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I am trying to make a steady hand game with latch.

It works perfectly fine in simulation and also work on a breadboard with real stuff. The mystery is it works when power is 5V and do not work when power is 9V or higher. All transistors are rated to much more than 9V.

I am using IRF530N and IRF4905.

Game works like this: you take the rod that rod that is connected to VCC, and you can touch 3 contacts, start (reset), the wire of the game (triggers fail led and latches), win game contact (opens the lock). When fail is latched the "WIN game contact" do not open the lock so you can't cheat.

For some reason when power is 9V when I touch the "WIN GAME" contact it triggers the "Fail latch". Almost as if the current floats from source to gate of third transistor, very strange. I tested all transistors, they seem fine.

schematic

simulate this circuit – Schematic created using CircuitLab

Here is the circuit enter image description here

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closed as unclear what you're asking by PeterJ, Olin Lathrop, Harry Svensson, Dave Tweed Dec 16 '17 at 21:41

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    \$\begingroup\$ This is what I would call a poorly presented schematic. \$\endgroup\$ – Harry Svensson Dec 15 '17 at 13:22
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    \$\begingroup\$ Yucc. That's one ugly schematic. Not only was it not cleaned up after parts were apparently moved around, but those dots and color changes really get in the way of seeing the circuit. \$\endgroup\$ – Olin Lathrop Dec 15 '17 at 13:44
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You likely have capacitive coupling from the drain to the gate in the third MOSFET when you attach the probe.

You really need to add a 100K resistor, (or whatever value keeps the gate of M1 beneath Vgs threshold when the gate of M3 is at battery volts) before the gate of the third transistor. That way any spike that makes it through is not enough to turn on the first MOSFET.

schematic

simulate this circuit – Schematic created using CircuitLab

Alternative is to use a diode and resistor.

schematic

simulate this circuit

Some ESD protection would be a good idea too.

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    \$\begingroup\$ Thank you. 100k resistor on the gate of third mosfet solved my problem! \$\endgroup\$ – dmitry8080 Dec 15 '17 at 16:41

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