Short circuit protection using 555 IC 5 V, 3.3 V

Question

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.

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

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. 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.