Timer

[JazzGui]

I am reletaively new at circuitry, and i need help. I am building an alarm system for devices, to protect agenst theft of specific objects. When the alarm trips, i want a mild buzzer to go off for about 20 seconds, to give time to turn it off if it is a false alarm. After 20 seconds, I want a loud siren to activate. How do i get my device to count for about 20 seconds and activate a relay?

 

[rjvh]

just out the top of my head

take a 555 in a astable mode (multi vibrator duty cycle 50%) and configure it so that it gives a positive puls of 5 seconds feed it in a 4017 and you buzzer is on the first output (not nr 0 but nr 1) and the siren is switched with a relay and a hold contact for the relay on the second output (nr 2)

the triger contact is a breaking with the + voltage contact and a making on the - voltage on the reset pin of the 4017 and to restore the situation (reset the alarm)you have to conect the + voltage back on the reset pin of the 4017 and undo the hold contact from the relay small breaking contact that let the relay fall off

one note the 4017 is not capable to switch a relay on its own so you need to do it with a transistor

see data sheet 4017

Robert-Jan