Why isn't this IR NEC signal being accepted by the receiving device?

Question

I'm trying to transmit an IR NEC signal to a portable air conditioning unit. I have the remote control that came with the unit, and it appears to use the NEC IR protocol.

I've setup a receiving circuit so that I can observe the signal transmitted by my circuit, as well as the signal transmitted by the remote control.

Here is the signal transmitted from the remote control (for the "Power" function, to turn on/off the AC unit):

This looks like the NEC protocol. It has the 9 ms initial pulse, then a 4.5 ms delay before sending the following, in LSB:

• Address
• Inverted address
• Command
• Inverted command

And then a final pulse at the end.

So I'm fairly convinced this is using the NEC protocol for communication.

Decoding the data from the signal shown above, we have:

• Address: 0x80
• Command: 0x9C

So I've setup my circuit to transmit the same address and command. This is the signal transmitted by my circuit:

But for some reason, the AC unit doesn't accept mine, at all. It's a pretty shoddy AC unit, so even when I use the remote control, I have to press the button a few times before it accepts the signal from the remote control. But with my circuit, the unit doesn't accept the signal, no matter how many times I transmit it.

When I compared the two signals, I did notice a slight difference in the pulse widths, so I adjusted my circuit to use the same pulse widths, but that didn't help - the AC unit still rejects it.

I'm transmitting with a TSHF5210 Vishay IR LED, with the carrier frequency at 38 kHz.

My receiving circuit uses a Vishay TSOP38238 IR Receiver. Both signals displayed above were captured with the same receiving circuit.

Am I missing something here? Any ideas on why my signal isn't being accepted?

Answer 1

It looks like it should work. The received signals shown look identical enough, but there may still be differences.

1. What is not known is the original IR light wavelength. Even if TSOP receiver can reveive the original and your replica signal, the AC unit may be more sensitive to another wavelength.

2. What is not known is the original IR carrier. Since it is NEC code, the best assumption is that it should be 38 kHz. The TSOP receiver can receive with some tolerance margin. If the orignal remote uses some other custom frequency, the AC unit may be sensitive to some other carrier than 38 kHz. I have seen some devices with non-standard carrier, maybe intentional as the timings change with carrier so it makes it harder to clone or replicate.

3. IR LED current may be too low so it's too dim. The current might be too high so it's too bright. TSOP might receive it just fine but maybe the receiver in the AC unit won't.

4. IR LED drive duty. Depending on your drive circuit, it is unknown how the carrier duty cycle drives the LED. For example if you use a standard BJT, they can be slow to turn off, so duty can be high and it may look like almost DC instead of 38 kHz. Typically the carrier duty cycle recommendation is 33% or 25% for NEC, to have bright pulses and definitive off time.

5. Almost forgot, the code you show is the initial burst and only sent once. The device may require that the button is held for some time and the initial burst is not enough, the button held down should send the repeat code. But that is yet another catch, some remotes send initial code burst and then the button held down repeat burst without code. Some remotes send the whole code burst when button is held down. I've seen a remote (not NEC protocol though) that no matter how shortly you pushed the button, the remote sent the code at least 3 times, and kept repating more if button was held down.