While doing a bit of home maintenance I left the internal door to the garage open. When the wife came home and opened the roller door to drive in, my 2 dogs did their usual greyhound impression and did the Harold Holt down the street. Sooner or later the little buggers are going to end up getting run over.

Easy solution in my head would be to grab a cheap 9V timer kit from jaycar, hook it up to a cheap reed switch... form jaycar... and then use a headlight/door open relay from any car. That way I could come in and out at night without disturbing people, leave the door open for 5 minutes while getting shopping in, but never again let the dogs out by leaving the door open.

Would a 9V battery be enough to trigger one of those relays do you think? I know I could buy something similar but I like to tinker.

I can probably help with advice on the electronics side but I'm struggling to understand exactly what you want the relay to do? Are you trying to make a door closer for the internal door? A little more clarification on what function you are trying to perform with your "hack" and I can probably help you.

Thanks for your reply. I thought the VW commmunity was slipping in not coming to my rescue. This place has saved me joining 101 forums on different subject matters.

The relay has the door chime in it.

http://www.ebay.com.au/itm/Door-Chime-Relay-4-95-97-VW-Passat-321-919-433-B-/...

In its original intended use it does that annoying bing bing bing when the car door is open. Thats what I want it to do with the interior access to my garage. Only I dont want it to bing bing bing as soon as the door is open, only if it has been left open for five minutes.

I'm guessing it wouldnt need the full 12 volts to trigger the relay but not sure if it would make sound. I want it to be small as possible so a 9V battery would be ideal. I think you can get 12V camera batteries but they are probably expensive. Or do I just get 2 of those 4xAAA battery holders and be done with it.

But then I might have problem with the timer kits as they are 9V.

Actually I may have just answered my own question.

This jaycar timer is 12-15V http://www.jaycar.com.au/productView.asp?ID=KA1732 

So perhaps the only unknown variable is whether I can solder without shorting tracks or cooking components.

I think using the car relay will require a lot more fiddling and probably cost more than going with simpler components. If you are using a 9v timer kit, use a 9v buzzer as well. About $3 on eBay delivered or Jaycar will have them too. 9v batteries are pretty good compact power sources. The bidders have relatively low current drain. No real reason to use a car door relay unless you want it to switch something as well (a relay is intended to switch larger current using lower current). Run a 555 timer circuit with a reed switch as the on trigger. Set with 5 minute delay and output to the buzzer. But you need to make sure that if the trigger is switched ie the door closes, the timer shuts off or resets. Like the reminder on a fridge to let you know the door is open.

These kits are pretty good to do. The chip is the most volatile component and you leave it til last or use a chip socket.