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I am trying to read some IR signals. There is a synchronizer emitting modulated IR signals. And my receiver must receive those signals and interpret them. Actually, there are only two different modulation: left signal 1

right signal 2

I put a photodetector in front of the synchronizer and get these two waveforms on the oscilloscope. These are 2 signals that my receiving circuit should interpret.

So in overal, transmitted IR is something like this:

IR transmit

My IR receiver is this. It's datasheet is this.

This is the schematic:

sch

output directly goes to microcontrollers interrupt input.

circuit

So here is the problem. Synchronizer, always transmit IR this way. Never changes. But my IR receivers output goes crazy most of the time. Some times it responds those coming signals like this:

enter image description here enter image description here enter image description here

and sometimes it outputs like this: enter image description here

I can never tell! Why does receivers output varies like this? Received IR signals or their RSSI never changes, I do not move the receiver or the transmitter but receivers output changes in time. Can you explain why is this happening and how can I fix it?

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    \$\begingroup\$ Are you modulating those input pulses with a 38kHz carrier? \$\endgroup\$ – Brian Drummond Jun 24 '16 at 23:45
  • \$\begingroup\$ That is not the correct receiver for that type of IR signal. As Brian indicated, you need to modulate your signal with a 38kHz carrier. The outputs you are seeing are possibly just false triggers from your IR signal kicking the receiver's AGC. \$\endgroup\$ – Mark Jun 25 '16 at 7:44
  • \$\begingroup\$ here is the thing: i can only receive. i can only read those ir signals and i have to do it right. So, should i change my receiver than? But all the IR receivers on digikey is 38 khz. \$\endgroup\$ – Alper91 Jun 25 '16 at 8:13
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    \$\begingroup\$ Then use an IR photodiode and roll your own amplifier using opamps. This is tricky thanks to issues with ambient light, and noise from LED lamps, and you'll need AGC, but it's the only way if you need to define your own signal bandwidth. \$\endgroup\$ – Brian Drummond Jun 25 '16 at 11:57
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    \$\begingroup\$ Not just carrier wave, but wavelength too. \$\endgroup\$ – Passerby Jun 28 '16 at 1:58
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As pointed out in the comments, the TSOP32538 expects a 38 kHz carrier wave. Per page 5 of the datasheet:

This series is designed to suppress spurious output pulses due to noise or disturbance signals. The devices can distinguish data signals from noise due to differences in frequency, burst length, and envelope duty cycle. The data signal should be close to the device’s band-pass center frequency (e.g. 38 kHz) [...] Some examples which are suppressed are: [...] Continuous signals at any frequency.

The table on the same page specifies a minimum burst duration of 6 carrier cycles, which would imply 158 us, but the note in Fig. 1 says this is only a recommendation. Shorter bursts, even bursts of only one cycle, are clearly detectable based on your Receiver Output (the empty spaces in the middles of modulations are long enough that each modulation is interpreted as two bursts).

However, the detection is inconsistent most likely because single pulses or pulses deviating too far from the carrier period may be classified as noise and suppressed. The widths of the pulses shown in the scope pictures vary from 25 to 50 us - it would take a good timing diagram to determine whether there is any pattern as to which durations are being recognized and which are not.

Further considerations:

  1. The modulation pulses are too short to modulate more than two cycles of 38 Hz carrier, so even if you could add it, I am not sure it would help much. There appear to be higher-speed infrared receivers outside digikey.
  2. Adjacent pulses that are detected are combined into bursts, and single pulses tend to be wider than pair. Even with consistent detection, this will make it difficult to distinguish the modulations, unless you can figure out whether there were one or two pulses based on the output burst width.

If you have no influence on the signal and cannot find a less suppressive model, there are many guides to making IR receiver circuits, though the result will not be nearly as compact.

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