Transistor as a switch 1

[grizzlybaird]

As a preface I will declare that circuitry is not my speciality by any means. I work in the Film business as a Steadicam Operator.

I'm trying to build a simple relay switch using a Transistor. I have a remote control focus system than has a camera start stop button. The system sends a .5v momentary signal to the camera to indicate start or stop. I would like to hijack this .5v DC signal to control an IR remote.

Using a low voltage, fast acting NPN transistor I built a simple circuit that works but might present a problem when I plug the system into a film camera.

I wired the + power supply (3v DC) for the IR remote through the collector and emitter of the transistor and wired the .5v DC output to the base and emitter.

With the pause button held down on the IR remote, every time the .5v DC pulse goes through the transistor the 3v DC power supply is momentarily connected and the IR sends it's command.

The problem is the transistor leaks power from the 3v DC into the base and thus into the focus unit which might create a problem when plugged into a film camera.

Is there a simple circuit that can be built to prevent this reverse power leakage?

 

[OperaHouse]

I can't actually picture this, but I can see two problems. .5 volts is not enough to turn on a silicon transistor. The voltage is likely higher and this extra transistor is placing extra load on the signal.

I would consider using a LM339 open collector quad op amp instead and reference the switching voltage at 0.25 volts. The input could be isolated with a 100K resistor. This solution only adds three resistors. Outputs could be paraleled for even higher currents

 

[grizzlybaird]

Thank you Opera House for your suggestion. I will try building this. Like I explained before, the method I used works but the base voltage might be higher than I tested. It is only a momentary signal so perhaps my multitester doesn't get the full read on it before it turns off.

My flawed design aside, what I would like to do is use the momentary signal out of the focus unit (PRESTON MDR) to activate the 3v DC without the 3v DC contaminating the Focus unit signal.

I will try the open collector method.

 

[Noway2]

One other suggestion. If are finding that base current and or voltage is causing you a problem, look at using an n-channel mosfet instead of a bipolar transistor. The gate (base) current is negligible and common off the shelf devices can be turned on with as little as 2V, but tolerate up to 20V.

 

[grizzlybaird]

Thanks Norway2,

2v is probably more than the momentary signal offers. It needs to be a circuit that can be triggered by 1v dc or less.
I do appreciate the help, As a member of a similar forum for Steadicam Operators I realize how frustrating it can be to deal with the likes of myself who are not educated in the field.

 

[grizzlybaird]

It should be noted that my circuit consisted of only the NPN transistor, all the feeds were wired directly into the transistor. A button cell battery through the Collector and Emitter and the momentary pulse through the Base and Emitter. I've tried to find a circuit design to suit my needs and was unsuccessful, I've since started looking for a circuit design using the LM339 but have been unsuccessful again.

 

[MacGyverS2000]

If the pulse is quite quick and you're using a standard multimeter, as you allude to, I imagine the pulse has a much higher voltage than you're reading. You need a scope, not a multimeter, for short pulses like that. I would bet the pulse actually goes up to a standard voltage inside the case, like 3.3V or 5V, or possibly even higher if a battery voltage is involved.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[grizzlybaird]

You may well be correct, I will have a friend at Panavision put it on the scope for me, but my problem will remain the same. I need to keep the pulse signal separate from the power source I'm trying to switch on and off.

 

[grizzlybaird]

Thanks OperaHouse for the LM339 idea. Looking a schematic from a Photo detector Circuit using the LM339, I'll replace the Phototransistor with the NPN transistor I'm using now. This should isolate the 3v DC and trigger whenever the pulse is present.

 

[OperaHouse]

This gives me an opportunity to try the photo upload. The circuit should look like this. If the input is higher than 3V, add an additional resistor from the - input to common. Also raise the reference switching volatge from 0.3V to 1.5V by changing the 10K to 100k.

 

[grizzlybaird]

Wow!!, thanks OperaHouse, That looks perfect, I'm going away for a week but I will build it when I get home and post the results.