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I have a cheap IR remote control from Ebay (like this one) and I´m straggling with the modulation of the IR signal. I use an BPV11F photo transistor as receiver and I get this signal with my scope

enter image description here

But this signal looks very identical to the signal which comes from my TSOP38238:

enter image description here

So I would expect to see some kind of modulation (the 38 kHz carrier signal) when I receive this signal over the photo transistor, like in this image.

enter image description here

My guess is that this cheap remote doesn´t use a modulation (is this possible?). Or do I have some misunderstanding?

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  • \$\begingroup\$ do you own a TV with its own remote control? \$\endgroup\$ – jsotola Sep 9 '20 at 6:15
  • \$\begingroup\$ Yeah. I can try it with this. Good hint. Thank you \$\endgroup\$ – Kampi Sep 9 '20 at 6:17
  • \$\begingroup\$ All my cheap remotes work happily. You might like to see their modulation waveforms: (1) "Rpi3 LIRC Library and UART IR Transceiver Setup Problem": raspberrypi.stackexchange.com/questions/103452/…. Cheers. \$\endgroup\$ – tlfong01 Sep 9 '20 at 6:53
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    \$\begingroup\$ Look at the top waveform : you see a very fast turn-on (falling edge) and a very slow turn-off (rising edge). That says your phototransistor is turning off very slowly : you haven't shown its schematic but I'm guessing the pull-up resistor is far too high to respond fast enough to show the 38kHz carrier. Try a 1K to 10K pullup. \$\endgroup\$ – user_1818839 Sep 9 '20 at 11:31
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    \$\begingroup\$ @BrianDrummond thank you for the tip. Changing the collector resistor to 1k "solves" my problem. \$\endgroup\$ – Kampi Sep 10 '20 at 6:23
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Yes, I'd have expected to see some 38 kHz carrier signal too.
Brian Drummond has guessed a likely cause, where the phototransistor detector has too-limited bandwidth to see the carrier frequency (perhaps 36kHz - 56kHz). It is difficult to see the rise time of the oscilloscope plot, but a guess is: 20us. Besides, photo transistors have slow response time when load resistance is chosen too-high:
phototransistor detector frequency response OPA505
A phototransistor load resistor \$R_L\$ should be chosen small enough so that 40kHz is within the passband. A larger-value load resistor limits frequency response so that carrier frequency is not seen. That BPV11F phototransistor data sheet says that you should get 100kHz bandwidth with a 100 ohm \$R_L\$, but that low value might give you a rather insensitive detector.

In addition, when probing \$R_L\$ with an oscilloscope probe, you should choose X10 attenuator setting, rather than X1 attenuator. The additional capacitance of X1 probe in parallel with \$R_L\$ will reduce frequency response even more.
Your phototransistor detector circuit may have such limited bandwidth, that it serves as a crude demodulator, yielding an output that looks similar to a proper IR-remote detector chip.

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  • \$\begingroup\$ I agree with Brian. That is a phototransistor and the circuit used to measure this is unknown. Most likely the charges at the phototransistor base junction just don't discharge fast enough to see the 38kHz modulation and the transistor is on for long time for each 38kHz pulse. The remote surely has a carrier, if the TSOP receiver can receive it, and the carrier is most likely 38kHz as the remote appears to use NEC protocol. If a photodiode, or even a standard red LED (due to lack of better detector) is fed with IR pulses, the scope would show the 38kHz carrier. \$\endgroup\$ – Justme Sep 9 '20 at 20:58

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