I'm designing an electronic door control system and I want to include a safety mechanism in the circuit to make sure there is always a way to open all doors in case of emergency.
Because a microcontroller might fail for any reason (bugs, power problems, etc.) I want to make this fail-safe mechanism using a standalone circuit.
The working I'm trying to achieve is as follows:
There is an emergency button. When this button is held for 10 seconds straight, the safety circuit will trigger and turn off the power to all doors, unlocking them. After 10 seconds the button may be released by the user but the safety circuit must remain active.
I looked for such circuit on the internet and found these two similar questions:
Which had pretty complicated answers.
But I also found this circuit:
Which is very simple and does exactly what I want.
When you connect the VCC to the 555 IC, after a set amount of time the output pin will go HIGH, and it will stay HIGH indefinitely.
The only problem is that when you disconnect the VCC the circuit will turn off. Which I have solved using the following circuit:
When the user presses the button, Q1 activates and the 555 starts working. After 10 seconds the output pin (3) will go HIGH. This does two things: first it activates the relay that opens all the doors, and second it activates Q2 which in turn activates Q1. Now if the user releases the button the circuit will still have power.
My problem is solved now using this circuit but I needed to know two things:
1- Is this circuit reliable? I need a reliable circuit because if this fails people could get stuck in a fire or other emergencies.
2- Is there any better solution I can use to achieve this?