Using PIC to read 12V/0V Logic signal and switch on 12V Lamp

[iso9001]

I have a sealed system in place that contains a processor sourcing 12V a switch via a sense wire. When the switch is toggled, the system sees 0V on its own pullup/sense wire and acts accordingly

When this all happends I want to turn on a 12V bulb. I can not get to the ground on the lamp, just the source wire.

I was thinking of using a transistor as a switch, but I cant get on the ground side of the lamp so thats not going to work. And there would be a voltage drop if I rigged somthing up on the source side. I'm using 16F872 parts, but I guess this doesnt HAVE to run into the pic at all

I'm just looking for a low parts cound simple and reliable way to do this. I'm also not certain how much current the lamp draws, but its a very small lamp, the kind that is in switches of car dashboards.

Does anyone make somthing like a solid state switch for 12V ? I found lots of low voltage / super low mA for switching rs232 signals.

Oh, ofcourse I can not change the original switch or change the signal the other system is monitoring. Maybe there is a way to use an opAmp? Maybe a schmitt Inverter ?

[Mark]

[iso9001]

Thanks Mark. I didnt have the setup in front of me to test when I wrote that. But upon some inspection I find a problem.

The switch is not a toggle. Rather its a momentary. So the otheer processor is probably watching for a HI_to_LO transiton. Which I can do just fine.

But now I'll need to use an INT to watch for change on B0, thats pretty easy. But, I have no idea how I am going to get 12V from my board using a 5V output from the pic ... (?)

I tried a NPN but I saw that the emitter only put out a little over the base voltage. I'm looking for "analog switches" on mouser, but they all seem to say that they can source no more then 30mA. I'm not certain but I would almost bet that my little stupid lamp is right about there. (I need to test that)

Ideas for getting 12V to light the bulb using Pic for control, while keeping part number and cost to minimum ?

[Mark]

Um, yeah. I already told you a PNP transistor and gave you a link that show a schematic. I think it would do you some good to get a basic book on electronics or study something online. I am not going to teach electronics here but will give you a little something. Look at the symbols for the transistors. Generally you need about a 0.7V drop across the pins where the arrow is show. The arrow points to the lower voltage. For an NPN the Base needs to be higher and for a PNP the base needs to be lower. So for the PNP circuit you will have to pull that low. The problem will be turning it back off so you will have to use an NPN transistor also. This is much like driving a motor with an H-Bridge configuration, that is controlling the high side. That being said, take a look at page ten of this PDF. http://www.northcountryradio.com/PDFs/column006.pdf

Go get yourself SIMetrix http://www.catena.uk.com/ simulation program and you'll be set.

[iso9001]

Yea, I was hoping for somthing a little simpler then a push-pull set up. Thats at least 5 extra parts I need for each board. Plus at least 12 more solder joints to check, potentialy have a problem with etc.

I was thinking that maybe someone makes a lamp driver, or some low pin count SO package that would just take my 5V in and drive it up to 12V out.

I cant find anything on google, I'll try digikey and mouser again.

[Mark]

Well, this is the chip that you want but it has eight drivers

http://www.allegromicro.com/sf/2987/

The transistor approach is very cheap. I am a bit stunned at you not wanting to use them because you are afraid of bad solder joints. We're not talking about BGA's here! Want something simple, just use a relay.

[PCM programmer]

What you want is generically called a "high side switch", specifically
with a logic-level control input.

Go to digikey and search for "high side switch" (with the box for
"in stock" checked). The BSP742T from Infineon looks like a potential
candidate.