How does the Light Sensor Work?

[spaldam]

I've been playing with the light sensor that's in the dash to see how it behaves, but it appears to be more complex then a simple voltage or on-off switch. I'm tring to find a way to fake it out into thinking it's light outside so that I can make a switch to instantly disable the auto-on's even after the car has been started, and the re-enable them by turning the switch back off.

If anyone has information or schmatics on the light senor, I'd appreicate it if they would share this information, or maybe just point me in the right direction as to were I might find this info. I though the electrical wiring book from the dealer would show this, but it's too generic with it comes to this sensor the be at all helpful.

Thanks in advance...

[SilverRide]

I`ve been thinking the same thing.
There has to be a way to electonicly disable the light sensor and install a toggle switch in the circuit.

The most logical way the sensor could work is for it to produce a small voltage during daylight.
if so, the solution wold be to feed the sensor curcuit a constant voltage equal to to it VIA a resistor set up when disabled. Perhaps even (small/flashlight) battery powered if the voltage is that minute.

The has to be some electronic experts here who can solve this! :)

[spaldam]

What I found is that who wires have power comeing from them. One is about 3.5 volts and the other is about 12.8 volts. The 3.5 volt one, however goes up to almost 3.9 volts when the light is detected. I probably need to do some different tests to see what the third wire does, but as far as I can tell it's probably just a ground of some kind.

[Blue Bomber]

Here's some info from the service manual:
http://www.imagestation.com/album/?id=4292526719&p=4255768083&idx=67

[tool462]

Originally posted by spaldam
What I found is that who wires have power comeing from them. One is about 3.5 volts and the other is about 12.8 volts. The 3.5 volt one, however goes up to almost 3.9 volts when the light is detected. I probably need to do some different tests to see what the third wire does, but as far as I can tell it's probably just a ground of some kind.

Sounds like it's a phototransistor. The 12.8 V should be supplied from the battery, and the other wire is connected to ground. The 3.5V line is the one you have to muck with. You should mess with covering/uncovering the phototransistor to find out the actual voltage that the lights turn on/off at. I'm guessing that it will be at different points on the rise and fall. I'm guessing the voltage feeds into an op-amp, so that when it's above the trip point, it turns on, and below it turns off.

To disable the auto headlights, you should just have to put a switch in the 3.5 V wire, with the output of the switch going to the voltage sensor, one input coming from the phototransistor, and the other from some current source. The trick here is providing enough current, but not so much you fry something...

[spaldam]

Ok, that make sence. It was at like 3.5 volts without light at like 3.8 volts with light, so it's a fairly small differance. Does that sound normal for a phototransistor? If that's the case, then a simple five prong relay switch sould do the job. Normal function for the relay would be a simple pass through, but when turning the relay on, it would switch to a 3.8 volt supply of power.

I'll have to go do somemore testing to make sure this sounds right, but if it works, then I'l need to figure out how to make a 12.8 volt source drop down to 3.8 volts.

[tool462]

Just make sure you crank up the power slowly. Hard to tell if the circuitry triggers on a current threshold or a voltage threshold, so be careful. Various phototransistors can operate over a huge range of voltages, so the numbers you gave seem reasonable.

[spaldam]

Maybe I should to same testing for amps then too?

& Blue Bomber I clicked on your link, but it just took me to your album cover, and I didn't see anything in there that said anything about a service manule.

[Blue Bomber]

Link fixed. :)