Help me build a relay & timer electronics project please.

May 14, 2006 4:15 PM

I'd like to trigger a relay that momentarily closes a switch based on the voltage of another wire. Maybe. I think. Please help me with this basic (I hope) electronics project.

Volkswagens with a hatchback have a "safety" feature that requires the flip of a switch in the driver's door before the latch on the hatch will function. The doors must also be unlocked.

I'd like to automate this process by tapping in to the wire that signals whether the Unlock button on the remote has been pressed. When this wire signals that I've pressed the button, I'd like a relay to close the hatch switch momentarily. So, this would be the same thing as me physically unlocking the car, opening the door, and pulling the little switch.

It might be necessary to delay the relay with some kind of timer - thus the switch would be thrown a few seconds after the unlock button gets pressed.

I have this controller, which already listens to the Unlock wire to accomplish its function. I have access to +12V, if that's necessary.

How do I go about building such a device?

posted by odinsdream to technology (6 comments total)

It sounds like you could just hook up the coil of a low power relay (ie a reed relay) to the output and hook the relay's switch up to the button.
posted by cillit bang at 4:34 PM on May 14, 2006

Thanks - I'm really not familiar with the technical names and details of stuff. I'm competent enough to know I don't want to mess something up - perhaps by putting too much of a load on the Unlock wire.
posted by odinsdream at 5:24 PM on May 14, 2006

A few suggestions...

First, make sure you know the drive requirements of the solenoid that controls the hatch latch. You'll need to use output components that can handle that amout of current at 12V. You can measure it with an ammeter. Alternately, you can measure its resistance and calculate the current requirements. (Assume 13 V, not 12... auto systems are 12V nominal but can vary a bit due to the state of charge of the battery and/or whether its being charged by a running engine. The only reason you need accuracy is to be sure you are OK if you choose a driver transistor on the edge.)

I'd suggest using a CMOS 555 timer chip set up in monostable mode with a two second interval. Use the output of the 555 to drive either a small relay (chosen for adequate current capability) or a driver transistor circuit with appropriate polarity.

Integrated timer chips like the 555 are commonly used to create time delays and are abundantly described all over the web for hobbyist level circuits.

You can also find them pre-integrated into hobbyist-level kits for things such as time delay relays... e.g. :

http://ourworld.compuserve.com/homepages/Bill_Bowden/page2.htm#delay.gif

Automotive circuits are funny. Often, you discover unintended consequences of making changes to them, as they endure a huge amount of design review and typically are influenced by a number of not-too-obvious factors.

Good luck.
posted by FauxScot at 5:27 PM on May 14, 2006

FauxScot; I don't believe the switch is related to the hatch handle except that the switch tells the computer to allow the handle to operate. How would I be able to tell what the relay must be able to handle?
posted by odinsdream at 5:47 PM on May 14, 2006

i'm trying to figure out why you can't just short the safety switch in the door (put a wire across it) so that it's always closed. if you are correct in that this switch is just an input to a computer or some other logic circuit that then drives the latch mechanism, then that should enable the latch as soon as you hit unlock. or am i reading this question wrong?
posted by sergeant sandwich at 8:52 PM on May 14, 2006

First of all, you probably need a better idea of the actual signals you are trying to automate, and how they are generated, in order to be successful. I'd suggest you get a schematic diagram for your make and model of Volkswagen, and spend an afternoon identifying the parts involved in the circuit, on your car.

I think what you'll want to do is wire a parallel circuit to the hatch solenoid, leaving the current circuit unmodified. You'll just be adding a circuit of your own, which can also activate the hatch solenoid independently, and this can be quite simple, if your functions are very simple. If you want the hatch to open every time you unlock the doors, you can mount a microswitch in the closest door, to sense the position of the lock mechanism, and use that signal to trip the coil of a momentary contact relay whose contacts ground the hatch solenoid. You want to pick a position in the door's lock mechanism travel that provides a signal at the appropriate time, and you want to use a microswitch with a directional roller [PDF file linked] like you would find on Style N of the link catalog page, so that the switch is only mechanically closed by the upstroke (unlock) of the door lock linkage.

Then, just wire the switch, the relay, and solenoid in series (you may be able to do this with a single wire, using the chassis as a ground), and whenever you unlock the doors, the hatch solenoid will operate. Obviously, this simple arrangement will permit the hatch to open if the vehicle is in motion, and the doors are physically unlocked, so you may want to add an additional circuit element so that your circuit only receives current connection when the ignition switch is in the off position, but I leave it to you to choose appropriately for such additional considerations.
posted by paulsc at 9:50 PM on May 14, 2006