I am trying to build a circuit for short circuit protection of 10 PCBs combined. These PCBs have 5 and 3.3 V.

I have tried building this circuit and have changed the circuit a little bit keeping the principle the same and the circuit works fine. It powers up the circuit when there is no short circuit and turns off the circuit when there is a short circuit.

The input of the 555 comes from the 3.3 V point on the PCB, and the MOSFET drain goes to the 5 V input of the PCB. The source is connected to 5 V supply in.

enter image description here

It misbehaves when I combine 10 of these circuits, and also I have to use a single reset button for all circuits.

  • \$\begingroup\$ sorry I forget to mention the reset pin is also there pulled up 10k and the a push to ground push button the trigger takes the input again when reset is pressed. \$\endgroup\$
    – zuhaib
    Nov 11, 2022 at 6:18
  • \$\begingroup\$ Welcome to SE/EE! Please take the tour to learn how this site works. -- You can edit your question, and you should, if you have new information. Do you see these textual links below your question? -- How does "it misbehave"? \$\endgroup\$ Nov 11, 2022 at 6:43
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    \$\begingroup\$ The PNP transistor should be NPN, and will require a base resistor to limit the base current. \$\endgroup\$ Nov 11, 2022 at 7:03
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    \$\begingroup\$ @jonathanjo The 555 is only used for the comparator and the flip-flip. The comparator can change the FF to turn the output on but it can't reset the FF -- and that's exactly what's needed for this application because you want the supply cut off until a manual reset is performed. \$\endgroup\$
    – ErikR
    Nov 11, 2022 at 11:26
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    \$\begingroup\$ @zuhaib How does the circuit "misbehave"? And when you say you "have to use a single reset button for all circuits" do you mean you want to use a single reset button but it's not working for you? \$\endgroup\$
    – ErikR
    Nov 11, 2022 at 11:31

1 Answer 1


A PNP emitter follower is a good solution for a 3.3 V CPU, because one less resistor is needed. At 5 V supply voltage, C-E can be reversed. This mode of operation is called the reverse-active region. (Major advantage of operating the transistor with its emitter and collector roles reversed, is that the saturation voltage drop is lower than obtained in the normal mode. This is why such devices were used long time ago as analog switches in low voltage control electronics in discrete applications. In reverse mode, the current gain is lower too.)
So here both versions work fine with a PNP transistor. Now the transistor works in linear mode. enter image description here enter image description here Each NE555 must have a decoupling capacitor of at least 1 µF. Do not connect the RESET pins directly, only via an NPN transistor to the push button. Any type of NPN transistor will do. I don't see any other problems right now. enter image description here


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