how transistor base touch sensor works????????

I was playing with a transistor and noted that when i touch the base light glows (How is skin giving it electric current??) i googled on this topic hardly found 1 link but there was only a diagram I have also worked with fingure touch techniques but this is a bit different it maybe capacitive

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"... when i touch the base light glows"

Heh! You must have one of those light emitting transistors(LETs).


It is funny.  You seem to be dismayed by the lack of information you have found on this topic.  Yet, at the same time, you yourself cannot be bothered to link to the diagram of the circuit you found, or to include a diagram of the circuit you built.  You know, so that persons reading your query might better understand what you are writing about.

But don't worry.  Some of the people who read this forum are clairvoyant, which means they know your answer before you've asked the question, and that can be creepy sometimes, but also very convenient too.

Small currents can find their way into the base of a BJT transistor just by touching the base with your finger.  I have seen it, and heard it, happen, hearing it in the case when some other part of the circuit was connected to a speaker.

I am going to suggest further experimentation.   Try examining the collector current in that same transistor, with the tools you have available to do that.  Also try touching the base of the transistor through some different components, like through a resistor, or through a capacitor.  Doing this might give you some clues as to what is actually happening.
back when I was super-early learning about how to use transistors I made a matrix controller such that two decade counters would select row and column of a matrix...with the hope of lighting one led at a time in sequence indefinitely; and it worked, mostly - all except if I put my hand near the circuit, where all the wires running off to all the leds from the transistors would somehow pick up my (presumably awesomeness) and make the circuit go haywire.
Wow! An effect from just bringing your hand near the circuit- that's pretty impressive! I guess there must have been some part of that circuit just flapping in the ether breeze, some input not adequately tied down, I mean if just the electric fields surrounding your hand could make it wobble.
rickharris3 years ago
This is a fairly general answer:

Your body acts as an antenna and will pick up all sorts of signals turning them into fairly small voltages.

Depending on what else is happening in the circuit an NPN transistor needs it's base to be 0.7 volts above it's emitter to conduct.


explains why.

So you can easily supply enough current to make the transistor turn on.

Your skin isn't giving it additional current. Your skin contacting the circuit is acting as a small capacitor. Collecting a bit of a charge thus triggering the transistor. Look up how capasitive touch works.