Replacing A Mechanical Relay With A Mosfet

Yes, one can replace a mechanical relay with a solid state device if one knows what is being turned ON/OFF.

Is this a clandestine operation?

No, it is a time lapse operation. I have removed the relay and connected the shutter wires to the LED and that works.

Geez - you're brave. I wouldn't connect a voltage source into an unknown camera circuit for fear of blowing something up. Ussually the camera is expecting "voltage free" contacts. Replace the relay with a reed relay - that's what alarm sensors use - almost silent.

Google 'solid state relay'

I've used these: http://www.aikenamps.com/VTL5C1.pdf an optoisolator with leads. This needs 20 mA with a 2V drop, so the resistor needed <= (12-2)/20e-3 ohm

At some voltage it may not work. An LM334 st at 10 mA with a resistor would increase the range. http://www.national.com/mpf/LM/LM334.html#Overview

Observe polarity with the LED.

Your saying that when you put volts (from across the LED) onto the shutter wires the shutter operates?

How about when you put the shutter wires onto the screw terminal of the relay's contacts, does that work?

Simple answer to your question is to get a solid state relay with clean, voltage free contacts to emulate the relay's function identically.

But if it is voltage across those wires that you need to trigger the shutter then you already have the answer. Forget the relay and use the LED voltage (that you have found somehow without releasing any smoke). Silence the relay by lifting one of its coil legs from the board or removing it completely.

Tell us more about the shutter wires....

The shutter wires give out 0.7 volts and the LED output gives 1.5 volts. I just removed the relay and connected the shutter wires to the LED. The camera has been doing a time lapse all night and nothing has been damaged.

So, if it is working reliably the way you've managed to sprag it why do you need a solid state relay solution?

IF you connect the two shutter conductors together does the shutter operate?

Is the connection across the LED, ie the working (interim?)solution, polarity dependent or will the shutter operate regardless of wire sense?

What type of camera are you using? Most cameras have built in timers these days.

What will you be taking time lapse photos of?

I no longer need this solid state solution, although it would be better to avoid damage to a more expensive camera.

The shutter does operate when the wires are connected.

The shutter operates regardless of the polarity to the LED

I am using a vivicam 9128 with a modified lens holder and a wide angle lens. No high quality camera that I have ever seen has a time lapse function.

I am making time lapses of the weather outside. I plan to create a time lapse over a month.

The circuit you had (with the relay) uses what is commonly referred to as "dry contacts" meaning that they have no other voltage or common connection to your timer circuit. And your dry contacts will generally be normally open and common meaning that the trigger happens when you connect the two together.

With that being the case, I would rather see a simple NPN transistor in a common emitter configuration used in place of your contacts. The collector would connect to your camera leadwire that is more positive and the other leadwire would connect to the emitter. Then supply a little current in the base (via your LED) and any potential for damage remains external.

While your current setup may be working, you are risking damage to an internal transistor which will then disable the ability to use the camera in this way.

Since your circuit seems to work, you might reduct the risk by placing another 100 ohm, ¼ Watt resistor in series with one leadwire. But we are still dealing with unknowns and it might not be enough or it may be too much depending on what else is inside. The advantage of some type of transistor is that it lets you control circuits without mixing currents, or in some cases voltages.

I actually tried your resistor idea and it reduced the voltage to about that of the shutter wires. But for some strange reason, the resistor being there stopped the camera from responding to the voltage output of the timer circuit.

The opto-isolator solution of mine is the best, It's effectively what you wanted, a MOSFET solution. You gain the isolation and it won't harm an expensive camera and the output won't be polarity sensitive. There are versions in a DIP package, but this would be easier for you to deal with. You could easily just place it on the PC board where the relay went with a resistor soldered inline. Place it with a resistor where the coil was, polarity sensitive on the LED side.

If you don't have and want to add a LED indication, you could do that too.

I'm still wondering how presenting 1.5V, independent of polarity, makes a trigger circuit that wants to see an open or a short actually works.

Any ideas people?

OP has an adventurous spirit. I like that. Blind to the perils he finds simple solutions. You gotta get lucky sometimes....

My guess is that the Camera manufacturer, aware that folk would connect all kinds of garbage to it, used a bi-diorectional opto-coupler for the input (like an IL766). Provide a pull up to one side and run the other side to the trigger input. The opto-coupler would then trigger on "some" voltage sources as well as shorts to ground.

Make some trigger sources provide voltage??? Agree with the luck. If I did I would have blown the **&^% out of it.

I like the opto-coupler suggestion. The 20 mA input current would agree with that, too. For the sake of another circuit possibility, this may just be going into the clamped FET input of the microcontroller. The 20mA in this scheme will be the input resistance and the clamping diode protection.