camera shutter trigger question

[Bob Chiang]

Hi all,

I'm planning to make a camera usb shutter switch using the instructions on this web page:
http://tinyurl.com/5b4vqa

Basically, you use the electronics from a servo to generate a dc voltage (from the servo motor leads) that can trigger a shutter via a usb connection and CHDK software.

The author says "... Remember the correct stick movement to generate the positive voltage. If your transmitter is programmable, remove the stick movement that gives you the negative voltage or be very careful when using remote trigger..."

I want the switch to be safeguarded against feeding reverse polarity into the usb connection. If I install a diode in line with the connection from the servo motor lead to the usb connector, will that take care of the problem? Should the diode be anything special? I'm pretty sure I have a small Shottky type diode from a brushed motor. Will that work? Would a small signal diode work?

Thanks,
-Bob

[dleroi]

Hi all,

I'm planning to make a camera usb shutter switch using the instructions on this web page:
http://tinyurl.com/5b4vqa

The author says "... Remember the correct stick movement to generate the positive voltage. If your transmitter is programmable, remove the stick movement that gives you the negative voltage or be very careful when using remote trigger..."

I want the switch to be safeguarded against feeding reverse polarity into the usb connection. If I install a diode in line with the connection from the servo motor lead to the usb connector, will that take care of the problem? Should the diode be anything special? I'm pretty sure I have a small Shottky type diode from a brushed motor. Will that work? Would a small signal diode work?

Thanks,
-Bob


Bob,

Off the top off my head (which means I'm guessing), I think you could use the "positive" motor lead, as shown, but instead of using the other motor lead, use the negative power lead (ground). That way, when you move the stick in the direction that gives you positive on the motor lead, you'll have pos and neg on the right pins and when you move it the other direction, you'll have neg on both, which shouldn't cause any problems.

Anybody find a flaw in that?

- Don

[AndyKunz]

Check the RC CAM website. He has a better (imho) setup that can easily be adapted to almost any digital camera, doesn't have the polarity issue, costs less, and can be used with something else on the same channel (rudder, for instance).

Andy

[mrdunk]

If I install a diode in line with the connection from the servo motor lead to the usb connector, will that take care of the problem? Should the diode be anything special? I'm pretty sure I have a small Shottky type diode from a brushed motor. Will that work? Would a small signal diode work?
yup. if you want to use a diode like this then i'd recommend a shotkey diode as they only have a small forward voltage drop. (around 0.6V.)

depending on the servo it is possible you never get negative voltages to the motor.
many will give a positive voltage on one wire and 0V on the other.
try testing it with a mutlimeter.

i'm slightly concerned by the instructions on that site when he tells you to connect the servo + and - wires to your camera USB port.
if i were you i'd check the voltage from the servo is exactly 5V.
i can't remember what the USB spec says a USB device should be able to handle but i'm fairly sure your camera won't be happy if you try feeding it too high a voltage for long periods...
http://www.usb.org/developers/docs/ would tell us what the official max voltage is on a USB device port but i'm too lazy to read it just now.


dunk.

[Bob Chiang]

Thanks for the comments and suggestions.

Don, what you suggest makes sense to me and I hope someone more knowledgeable will verify the idea.

Andy, I have looked at the RC CAM site but all I've seen are PIC based circuits. They are simpler, but I don't have any experience with PIC chips and I don't have a programmer. I think they also rely on wiring directly to the shutter switch. I direct wired my first AP camera (an Aiptek Pencam) with a PIC based switch that I bought from an Ezoner, but I'd rather not open my current camera (at least not yet).

Dunk, the servo board I have puts out 4.95 volts with the transmitter stick in one direction, and reversed polarity with the tx stick in the other direction. Don't they all have to do this to get the servo motor to run in both directions?

With a diode in-line, I get 4.66 volts with a TX control input. With the opposite TX input the voltage increases over a few seconds to approximately -400 mV and then drops to approximately zero and then starts rising again in a cycle as long as I hold the TX input. Similar behavior with two different diodes. Will this reverse voltage hurt the camera?

-Bob

[mrdunk]

Dunk, the servo board I have puts out 4.95 volts with the transmitter stick in one direction, and reversed polarity with the tx stick in the other direction. Don't they all have to do this to get the servo motor to run in both directions?
yes. but what you have to be careful of is that the voltage does not drop below 0V on either pin when you reverse.

With the opposite TX input the voltage increases over a few seconds to approximately -400 mV and then drops to approximately zero and then starts rising again in a cycle as long as I hold the TX input.
yours apparently does drop below 0V (-400mV) so yes, use the diode.

With a diode in-line, I get 4.66 volts with a TX control input.
that should be fine.
if i remember the USP specs correctly the data pins on a USB device should be a +3.3V signal but be 5V tolerant.
so if you want to do things by the book you should only send +3.3V on your data line but don't worry about it because your camera should accept up to +5V on D+.
as your servo with diode in line is +4.66V you should be fine.

did you check the voltage you will be sending to USB pin 1 (VCC) ?
according to the link you will need to connect the servos power feed here.
i'm guessing this is to enable the USB port.
if it is over 5V then you better check on the USB specs to see what the official voltage range is on this pin. (i'm afraid i don't remember the USB specification for that one...)


dunk.

[AndyKunz]

Bob,

Mr. RC-CAM will burn a PIC for you. You'll be doing fewer mods using his circuit than trying to adapt the servo amp like your original post.

Andy