555 Timer Contest

Impulses detector - Watchdog security

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I use a 555 Timer as impulse detector and I transform it in a watchdog for a IoT relay board. The watchdog serve to high protection, if software is blocked, send turn off for all 4 relay.

The IoT relays board was designed to command relays over internet and the concern was if turn ON a relay and then nobody not turn it OFF. The implementd  software use allways timers for each relay. If I turn ON a relay and for any reason I forget to turn OFF this relay for sure it will tuned OFF in assigned time. As a supplimentary protection if the microcontroler goes blocked in some software bugs a module with LM555 timer is added, to close,  by hardware, all relays. The module with 555 timer, expect a rectangular signal, to not involve in relay command. When this rectangular signal is missing and the clock line remain TRUE or FALSE longer then 1 sec the timer put the OUT pin to ground and through Shottky diode put the base of transistor to ground. This detemine low voltage on relay and turn OFF the respective relay.

detector lipsa impuls hack.jpg

Schematic for Watchdog with LM555

JPEG Image - 186.42 kB - 12/15/2021 at 19:05


  • 1 × LM555C Clock and Timer ICs / Timer ICs
  • 5 × BC547 Discrete Semiconductors / Transistors, MOSFETs, FETs, IGBTs
  • 4 × 1N5817-T Discrete Semiconductors / Diodes and Rectifiers
  • 1 × capacitor 6.8uF
  • 1 × capacitor 22uF

View all 12 components

  • 1
    Schematic description

    Schematic is for 4 Relay board and 555 timer acting to block signal from microcontroler and turn off all relay when impulse are missing for more then 1 second.

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