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I've made a circuit with a touch switch module turning on/off a LED strip via a transistor, and the whole thing works, but in the "off" state the LED strip still shines a bit. touch switch led circuit Touch switch module I'm using: link to aliexpress (I've configured it to work in latch mode)

The touch switch module is supposed to have 0V in off state, so the transistor should be in a "closed" state. Why is it allowing some current to flow through emitter-collector?

What can I do to make it turn off completely?

In the "on" state everything is fine, and touch-switching works well.

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Your sensor is trying to drive the transistor's base to 5V in the off state (because with a 5V supply the sensor can't output more than 5V), and its emitter is at 12V. (Given that the transistor is not on fire, it's safe to assume the emitter-base voltage is not actually 7V, and the output resistance of the sensor is limiting the current. This should not be relied upon; you should put a resistor in series with the base.) This forward-biases the emitter-base junction, driving the transistor into saturation or forward-active (depending on circuit conditions) and allowing current to flow.

If you want to use a 5V signal to switch 12V power, you'll want to use an NPN transistor on the low side; this transistor as it is won't turn off until you apply 12V to it.

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  • \$\begingroup\$ Do you mean I should use NPN instead of PNP? I didn't understand what is "the low side". So if I keep using this transistor - can I fix the circuit by adding some resistors? So the base-emitter voltage should be less than 1.4V for transistor to shut down? Didn't realize PNP is so different from NPN, thx! :) will read more about it :) \$\endgroup\$ Commented May 13, 2017 at 0:33
  • \$\begingroup\$ The emitter-base voltage should be less than 0.6~0.7V for it to turn off. Likewise, in an NPN transistor, the base-emitter voltage should be less than 0.6~0.7V to turn off. It's easier to treat them as current-controlled devices though. No, you can't fix it with resistors, you would need a level shifter of some sort which is more complicated than you need. "the low side" means put the transistor on the low-voltage side of the LEDs; NPN transistors are typically used with their emitters at or near ground potential. \$\endgroup\$
    – Hearth
    Commented May 13, 2017 at 0:36
  • \$\begingroup\$ One way to think about it is like this: ignore the collector of the transistor and just treat the emitter-base (or base-emitter) junction like a diode, directed in the same direction as the arrow in the symbol. If that diode is forward-biased, the transistor is either in saturation or forward-active mode, and it is conducting. \$\endgroup\$
    – Hearth
    Commented May 13, 2017 at 0:39
  • \$\begingroup\$ Can you recommend some popular NPN transistor with 1A or more max current? :) To replace this TIP32. \$\endgroup\$ Commented May 13, 2017 at 0:49
  • \$\begingroup\$ Sure, I'd suggest a TIP31 or if you want to really over-spec it, a D44H8. But you can also always just use Digi-key's parametric search, instead of having to ask around! \$\endgroup\$
    – Hearth
    Commented May 13, 2017 at 0:54

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