Transmission of servo (pwm) signals through water and clear plastic

I am just putting the components away for something I was trying with the original Darnell XXI. I originally needed to send a servo signal through a clear, flat plastic hatch. I dropped the idea later as you will see in the Darnell XXI thread.

But the method I tried worked well and was very simple.

The sender unit (connected to the receiver) was just an Infrared LED with a 30 ohm resistor in series (just to limit the current. The receiver unit (connected to the servo) was a photodiode with a 1 megohm resistor in series. (The LED and photodiode were a matched pair but I can't remember where I bought them from. Ebay probably.)

The early trials were surprisingly successful with the signal being happily beamed through several mm of plastic or even water.

If you used a separate 5v supply at the receiver end then that is just one wire to transmit a servo signal.

David

Is that all it took was a resistor connected directly to the infrared led? No complicated circuits? Am misunderstanding?
Thank you for posting this! Makes the brain work.

Yes, it really is that simple. I've just connected it up again to remind myself.

One side of the photodiode (the side without a flat on it) is connected to +5v.
The other side of the photodiode is connected to the white signal wire of the servo and one end of the 1 megohm resistor. The other side of the resistor goes to earth. (It's a pull-down resistor.)

To "connect" the signal wire you would just need to push the led and diode into the end of ,say, some silicone tube.

It should have wide application to model subs but, in my case, I bunged everything in a single WTC so the need for it disappeared!

It is not particularly original - I'd seen the idea somewhere on the web in connection with robotics perhaps as an "optoisolator". The real optoisolaters are no good to us because they are encapsulated in a single device.

David

Thank you David! I have heard that running a servo a long distance from the receiver causes problem with interference. This might be good for keeping a servo in a remote location on the sub. Definitely something to play with!

Your comment has made me think of something else.
Long servo leads can indeed give interference. I wonder if the light signal can go down a fibre optic cable?

David

Exactly what I was thinking!