Help me reverse-engineer an obsolete IR receiver? - Page 1
Forum section: electronics / repair

author: cvanc
author_rank: Frequent Contributor
timestamp: August 27, 2019, 06:07:39 pm
content: Please see attached schematic.In the upper left corner is an input for the IR receiver. The receiver assembly has been lost and I need to build a replacement. This is all vintage 1983 or so.Photos show the handheld remote and interface box (which I have) and the IR receiver on a long pigtail (which I don't have). The schematic is for the interface box.I'm thinking a simple optotransistor should work, but I'm open to suggestions. Thanks.

author: Renate
author_rank: Super Contributor
timestamp: August 27, 2019, 06:25:28 pm
content: I would normally have thought this would take one of those little IR photodiodes with a built-in 38 kHz demodulator.But since there is only 12V and input (no ground) I'm thinking that it must be a bare photodiode or photo transistor.Just try using a photodiode, cathode to the 12V.Use your remote point blank.You can work on improving sensitivity later.

author: floobydust
author_rank: Super Contributor
timestamp: August 27, 2019, 07:19:11 pm
content: The IR input is oddball with no pullup/pulldown and it does not show a GND connection. Just two wires, +12V and IN?With that a photo-transistor (or anything) would not work, nothing to take the 4093 input low.And it would have low gain, not decent range with a photo-transistor, and 5V would have been enough no need to have a boost converter to 12V. The receiver module must have a pre-amp.With the remote, if you can measure the carrier frequency or the chipset it uses.

author: Renate
author_rank: Super Contributor
timestamp: August 27, 2019, 09:27:16 pm
content: Yeah, it is strange that they jump through the hoop of the DC converter for the 12 V.For such a (relatively) high voltage I presume that a reverse-biased photodiode plays a part.The lack of a ground is strange.What is the jack for the receiver? Does that have the ground.The question is whether there is a 38kHz (or other frequency) demodulator built into the pickup.Since we are looking as stone-age stuff, I would think that they don't deal with data as fast as the raw carrier.I have a little protoboard and software on my PC to decode IR streams.

author: cvanc
author_rank: Frequent Contributor
timestamp: August 27, 2019, 11:04:22 pm
content: Quote from: Renate on August 27, 2019, 09:27:16 pmHere you go...

author: Renate
author_rank: Super Contributor
timestamp: August 28, 2019, 12:33:25 am
content: I guess that must be a 3 conductor coaxial connector.They seem pretty cavalier about using a painted chassis for ground.I've used a bunch of IR receiver/demodulators, something like this:http://www.vishay.com/doc?82459The original version was probably all discrete.You've got the remote, put a scope across the output LED and see what the carrier frequency is.You could probably tear apart your families VCR, add a 7805 and make it work.

author: floobydust
author_rank: Super Contributor
timestamp: August 28, 2019, 02:53:30 am
content: In the early 80's Siemens was king for IR photodiodes, like SFH205 and you would use a transimpedance amp and some filtering, AGC- all of which was a big hassle. It was Telefunken, then Siemens, now Osram that makes them. This was likely inside the missing original receiver.I would first try a vanilla consumer IR module supplied quiet 5V from that 12V. The original receiver module must have gotten GND from the connector housing.I wonder if the IR receiver does not demodulate because of U1211, U1209 seem to be a synchronous demodulator divided down off the 5V serial data clock. Hard to know the serial data rate at J1201, if it is super fast thenVishay TSMP58000is a receiver module with the raw carrier output 20-60kHz.

author: Renate
author_rank: Super Contributor
timestamp: August 28, 2019, 03:14:19 am
content: That is a wacky circuit.All the 5V that powers everything (including the 12V DC converter) is being fed in J1201 with a superimposed processor clock for the 8085A!