I got a trouble when I powered ON a circuit with 555 which is used in monostable mode. The schematic is as follows.

Fig. 1 Watchdog timer circuit with 555 Fig. 1 Watchdog timer circuit with 555

Expected behaviour

Trig in is connected to a digital output pin of a microprocessor. The microprocessor sets Trig in to Low every several hundreds of milliseconds for few milliseconds. Watchdog out is supposed to be Low only when the microprocessor fails to set the Trig in to Low within 1.034 [s]. Trig in is pulled up to Vcc via 47kΩ, therefore output of 555 is supposed to be Low statically when this circuit is powered ON.

Actual behaviour

Although Trig in is pulled up, sometimes output of 555 somehow turned High for one shot.


  • Because of two inverting buffers 4049, there can be time delay in rising voltage potential at pin2 of 555 compared to pin6 of 555. When 555 wakes up, voltage potential of pin2 can be still lower than 13 Vcc. Then output of 555 becomes High for 1.034 [s].

Unclear points

  • Purpose of diode D1. Why is it connected to pin6(THRS) and pin7(DIS) of 555?
  • Purpose of capacitor C1. Is it to prevent Vcc to be applied to GPIO of the microprocessor?
  • Purpose of two inverting buffers U1 and U2. Does it need power (current) to drive trigger input of 555?