

Here's an electronic switch that allows the current to keep flowing once it is turned on, even if it is turned OFF later. When you finish wring up the project, turn power ON and see if the LED lights up.

Don't be surprised that it doesn't light up: it shouldn't light at this moment.

Now press S1, and the LED lights up.

Release S1. The LED is still on.

Can you explain why?

When S1 is closed Q3 conducts to light the LED. At the same time, Q1 and Q2 also conduct.

Check the current flow on the schematic, and you'll see that once these two transistors conduct, they keep on even after you release S1.

As long as these conduct, Q3 is also kept on and the LED stays lit.

Turn power OFF for retrial