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DIY not gate


i tried building a not gate (1 input only), not one i saw on the internet but i tried to make one myself. The problem is i didn't succeed. Now, i know there are loads of examples of how to make one on the net, but i opened this post because i'd like to know why mine doesn't work, so i can improve, hopefully :), my electronics skills.

Here it is:


..the LED doesn't light up. how come? Oh btw i forgot to mention that the base of the PNP is connected to an MCU, and i wanted the LED to light up when the pin goes low.

Thank you for the answers, in case i forget to thank you later :=D ! 

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Well, on your circuit, you will have a similar current wasted when the pin is low. As you stated, it is very little current (less than 0.5 mA).

The one transistor NOT gate is an OK solution. Practical NOT gates inside ICs have all sorts of extra stabilization and protection. However, if all you need is a simple inversion of your signal, this works fine and only uses a transistor and two resistors.

Alright. I think i'll stick with the original idea since the signal will be low only for 20% of the time. Being the two results identical, i'll waste a little less current with it. 

Thanks for the answers! 

Well, it's not a perfect solution, but in theory it should work. Base of PNP is pulled low by micro, PNP turns on, voltage at MOSFET gate rises above threshold, MOSFET allows current through LED.

If you give us the resistor, transistor and LED values we can probably figure out where this is going wrong. Assuming the resistor values are reasonable, I suspect that the PNP isn't able to push enough voltage to the MOSFET gate to reach the gate-source threshold, and as such the MOSFET won't turn on. Having the LED between the source and ground further complicates matters by raising the source voltage, so the gate voltage has to be even higher than usual.