I need more than a light detector.

[jsolos]

Hi, I have been looking for a light detector circuit that can drive a MOSFET or something with more power. Most circuits I see can only drive a small amount of current like an LED.

The goal of this circuit is kind of a video camera hack. I have a helmet cam hooked to my camcorder. I need something to monitor the power LED on the camera and cut the power to the helmet cam when the camera is off (if not it drains my batteries) and back on as the camcorder starts up. I think the helmet cam takes about 500 ma at 9 volts.

Monitoring the power LED on the camera seems the easiest way without messing with the camera and it is completely isolating the helmet cam circuit. Also, I figure you can move it to any camera since they all have a power LED.

Please let me know if you have a circuit (or another suggestion).

Thanks!

 

[Roff]

Are they both running off the same battery?

 

[jsolos]

Yes, both are running of an external 12v battery. I'm controling the camera from an LANC waterproof remote so I can't really cut the power easily.

 

[silvarblade]

for switching with mosfets you need the gate voltage to be higher then the DS voltage inorder for it to remain on and since both are running at 12v i think there is going to be a problem.why dont you use a BJT?

 

[jsolos]

Anything that can handle that kind of current would do fine. Do you have any sample schematics?

 

[Roff]

If both units have mechanical power switches, you could just leave them both on, with both connected to the battery, and simply use a mechanical switch to turn them both on.

 

[ericgibbs]

Anything that can handle that kind of current would do fine. Do you have any sample schematics?

hi,
Look at the data for theses power FET's

IRF520A, IRF740, IRF840.

 

[jsolos]

You can't just mechanically cut the power to both or else you have to open the case and restart the camcorder for the LANC remote to take over.

Here is someone else's solution to the problem. Only drawback is you can't use an external mic anymore.
Automatic remote ON/OFF using the MIC jack on your camcorder / Low Voltage Solid State Relay

I would still like to explore the light detector since it optically isolates the cam corder form the helmet cam. Keep the ideas coming.

Sorry, I'm new to the site and it looks like most of my replies are sitting in the moderator queue. I'll try again later...

 

[jsolos]

OK, I'm back on line so if anyone has a light detector circuit that drives a FET or power transistor I'd like to see it. Thanks!

 

[Roff]

Apparently the helmet cam does have a mechanical switch that you can leave ON, or am I missing something?
Do you have access to power MOSFETs?

 

[gracet]

Splice into the lanc circuit, specificly the power line from the camera. It is run at about 5to7v when the camera is on, and 0 when the camera is off-standby. The load of the circuit is said to be limited to 100ma , enough for switching, perhaps a tiny relay.

 

[jsolos]

Apparently the helmet cam does have a mechanical switch that you can leave ON, or am I missing something?
Do you have access to power MOSFETs?

No, the helmet cam does not have a mechanical switch, just a 9vdc plug. All these plugs, batteries, camcorder, and connections are in a waterproof case in my backpak. That's why I can't get to them easily. As I said before, if I cut the power to both, I have to open the case and restart the camcorder. I could put a waterproof switch somewhere on the helmetcam but it's not a great solution to have to start the camcorder with the LANC remote and then power up the helmet cam everytime you want to record a 5 second segment.

Yes, I have access to some power MOSFETs. Just not sure how to work them into an existing light detector circuit in place of a transistor.

I'll look at the other suggestion of tapping in to the LANC remote...