15 seconds Timer delay with relay on for 4 seconds on expiry of timer

[Aggie]

Hi,
Need help to make a simple circuit which functions as mentioned below.

1. Should have a trigger input ( high or low)
2. Should have a reset switch
3. should have a output to activate a relay(high)

On trigger a 15 seconds timer is activated, if reset switch is not pressed within 15 seconds, the timer will expire and activate the output for 4 seconds and reset itself after 4 seconds.

This operation is required to monitor a vault door, if the door is opened to enter the vault once inside he has to close the door behind him, and reset the timer by pressing the reset switch inside the vault.

If in case the door is not closed behind once inside the vault the reset switch should not function. The 15 seconds timer will expire and activate the output for 4 seconds and reset itself.

Please let me know what would be the best way to do this or if there is any circuit that can fulfill these logical functions.

thanks

 

[crutschow]

Two 555 timers (or one 556) connected as one-shots (monostables) should do what you want. The first 555 has a 15 second delay, which triggers the second which has a 4 second delay. Here is a representative circuit. The 555 has a reset input which, when connected to ground, should do the reset function you need.

 

[ericgibbs]

hi,
This Tool may help with calculating the components for the timing Monostable/s.

https://www.electro-tech-online.com/tools/IC555VM1.php