Is there a way to trigger a 5V relay that is more robust than just feeding it with 5 V?

Question

I'm using a ESP8266 to activate a siren. The siren is activated thru a 5V relay, and the 5V relay is connected to a transistor, the transistor is connected to a pin of the ESP8266. The power supply of the ESP8266 failed and it hung, in this state the pin went high and the relay was triggered.

I was wondering if there is someway to trigger the relay with some kind of code, not just voltage, as a safety mechanism to avoid activation due malfunction. My first idea was replacing the transistor with a cheap microcontroller, like an ATmega 88p.

Is there a more robust way for doing this?

Answer 1

For critical systems, a failsafe circuit is used that has to be toggled continuously to activate the relay.

The GPIO pin is toggled in the main software loop (typically) after doing all the normal processing. If the loop stops, the toggling stops and the relay is deactivated.

^{simulate this circuit – Schematic created using CircuitLab}

C1 blocks DC from the GPIO pin, C2 and R1 form a low pass RC filter that determines how long the output will stay active when the toggling stops (and removes the ripple from the AC component that is passed by C1 and rectified by D2).

You can simulate this using the above schematic or recreate in LTSpice or similar to see how the circuit works.

The values of C1, C2 and R1 are dependent on the GPIO toggling frequency.