As I'm a beginner I was playing with transistors connecting them parallel, serial, making AND/ OR logic gates and so on. I connected all collectors to + and each transistor emitter to another one's base.



The wire of the first base acts a little amazing for me. When I stand up, move my body, touch it, move my hand in the air on top of it, move a paper in the air on top of it, LED turns on and after a while turns off. When I place my finger on it, a coil, sheet of metal, antenna, LED turns and remains on and doesn't turn off after a while. I guess it gets very low electric induction from the environment and makes current in it and with amplification of transistors led with turn on. But the more confusing thing is that when I connect a diode to the wire of first base and place my finger there, it turns on :| while no current should go there. How is it possible? What if instead of 5 transistors, there will be 100 transistors? Can it be used to detect any small movements in a room?

Please test it and I have a movie of it if you wanted I upload it. Thanks

  • \$\begingroup\$ What transistors did you use? \$\endgroup\$
    – Alex I
    Commented Sep 24, 2015 at 18:41
  • \$\begingroup\$ It's the same principle as a Darlington transistor. You've got lots of current gain, and a floating base. It should be no surprise that you're getting unpredictable LED behavior. \$\endgroup\$
    – Matt Young
    Commented Sep 24, 2015 at 18:41
  • \$\begingroup\$ @AlexI BC548 transistor \$\endgroup\$ Commented Sep 24, 2015 at 18:47
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    \$\begingroup\$ How do radio transmitters work - there is no current between transmitter and receiver because they could be on different planets. Look up electric fields from AC wiring and see what voltage actually may fleetingly exist on your body when you move around. \$\endgroup\$
    – Andy aka
    Commented Sep 24, 2015 at 18:51
  • \$\begingroup\$ @MattYoung Thanks for introducing darlington transistor that I've never heard about. So it will be sensitive to very small currents. that was surprising to me as I made a touch button coincidentally :) \$\endgroup\$ Commented Sep 24, 2015 at 19:01

1 Answer 1


Perhaps a more interesting question is why the LED is 'off' under some conditions. You've made a kind of expanded Darlington configuration, which means the current gain is the product of all the current gains. Normally the current gain of one transistor is of the order of 100.

Since transistors have leakage you might expect that the leakage of the first transistor would be multiplied by 10^8, so even if it is <100pA it will turn the LED on.

The reason this circuit does something interesting (respond to the base of the first (top) transistor) is that the gain of a transistor is not constant with collector current. It drops both at very high and at very low current.

  • \$\begingroup\$ So because of the leakage between base and collector I can't attach 100 together otherwise LED will remain On. Am I right? the last part was nice I didn't know it. "It drops both at very high and at very low current." tnx \$\endgroup\$ Commented Sep 25, 2015 at 19:30
  • \$\begingroup\$ You can attach more together (more than about 15 won't work because the voltage drop will be too high) but more than maybe 4-5 won't be more sensitive. If you want really sensitive circuits you can try playing with a 2N7000 MOSFET but they are easier to destroy, so get a bunch. \$\endgroup\$ Commented Sep 25, 2015 at 20:12

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