So remote controls are pretty awesome and they're everywhere, TV, DVD, VCR, Stereo......I think the list is pretty endless. Remotes are pretty reliable but when somthing doesn't respond it's really annoying, there's a lot of remote checkers out there but they're pretty expensive and don't really tell you anything useful, or show you anything cool. Time once again to break out the DrDAQ.
IR is at the long end of the visibile spectrum, just past red, and since the DrDAQ has a light sensor it might be able to pick up IR signals. I found a remote and setup the DrDAQ in picoscope and had a quick capture, and it worked!
(the 100Hz ripple at the bottom is the ambient from our strip lights). This was me holding down a button on the remote, I reckon from that capture that this remote sends out 1 data packet and then a repeat pulse every 100mS, so we've checked that the remote sends something out (and that's pretty much what those remote checkers do) but I think we should get in a little deeper.
So it looks like the data coming out is in a 100mS window so switch down to timebase 10mS and go again
that looks ok but I don't think the response of the light sensor is a bit limited for us to actually see what's going on, time for a better external sensor.
It's time at this point to do a little reading about remotes, a lot are modulated at 38KHz, why? Well, there's plenty of IR floating around ambient so if a remote just turned on and off the IR Diode then it would just be adding a little bit to ambient and the devices would need to detect this, not very easy, what is easier is to look for a known signal. As it happens 38KHz is a standard frequency used by many remote manufacturers and there's lots of IC's that will look for this signal for us and give us a nice digital out of 'present' or 'not present'
We grabbed a Vishay TSOP4838 and set to it. Since the signals that we are looking at are fast the 'scope' input is best to use for this sensor. So, in a bit of a change we'll take some power from one of the external sensor sockets and wire the sensor into the scope input. Since there's a lot of range on the scope input and it's very high impedance there's not much need to worry about an output buffer or to make the signal smaller, 0-5V straight out of the sensor will do fine. Really I should've done this on stripboard or just with some heatshrink and wires, but I'm addicted to PCBs!
So I connected up the BNC and did the capture again to compare both the waves. So it was individual bits we were kindof seeing with the onboard sensor but the TSOP4838 gives a much clearer output. This sensor is inverted so low means that there is 38KHz being transmitted, I guess with this remote that there is the long start pulse and then the individual bits are a long off time or a short off time for 0 and 1.
It's easy to see now if we use a reference waveform that all the buttons on the remote give out a different code, that's pretty cool. Now I wonder if we can make the DrDAQ give out a captured wave and modulate it by 38KHz to get it to control devices?????