Page 33 - Electronics Starter Kit_EN

Wiring diagram

7.4 Circuit Explanation

When you touch ON probes you are making a resistor between 5V and the base of TR2, when
TR2 works it will allow the current to go out from the base of TR1 (PNP transistor) so it will be in
the saturation mode and the LED will light up, but we need a feedback current to keep the LED
ON when you release your finger, we get this feedback through R1 resistor it will keep TR2 on
the Active Region mode and TR2 will keep TR1 on the Saturation mode.

When you touch OFF probes you are making a resistor between 5V and the base of TR4, when
TR4 works it will make TR3 on the saturation mode, when TR3 be on the saturation mode, it is
connected directly to GND so all of the Feedback current will go directly to GND, TR2 will be on
Cut-Off mode accordingly TR1 so the LED will turn OFF.

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