Automobile rear window defrost control

[kadams4458]

Here's an interesting one. I've been searching the web for a couple of hours now, and I'm no closer to figuring out what to do.

My car currently has a simple toggle switch controlling the rear window defroster. If you leave the switch in the on position, well, it's gonna be on and sucking down power whenever the car is running. I've never been too keen on this.

A car that I owned previously used a momentary contact switch to activate the defroster. The first time the button was pressed, a cute little LED lit up and the circuit did its thing for 15 minutes and them shut down. If you activated any time after the initial cycle, it would shut down after 7 minutes each time until the car's ignition was shut off. Of course, at any time you could just tap the momentary switch and turn it off. I really liked the way that one worked and I want my current one to work the same way!

Can anyone tell me where I should head with this? I could figure it out if it involved a timed cycle that lasted for 15 minutes every time, but I'm clueless when it comes to having the subsequent cycles last for half as long.

*Edit* I haven't been on this forum for a long while, but I definitely had more than one post. What's all of that about?

 

[Nigel Goodwin]

The obvious solution is to use a PIC (or other micro-controller), this will allow you easy and accurate long term timings, and you can set subsequent operations to be different times quite easily.

A single 8 pin chip and a few support components (power supply, relay driver, switch and pullup resistor) are all that's required.

You could also add extra functionality quite easily, such as pressing the button twice gives twice the time delay.

 

[eblc1388]

The first time the button was pressed, a cute little LED lit up and the circuit did its thing for 15 minutes and them shut down. If you activated any time after the initial cycle, it would shut down after 7 minutes each time until the car's ignition was shut off........ I could figure it out if it involved a timed cycle that lasted for 15 minutes every time, but I'm clueless when it comes to having the subsequent cycles last for half as long.

Just a thought. A 555 timer would have twice the timing duration when first powered ON. Subsequent timing cycle would be normal as designed.

I'm glad that the clever designer had use this condition to its advantage.

Perhaps the first push of the switch power ON the 555 circuit and it self latched until ingition is switched off.

 

[stevez]

You might also add some kind of sensor that would indicate the opacity of the glass - possibly an LED and photosensitive device. You could reduce the power as a function of time or as the system voltage draws down for other reasons (engine speed, other loads).

Back to the sensor - you'd have to pulse the LED and your photodevice would need to respond only to the pulsed light - so as not to have the photodevice confused by daylight.

 

[kadams4458]

I'm kind of leaning towards PIC for this. My past experiences with a few 555's has left a bad taste in my mouth. (I guess I might be expecting too much from them, but I've never managed to build anything with a 555 that worked properly for very long. But then, I don't really ever know what I'm doing.)

My problem with using a PIC is that I've never used one before. Oh well, now is as good a time as any to start. I think I'll start researching. Anyone care to suggest some good resources?

The little black box known as a BCM controlled the defroster circuit. I dismantled one to see what made it tick, and there were no 555's in there at all. Just a lot of SMD stuff that prompted me to buy a bigger magnifying glass.

As far as sensing the opacity of the glass is concerned, that's a pretty neat idea, but I'd like to try to keep everything as simple and compact as possible. Plus, I don't think anything like that would be possible without mounting stuff on the outside of the car, and that just violates my sense of aesthetics. The idea could come in handy at some point in the future though, so I'm going to file it away with all of the other stuff that I keep in my brain.