555 timer designs using relay as output switch

[jacksparo]

555 timer relay switch, the relay will energize for a certain period of time and will take a longer time before it deactivated even if the input supply is disconnected, please help and indicate each component purpose. many thanks.

 

[Hero999]

The 555 isn't suitable for this because there's nothing to keep the relay powered when the power supply is removed. You could use a capacitor but the delay will be neither predictable nor easily adjustable.

How long do you need the delay to last for?

You'll need a very large capacitor for a delay longer than a few seconds.

A simpler option is just to buy a timer relay which is also how it should be done.

 

[jacksparo]

i would the relay to deenergize for about 15-20sec after the power input is removed.. can i ask a schematic design for this one sir..
thanks

 

[Hero999]

http://datasheet.octopart.com/8600-0240-0000-Finder-datasheet-539064.pdf

 

[jacksparo]

if i use the attached circuit, where can i put the capacitor for the delay of the relay to deactivate after power input is removed.. thanks

Attachment edited by moderator.
In future please crop the image before posting - I've trimmed the white space.

 

[Thomas W1]

if i use the attached circuit, where can i put the capacitor for the delay of the relay to deactivate after power input is removed.. thanks

I think Hero999 meant across the power input supply, i.e. from +12V to ground.

 

[Hero999]

As I said before, the capacitor needs to be enormous for that to work.

What's the resistance of the relay coil?

Yes, the capacitor obviously has to be in parallel with the power supply otherwise the relay will turn off as soon as the power is disconnected.

Even if it's high resistance reed relay with a resistance of 1K you'll need a 22,000uF capacitor for it to hold for 20s and that's before the 555 timer's current draw is taken into account.

You could get rid of the 555 and add a trimmer resistor in parallel but it will be less reliable as the delay will depend on the power supply voltage and temperature.

What's the load current?

A typical read relay will have a maximum current rating of 500mA.

 

[Hero999]

1) No.
2) Read the 555 datasheet and look for 555 timer tutorial on Google.
3) This is no a free design service. We won't simply design a circuit for you. The purpose of this forum is help people learn how to design circuits for themselves. If you don't want to learn then why not just buy an off the shelf timer module similar to the one i linked to previously?