Headphone jack IR transmitter not working reliably

Question

I'm working on a project to build a remote control app for some IR controlled toys like DeskPets, Robosapien, TOMY etc. including an IR transmitter that (hopefully) has a very high range.

I built an IR receiver that works with IrScrutinizer to capture the signals and export them to wave files.

I built 2 transmitters (one passive, one with a headphone amplifier powered over a USB port, later to be replaced with a LiPo battery). The LEDs are connected antiparallel to the left and the right channel. However these are not working reliably enough.

Both can send signals, however only if I play the recorded wave file using VLC media player and that only works a few times, leaving me wondering whether I broke the LEDs. A protoype app for Android successfully sent a signal once after countless attempts.

I have 2 theories:

1. I forgot to set up specific settings in order to correctly play back wave files. Maybe VLC does something right when playing back a wave file. Playing back the captured signal directly via IrScrutinizer also works. I downloaded a soundboard app for Windows and playing back the same wave files didn't work.

2. The wave files themselves are unreliable. While capturing the signals I noticed the timings are off (pauses are for example between 3395 microseconds and 3401 microseconds long). I didn't mess with the timings before exporting the wav files, but maybe if I fix the timings the signals could be read more reliably.

I hope you can help me fix this problem so I can continue with my project.

Answer 1

IR remote waveforms are not simply a coded sequence.

Rather they are a carrier such as 38 kHz which is switched on and off in the coded sequence.

Typically an IR receiver removes the carrier and outputs only the coding. Even if it did not, the carrier will not "fit" through a phone's audio path, but rather will be removed by the decimation filters in the ADC.

An IR sender requires a circuit to generate the carrier, the phone is only able to provide information used to key that carrier on and off, and maybe if harvested in various hacky ways, the electrically power.

You'll need to do some research yourself to find a suitable published circuit.

Any success without a proper carrier is a lucky accident.

Answer 2

As others have said, unless your audio output is capable of generating a carrier frequency, reception will be very unreliable. These carriers are typically around 36 to 56kHz.

Connecting two anti parallel diodes to an output will give a transmitted frequency twice that of the audio file. Unless you allow for that, the signal won't work.

Answer 3

From the comments:

One LED's plus pin is connected to the left channel and the other LED's plus pin is connected to the right channel. The minus pins are connected to the other respective channels.

That means that they'll only light when there is a difference between the two channels. If you have a mono recording or a stereo recording with the same signal on both channels then you will have no voltage difference between L and R. I don't understand how you got this to work at all unless you have zero volume on one channel or you inverted the signal on one channel to maximise the difference in L and R.

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. (a) What you've got. (b) What you need.

If you've got the same signal on L and R then you would have to wire as shown in Figure 1b but would need a higher output level than if you were running (a) with anti-phase signals.

The signal on the right channel is inverted to the signal on the left.

That is critical information and should be mentioned in your question.

So i do get the voltage difference necessary to send the signal.

Yes, you would.

I forgot to mention: If the mono signal solution + different circuitry is more reliable, I'll absolutely switch over.

No, your complimentary (inverted) signal gives twice the voltage to the LEDs in (a) than you would get in (b).

---

Standard IR transmission uses 38 kHz modulation on the IR light. Your phone's audio won't go above 20 kHz so I don't know how this is working at all.