red LED on = no reset
red LED off = reset (active in LOW)
The switch turns off the regular pulses. You can test the response if the MCU freezes in LOW or HIGH.
This is an example of external watchdog timer**.
It looks complicated, but it's easy.
They are two identical circuits.
The 555 on the left detects LOW - when the signal freezes at LOW, the yellow LED goes out
The 555 on the right detects HIGH - when the signal freezes at HIGH, the green LED goes out
yellow LED on pulse HIGH
greed LED on pulse LOW
Arduino UNO blinking BUILTIN LED every 500ms is eaual to MCU sending pulses on D13 every X ms.
If the pulses stop - the MCU freezes - the missing pulse detector resets the Arduino PIN.
RESET PIN is active in LOW - red diode means noRESET.
The switch turns off the regular pulses. You can test the response if the MCU freezes in LOW or HIGH.
Two IC only NE556N and 7404/7414
Two NPN transistors.
2xNE555 in one NE556N
Inverter 7404 od Schmitt inverter 7417
Transistors BC337 or MPSA06 or similar.
I know thar two diodes and an inverter can be exchanged for a NOR gate from a red LED. I used inverters and i have six in one chip, no reason to add more IC.
---------------- real application test:
The 20uF capacitor is connected to the RESET pin. If capacitor discharges the Arduino resets.
**AVR processors in the regular Arduino support a hardware watchdog timer. No need to connect anything externally. You just set the time to reset and if you don't set it again, it means your program has stopped and the watchdog will restart automatically.
