external watchdog using NE555

[derakhshanPazhuhesh]

Hey there
the following circuit is a pulse missing detector, I want to use a RMT signal output from a ESP32 GPIO to feed the input of this circuit, thereafter, when the microcontroller is stopped working the circuit send a low voltage level to EN pin and so the microcontroller got reset, but the issue here is while the boot up, the voltage level shall be high so the boot happens but the output of the circuit without input signal is always down and stop micro from boot up, is there any solution to it?

[KerimF]

At boot, you may need to turn on T1 for a short time (how long depends on your code). This could be done by adding a resistor and a capacitor (connected in series) between the supply voltage and the transistor base. The resistance value could be 10K. You will need to choose the value of the capacitor to keep the MCU enabled till the GPIO starts enabling it.

[KerimF]

I noticed that the NE555 is supplied with 12V. Its output is therefore close to 12-1.4 = 10.6 V (12-1,4 = 10,6 V, in French)!
I guess your actual supply is 5V instead of 12V, right?

[derakhshanPazhuhesh]

yes, I am using 5 volt and not 12 volts, actually the MCU is working with 3.3 volts, I will add a voltage divider or something at the output, but I didn't get idea of the resistor and capacitor in series with the base of Transistor, can you elaborate it or give a schematic maybe

[xvr]

ESP32 has internal watchdog. Why you not use it?

[PGPG]

the following circuit is a pulse missing detector

For me this circuit looks like positive pulse missing detector, but not negative pulse missing detector. So if microprocessor is stopped working with this output set positive than missing pulses will not be detected.