Claudio Buie
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.
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555 Timer Contest
Impulses detector - Watchdog security
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