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BJT Touch Switch

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Exam Circuits

Build Three Circuits

In each circuit, the current through your finger might light the LED.

1) A Simple Circuit

In the left circuit, the current is too small.

2) A Transistor Switch

In the middle circuit, your tiny finger current is amplified about 300 times by the transistor. The LED might come on dimly. If you wet your finger, it will work better.

3) A Darlington Pair Circuit

In the right circuit, your tiny finger current is amplified 300 times by the first transistor and another 300 times by the second. (300 x 300 = 90000 ! ) This circuit works very well.

Three Touch Switches

Touch Switch Layouts