Using transistor as a relay.

[Colt45ws]

Im attempting to figure out how to install door switch panels in my car from a newer model year. Everything is pretty much just wired straight up just with different plugs.
Except for the Lock/unlock switch.
On my model year only the up-optioned vehicles like mine received a DDM (Driver Door Module) which is needed for keyless entry and the like. So some vehicles had the lock/unlock switch directly running the lock actuators by switching +12v to them and on the up models it would still switch +12v but only to a pin on the DDM which would then take care of actually running the lock actuators. Basically the switches are designed to switch positive.
On the newer years, all vehicles receive a DDM so it simply sends VREF out on the lock and unlock wires and the switch itself when pressed shorts the corresponding pin to ground. They are only designed to switch to ground.
So I need to have a setup I can use ground switched to switch positive. I confirmed the current through the switch on my car is around 35mA to the DDM.
I have looked on the internet for how to do something like this, but I just get confused.
Using what is given at http://www.rason.org/Projects/transwit/transwit.htm it looks like I would need a 28-29KOhm resistor as R1 and 280-290KOhm for R2 for the shown 2N3906. Does this sound right? I selected 13V as a middle ground input but there is a pretty wide range of voltage in a vehicle; do you think it would stay "saturated" in say the 10-15V range?

[PCBONEZ]

Re: Using transistor as a relay.

Wiring diagrams would help. - Hand sketch will do.

This DDM thing is in parallel with the switches so either works the lock?
.

[b700029]

Re: Using transistor as a relay.

Using a MOSFET may be more convenient since they don't need biasing resistors when used as switches.

[Colt45ws]

Re: Using transistor as a relay.

This is what I drew up last night using my original diagram and pasting in the newer switch design then wiring it up with relays. I think some sort of solid state switching action could be much smaller and less complicated as far as actual operation. Relays are way over kill for the actual 35mA going to the DDM.
http://colt45.ws/cvn/DDM_lock_revised.gif
The DDM is what actually operates the lock mechanism. The switch just tells it to do so. It basically a computer. It probably has relays of its own internally that operate locks.

[PCBONEZ]

Re: Using transistor as a relay.

I think I'm lost.
Have a drawing before the mods?

Is the light bulb lookin' thing in the "Door Lock Switch" representing the lock's solenoid?
.

[Colt45ws]

Re: Using transistor as a relay.

My car: http://colt45.ws/cvn/DDM_lock.gif
2010: http://colt45.ws/cvn/2010locks.jpg
The light bulb is just a light bulb. I scaled it up because the screen shot is small so it lacks a lot of detail.

[Colt45ws]

Re: Using transistor as a relay.

Im liking the MOSFET idea! It looks like I need a P-Channel type.
Would something like this work?
http://www.mouser.com/ProductDetail/...jjUTbfift7g%3d

I drew a schematic again. I tried to to a better job this time.
http://colt45.ws/cvn/DDM_lock_FET.gif

[japlytic]

Re: Using transistor as a relay.

I have seen high power solid-state DC relays from Jaycar.
The AC and DC types of solid-state relay are not interchangable.

[Colt45ws]

Re: Using transistor as a relay.

I also seen solid state relays as well, but they were not cheap. If I was going to do that, I might as well get regular relays..
Also, I just realized my FET circuit will not work. It looks like they would have to be fed source voltage at the gate in order to actually turn them 'off'.

[joshnz]

Re: Using transistor as a relay.

Why can't you move the grounded line from the switch to +?

[Colt45ws]

Re: Using transistor as a relay.

Then the light wont work.

[joshnz]

Re: Using transistor as a relay.

oops didn't take not of that.
I'll be back soon i'll cook up something for ya.

[joshnz]

Re: Using transistor as a relay.

Here is something using common components.

[PCBONEZ]

Re: Using transistor as a relay.

I don't see any functional difference in the two kinds of switches.
.
Why wouldn't this work?
.

[joshnz]

Re: Using transistor as a relay.

I don't see any functional difference in the two kinds of switches.
.
Why wouldn't this work?
.

It's to do with the light on each switch from what I seen.
One system is pull low to do it's thing the other is pull high.

[PCBONEZ]

Re: Using transistor as a relay.

One system is pull low to do it's thing the other is pull high.

When all you do is change the switch that doesn't matter.

It's to do with the light on each switch from what I seen.

Looks to me like control of the light bulb is via something in diagrams 10-9 and 10-10 which aren't shown.
-
If that isn't true then looking at the original drawing it looks like the light would be on the whole time the ignition switch is, including when engine is running.
.

[mathog]

Re: Using transistor as a relay.

I also seen solid state relays as well, but they were not cheap.

Well, that depends on how much current is required. If you can drive the load with a a couple of hundred milliamps, a Clare LCA110 can be had for $1.90 from Mouser. It will only do 240 mA max though, in the DC configuration (both Mosfet's in parallel.) 1A and 2A parts are available from other manufacturers, and they cost less than $5. I really like the solid state relays because there's no way to blow out the mosfet gate, since these are driven optically by an LED inside the package. It is, however, possible to blow out the LED if you forget to put a resistor in series with it, and some relays could be damaged by too much reverse voltage at the input. (So don't wire it up wrong!)

[Colt45ws]

Re: Using transistor as a relay.

The light bulb is always on in run or acc. That is all the +12v feed is for. The switch is connected to the ground side of the bulb.

[Colt45ws]

Re: Using transistor as a relay.

Here is something using common components.

This is perfect! Thank you!

[Gordon01]

Re: Using transistor as a relay.

So I need to have a setup I can use ground switched to switch positive. I confirmed the current through the switch on my car is around 35mA to the DDM.

Much simpler idea:



As FET you can use: IRF5305, IRF4905, IRF5210, IRF9Z24, IRF9Z34.. or just about any P-Channel MOSFET in TO220 case(because you need very small current). TO220 for easy air-wiring and you can screw it to something.
You can reduce R1 and R2 if you want... just need to R1/R2 = 10/1