This little project was created for a friend of my father. He is involved in a group that build home made cars. One of the ADR's (Australian Design Rules) requires there to be some form of warning to let the occupant know that they should fasten their seatbelt.

Some vehicles have implemented sensors in the seat and seatbelt buckle to check if a person is occupying the seat and if they are wearing a belt. The rules don't actually call for this level of sophistication. Instead, all that is required is a light to illuminate for a set period after ignition.

This circuit does exactly that. The 555 timer is set for one shot and looks for the ignition signal before illuminating an led for around 15sec. I could have used a PIC micro for this project but sometimes simple is best. That's especially true when operating in the automotive environment, noise & voltage spikes introduce all sorts of gremlins to affect your microprocessor.

For a great resource on circuits using the famous 555 timer follow this link:
http://home.cogeco.ca/~rpaisley4/LM555.html