Question about triggering transistors

[CPW]

I have never been able to get a clear answer on this.

Can I use a separate power source to trigger a transistor. By this I mean a positive voltage from a source other than the circuit the transistor is in. The ONLY connection from the separate power source to the circuit with the transistor would be the positive voltage to the base of the transistor.

I have tried this and it doesn't seem to work, but I've been told it does.

If this doesn't work, what is a way that would allow me to use a positive voltage from a separate power source to trigger a transistor?

Thanks.  ;D

[audioguru]

Electricity current flows only when there is a source wire and a return wire.
Frequently two power supplies are used and their returns are connected together.

[CPW]

Electricity current flows only when there is a source wire and a return wire.

That's what I thought. After I posted, I tried using a return wire and it seemed to work. However, something strange is happening. In my project I'm working on, I basically have a custom made USB device controlling transistors. I tried using the return wire, but it wasn't working correctly. So I removed it. Now it works. I only have positive voltage wires connected from the outputs on my micro in my USB device to the transistors. There is absolutely no return wire. I looked it all over, and I can't find any way I would be completing the circuit. But it works. That's what I'm confused about.

I have a battery powered circuit with ten transistors. The ONLY wires connected fromthe microcontoller in the USB circuit (USB circuit is powered by USB port) to the transistor circuit are the ten wires connecting the micro's digital outputs to the bases of the ten transistors in the other circuit. Since one circuit is powered by a battery, there is no way they have the same ground because they are plugged into the same house circuit (they aren't plugged into the same house circuit, I mean). And here's the really wierd part. I can't measure any connectivity between the emitter and collector of any of the transistos even when they seem to be working. But let me clarify something. It's hard to explain my project, but I'll do my best. I took apart the controller for a semi-decent radio-controlled car (it has multiple steering steps and stuff, but it's still not like hobby-class or anything). I wanted to be able to control the car with a joystick on my computer, so I studied it and figured out how it works. I'll still not clear on how it does it, but I figured out the basic idea of how the positions of the controller's (the rc car's controller) steering wheel and throttle trigger are found. For both the steering and throttle, there is a curved group of metal contacts. There are four rows, with multiple small rectangles spaced out over the curve. As the steering wheel or throttle trigger is moved, it slides a metal plate over the contacts, completing a circuit between the ones it is on. The bottow row is one long solid contact that is the common. The rest of the contacts are connected to positive voltages. The metal plate connects them to the common contact, and somehow the controller can figure out which contacts are "connected" to the common contact and calculate the position of the steering wheel or throttle trigger. Since multiple contacts can be "connected" to the common one via the metal plate, I'm not sure how this works since each contact is connected to a positive voltage. What I'm trying to say is I don't know what method the conroller's micro uses to determine what contacts are shorted to the common one. It can't be based on voltage because there would be multiple voltages connected to the common and I don't think that would work. But whatever it does, I figured out that all I needed to do was have a microcontroller control transistors that would do the function of the metal plate by connecting the contacts (actually, I connected the transistors to the wires, not the actualy contacts) to the common one to simulate the different positions of the steering wheel or throttle trigger on the conroller. Right now, it is working great. I wrote a Visual Basic application to communicate with the joystick and with my USB device that controls the transistors. It works great and its kind of fun to mess around with.

But anyway, my whole point is that I have no return wire whatsoever, but the transistors are still being triggered or something. I can't measure connectivity with my multimeter across the emitter and collector, but my circuit is working. It's really weird. If I try to use a return wire, it messes things up and causes the steering servo on the car to jitter a lot and I think it's confusing the controller by messing things up somehow. I can't figure out what method the contoller is using to figure out which contacts are shorted to the common, but whatever the method is it would seem that I need the transistors to work right. I can't measure any voltage on the emitters though.

Sorry about such a long explanation, but I can't find a way to make this really clear. It's really weird. It's like the transistors aren't working, but they're definately affecting my circuit. Because it works. Really weird...

Anyway, thanks for the answer to my original question.  :)