by Andrew Davie » Wed May 16, 2007 1:28 am
Well I've just surfaced after a very painful and slightly embarassing two days trying to get a simple IR pair working. Since my first attempt at running the motor synch circuit as it is newly installed on my single board TV didn't work, I thought -- well, let's have a look at the IR signal as I hand-spin the disc. Nothing.
So, I took out the IR bits and created a stand-alone version of just that part of the circuit on a breadboard. Just the IR transmitter, the 680 ohm resistor, the IR receiver, a 100K pot (what I had handy) and a 33K resistor in series with that.
Nothing.
I bought some new IR send/receive components. Nothing. I checked and re-checked. Basically, as I'd mixed up the components and they don't really have much of any markings to help out... I had a pile of IR transmitters, none of which I knew worked or not -- and a smaller pile of receivers -- ditto. I had assumed that something I'd done blew them up -- and had no real way to tell (I can't seem to get my multimeter diode function to work -- perhaps it's not putting enough voltage out?).
I thought to check the setup by placing a LED across where the IR transmitter goes, and similarly across the receiver -- just to see that there was a reasonable voltage/current there. In hindsight I could have used the multimeter, but in any case, the LED lit (not too brightly), and so I figured that was OK (I understand the specs of the transmitter and receiver are about 1.3V). I calculated the correct resistance for the transmitter, noting that the 680 ohms was on the large side, but should be OK.
I've been frustrated in this for a couple of days, but tonight I had a bit of inspiration! A long time ago I noticed that my old videocamera when pointed at a TV remote actually showed the remote led brightly in the viewfinder, even though it wasn't visible to the eye. So I'd figured the camera was showing the IR downshifted to a visible wavelength on the viewfinder. Dug up the old camera, pointed it at a transmitter IR LED and *voila* I could confirm that LED worked. From then, things got much simpler. When I plugged in my original IR LED it worked.... and then a minute or two later it did not work. Very strange, I appeared to have blown that one up while testing. I'd done this a while back, too... perhaps adding/removing components to the live circuit is a no-no?
In any case, now that I had a working and visibly working (with the camera) IR transmitter going, I could concentrate on the receiving side of things. Plugged in the receiver. Nothing.
I figured I'd blown that up too, but it's the only one I have left that seems suitable. I have a bunch of others, but they reacted strongly to the lamp I'm using (last I had things working) so I'm avoiding those.
After trying installing the receiving LED leads backwards ... things started working. When I disassembled my original working breadboard circuit, I carefully 'tagged' the negative leads of the wires with black tape. It appears that I did this for the wrong wire with the IR receiver. This is very annoying, because I was trying to be careful -- and this mistake has cost me a few days.
Now I have a working IR pair, running in the small subcircuit. The problems were two -- firstly, because I couldn't SEE if the IR was transmitting, I had no idea if the IR LED was blown or not. I had several IR LEDs which (it turns out) were blown. These have now been thrown away. Secondly, I had the receiver plugged in backwards, because of my stupid tagging mistake.
OK, so now I'm happier... a working IR signal, giving a good 12V when they're in proximity, and 0V when not. Next step is to connect these back to my single-board motor control circuit, and start to trace the rest of that circuit knowing that the IR side of things is working.