i'm new in this domain, but I like a lot. So, I have a question: I have a circuit with 20 LED (series + parallel 50mV, 3V), and I want to stay HIGH all time, except when I get a signal from another circuit. I want to do this without use a microprocessor, but I didn`t know how. I try with MOSFET N-Channel, P-Channel, but didn't get solution for my problem.

Thank you and sorry for my bad English. I work also on this ;)

  • \$\begingroup\$ Can you just use a normally closed push button? \$\endgroup\$
    – Transistor
    Mar 3, 2021 at 20:05
  • \$\begingroup\$ I wanna do it with a signal, not a button. \$\endgroup\$
    – SkePsis
    Mar 3, 2021 at 20:12
  • \$\begingroup\$ But your question states "when I press a button". \$\endgroup\$ Mar 3, 2021 at 20:53
  • \$\begingroup\$ Oh, sorry, my bad. I wanna use a signal from another circuit \$\endgroup\$
    – SkePsis
    Mar 3, 2021 at 20:55
  • \$\begingroup\$ @SkePsis By "stay HIGH" do you mean "stay ON?" High isn't clear in English, but On would be when talking about LEDs. Also, when you say "get a signal," what exactly does this signal look like? Can you describe it, in detail? Is it single-ended or differential? What source impedance does it have? Is it voltage or current? What's the shape? Etc... \$\endgroup\$
    – jonk
    Mar 3, 2021 at 22:12

1 Answer 1


I do not know what 50mV means (isn`t 50mA or 50mW?), but for inverting logic just use one transistor more. Zener diode can be avoided if you are sure the Vcc does not exceed 20V (but rather count with 16V).

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  • \$\begingroup\$ Thanks you, can you please explain me why use zenner diode? What exactly do ? Let me explain if i get your circuit: when Q1 is 0( no signal) the current flow in R4 and R5 and turn on M1, and leds get up. When i have voltage on q1, al of voltage " goes to ground" ? And yes, i wanna write 50mA and 3V like led atribute \$\endgroup\$
    – SkePsis
    Mar 4, 2021 at 5:39
  • \$\begingroup\$ Zener diode clamps the voltage to safe level, so if the Vcc is 24V the gate-source voltage of M1 is still 12V. \$\endgroup\$
    – user208862
    Mar 4, 2021 at 6:18
  • \$\begingroup\$ Basically you are right, when Sig=0V Q1 is off, at Q1 collector is enough voltage to turn the M1 on. If Sig=5V Q1 is on and at collector Q1 is about 0V what M1 makes fully close. In Mosfets is better to follow voltages beacause they are voltage controlled, the gates take/release currents during voltage transitions (at gates) only. \$\endgroup\$
    – user208862
    Mar 4, 2021 at 6:40
  • \$\begingroup\$ Btw, this circuit will works for Sig up to few kHz, for higher switching frequencies the driving of M1 must be rebuild. \$\endgroup\$
    – user208862
    Mar 4, 2021 at 6:54
  • \$\begingroup\$ It`s ok for me, i search more information about MOSFETS, is a new subject for me. Also, what program you use for simulate circuit, see voltaje, current and things like this ? Thank you for your time and explication! Have a good day sir! \$\endgroup\$
    – SkePsis
    Mar 4, 2021 at 9:14

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