Transistor Circuit, Lights with Finger Contact?

Thread Starter


Joined Nov 24, 2009
I've built a simply transistor circuit in which a small current flows through two terminals when they are both placed in water, opening the bridge between the collector and emitter on an NPN transistor to light an LED.

I noticed a weird anomaly, however. If I touch the point marked "Problem Here" in the image, without anything being in the water, and the two terminals separate, the LED still lights up.

How is this possible? Why is the transistor opening the bridge between the collector and emitter in this situation when there is no current supply!? I'm very interested in knowing how this is happening :).




Joined Apr 20, 2004
Your circuit supplies current through the resistor to the base of the transistor, turning it on (a more convention term than "opening the bridge"). The transistor is completely indifferent to what the cause of the current is - enough voltage to forward bias the junction and it conducts. Your finger is not a good conductor (the current runs across the skin), but seems to be good enough.

Here is a link to our Ebook that might help you see what is going on -