to the electrical guys..

[twistedsymphony]

"I don't know why everyone is ignoring my 555 timer suggestion"

Right, the 555 is easy to use & is very versitile. I used one to build a sawtooth generator in college & used one once to build a pest repeller years ago. It's just a bit of overkill to use it for a timeout circuit (IMHO).

The real question is what to use to trigger the light to come on in the first place. I assume that you're not interested in a switch on the dash since they're hard to find in the dark. The most logical trigger in my mind would be the dome light. If the dome isn't triggered by turning off the ignition key (can't remember), it'll still come on as soon as he opens the driver door. You can still use the 555 astable multivibrator (that really is it's official name) to time it off at a different rate. It's also possible to use it for dimming an LED as well (PWM) but that would be more complex to design.

whoa whoa whoa...

I think you're missing HOW I was suggesting to use the 555 timer...

"Monostable mode" does EXACTLY what we're looking for here..

it gets a trigger, then it outputs a solid on signal for X amount of time.. then it turns off the output and waits for a trigger again.

this is exactly what we're looking for. the signal is when the ignition turns off. when that happens the LED comes on for X amount of time... then the LED turns off and waits for an signal again.

If you attach it to the dome light for a signal then the design has already failed because the intent is to help the passenger find the interior door handle BEFORE the door is opened... the dome light comes on AFTER the door is opened... at which point you don't need to find it anymore

As for the trigger...
the 555 timer has a trigger input... the cool thing about how it works really nicely with this setup.. is that the 555 is activated on the "trailing end" of the trigger pulse. what does this mean?

I means when the car is off the Ignition/ACC signal is at 0V... then when you start the car it goes up to 12V, This is the LEADING edge of the "trigger pulse" (it just happens that this single "pulse" lasts the duration of the car ride). When you turn the car off the Ignition/ACC signal drops back down to to 0V, this is the TRAILING edge of the "trigger pulse". Since the 555 does it's thing on the trailing edge of the trigger pulse it turns on the LED the instant the Ignition/ACC signal drops down to 0V. PERFECT

A 555 timer triggered from the ACC line in the VX and wired in monostable mode with an output to an LED would work like this:

-The VX is off and so is the LED
-you start the car and go for a drive... the LED is still off
-you park it and leave it running... the LED is still off
-You turn the key off ... the LED comes on and stays on
-the LED turns off after a set amount of time.
-repeat

I don't know but IMO that seems to be EXACTLY what we're looking for an EXACTLY the kind of application the 555 was built to be used for.


If you want to add a fade effect to that where the LED comes on instantly stays on for a few seconds, and then fades out, you'll just need to add a transistor and capacitor between the output of the 555 and the LED.