I have the following example circuit:

enter image description here

I do not understand why this lights the LED. For my main project I am building a circuit that has a button connected to an output as well as the collector of the transistor as shown in the diagram. The circuit works as expected when I put the LED on the collector instead of the emitter, but this solution will not work for my overall setup. Even a 1M Ohm resistor allows current through. Am I just using transistors wrong?

  • 1
    \$\begingroup\$ Draw a proper circuit and not a cartoon. \$\endgroup\$
    – Andy aka
    Commented Jan 23, 2017 at 8:46
  • \$\begingroup\$ @Andyaka fixed. \$\endgroup\$ Commented Jan 23, 2017 at 9:02
  • \$\begingroup\$ So, roughly speaking you are going to see \$I_{led}\approx \frac{5\:\textrm{V}-V_{led}-V_{BE}}{R_1}\$. You have a resistor, in series with a BJT diode, in series with an LED. And you have enough voltage present to overcome the diode drops. So current flows. No surprise. Why can't you put the LED into the collector leg? \$\endgroup\$
    – jonk
    Commented Jan 23, 2017 at 9:10
  • \$\begingroup\$ If you replace that BJT transistor with a MOSFET transistor then yea it shouldn't light up. \$\endgroup\$
    – Bradman175
    Commented Jan 23, 2017 at 9:49
  • \$\begingroup\$ I think I made the LED backwards \$\endgroup\$ Commented Jan 24, 2017 at 2:07

1 Answer 1


enter image description here

You mean there is no supply at the collector! The emitter is the pin with the arrow however, it matters not because, you can put a meter on a transistor and find a diode between collector and base and emitter and base and this is why your LED lights.

Current from the +5 volts passes through R1, passes through the base-emitter diode and then onto the LED (which it illuminates).

  • \$\begingroup\$ Oops, that was supposed to say collector. I must be tired. Let me fix that. Also, yes, I see why it isn't working. I guess I will just have to redesign. I might delete this question as it doesn't provide much help for others. \$\endgroup\$ Commented Jan 23, 2017 at 9:22
  • \$\begingroup\$ Don't delete the question please. \$\endgroup\$
    – Andy aka
    Commented Jan 23, 2017 at 10:11

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