How to sink low voltages?

[sgtpepperaut]

Ok i am a little stuck on this one some help would be appreciated:

i have a 0.7V source that is coming out of another circuit (car controller) that needs to be sinked. If wire is simply connected to ground the doors will lock. If the wire is connected to a 470 Ohm resistor the doors will unlock.

I have tried an optocoupler but it appears that even under saturation conditions the doors will only UNLOCK. I am assuming that the cars controller is simply reading the voltage on that wire and locking unlocking depending voltage on that wire.

How can safely (protecting both circuits) have a digital pin control this sinking current (which is less than 10ma)...

Can i sink them directly into the DPs? is there a way i can only use one DP (pwm?!)? transistors didnt work either ...2n222  transistors but the doors would only UNLOCK...meaning the voltage is not dropped enough?

any help is appreciated.

[runaway_pancake]

i have a 0.7V source that is coming out of another circuit (car controller) that needs to be sinked. If wire is simply connected to ground the doors will lock. If the wire is connected to a 470 Ohm resistor the doors will unlock.

For the sake of argument, we'll assume that what you're doing is OK.
What's the other end of the resistor going to?  If it's Ground then leave that connected, and then...
Get a 5V "reed relay", one with a coil resistance >= 170? (Radio Shack has a good one, but get a diode for that, too), and connect one of its contacts to the 470? junction and the other to Ground.
[The relay switch will short out the 470? when the coil is energised.]

[sgtpepperaut]

Well the other end is coming from the car controller. Maybe I didn't express my problem well enough but your solution seems to always leave the cable to ground with the 470ohm resistor in between which is not what I want.
Lets try again.
I have a wire in my hand coming from that car controller at 0.7V to ground. When u ground that wire doors open. When I put a 470ohm r in inline then ground the doors lock.
When I have the wire in my hand nothing happens (duh)

[Grumpy_Mike]

I have tried an optocoupler but it appears that even under saturation conditions the doors will only UNLOCK.

It sounds like you need to use two FETs rather than a transistor. With a FET you can get much lower resistance to ground.
Wire one FET with the drain to the resistor and source to ground. Wire the other with source to ground and drain to the other end of the resistor. Use logic level FETs.

[runaway_pancake]

Well the other end is coming from the car controller. Maybe I didn't express my problem well enough but your solution seems to always leave the cable to ground with the 470ohm resistor in between which is not what I want.
Lets try again.
I have a wire in my hand coming from that car controller at 0.7V to ground. When u ground that wire doors open. When I put a 470ohm r in inline then ground the doors lock.
When I have the wire in my hand nothing happens (duh)

So the resistor is between the "car controller" and Ground - and the doors lock
Leave that connected anyway and short the "car controller" to Ground (that's what the relay contacts will do, as I described it) - the doors unlock

[sgtpepperaut]

Having that connected all the time  would send constant signal to the cars computer to lock the doors.... Which it will gladly repeat over and over again..
The cable needs to be not connected to anything for no action.... There are three discrete states here. Not two

[runaway_pancake]

Having that connected all the time  would send constant signal to the cars computer to lock the doors.... Which it will gladly repeat over and over again..
The cable needs to be not connected to anything for no action.... There are three discrete states here. Not two

OK

You could still use two reed relays and you wouldn't have to get transistor polarities and all that right.

[Grumpy_Mike]

There are three discrete states here. Not two

That is why I said you need two FETs.