Re: IR receiver and phototransistor examination of carrier and stripped carrier :
"Traps For Young Players" file.
Hi - I am new to the blog and forum.
The video was very informative. I don't have the fancy pants equipment but I wanted to view the output of some of the remotes around the house on an analog scope. I scrounged up a photo tranny and an IR receiver from the junk box. This note is just to add to the "TFYP" file on this aspect of the subject.
There are two things that I bumped into while trying this out.
1) IR receivers - I was unaware of what these things do, so the video encouraged me to try one out. The trap here is that the pin-outs are not uniform across manufacturers. I had a few that had been removed from consumer gear but they have no part number to look up. My solution was to remove another one that was still in place on some junk board, but paying attention to the circuit traces as to which ones are the likely suspects ( example, an electro cap across two pins was good evidence that I had Vcc and ground). So my advice to a beginner on this is to pay attention when you remove it and store it away with a bit of documentation.
2) Detecting the carrier. My first circuit for this was a simple grounded emitter ( Vcc - 2k2 - photo-t collector , photo-t emitter grounded). This detected IR from a remote but I didn't see a carrier. I then tried 4 other remotes - same results. The output looked about the same as that from the IR recevier. I thought that I was unlucky and none of my remotes used modulation. But unlucky 5 times ?? I cranked up the volts/div on the scope and could see tiny sawtooth waves on the low going parts of the signal. I started to wonder if the lone photo-t was not fast enough to follow 38 khz. I googled 'phototransistor circuits' and got a few hits that were real app notes. When I built the circuit in the Sharp app note ( figure 11a ) the response was fast enough that I could see ( and measure the period of ) the carrier (36 k in 5 out of 5) . That made for a fun evening. Thanks.
If the URLs don't work, google
'Photodiode/Phototransistor Application Circuit' for the Sharp app note and
'application note an-3005 design fundamentals' for the Fairchild one
Fairchild app note
http://www.fairchildsemi.com/an/AN/AN-3005.pdf?
Sharp app note ( this had the solution that I needed )
http://physlab.lums.edu.pk/images/1/10/Photodiode_circuit.pdfGR