IR Modulation

I’m looking into event synchronization with a modulated IR signal, so I’m glad to see the 56kHz IR detector module available by itself. So, time for the newbie questions:

  1. This detector is sensitive to 56KHz modulated IR light. Does that mean that the output is normally high, even in the presence of some ambient IR light, but drops low when it sees IR light pulsing at 56KHz, i.e. a PWM controlled IR LED with a 56KHz frequency and a 50% duty cycle? I want it to be true, but I’m having trouble believing it’s that simple.

  2. The Vishay TSOP34156 detector/demodulator has an internal frequency bandpass filter. Is there such a thing as an IR emitter with an internal modulator (i.e. 56kHz flashing IR LED), or do I need to take care of the flashing on my own?

I know you guys are modulated IR experts. Actually, did the original (square) IR beacons use a modulated signal or just lots of brightness?


Hi, Adam.

It really is that simple, but there are a few things to keep in mind.

The IR sensors are set up for bursts of the frequency they are looking for, so if you keep shining 56 kHz at them, they will eventually tune that out, too. We like the TSOP34156 because it doesn’t do that as much as other units, which we need for our IR beacons since we’re (almost) constantly transmitting on the other side of the same PCB.

The output does not exactly match the envelope of your transmitted signal. For example, a 10-cycle burst is about 180 us long, but the output pulse from the sensor could be 108 us to 288 us. The delay time can also vary by over 100 us, so that might limit your synchronization application.

I don’t know of any IR LEDs with built-in modulators. In general, it’s important to flash the LEDs really brightly, like 1 A through a 50 mA LED kind of brightly. That means you have to keep your total duty cycle under 5%. Good LEDs are optimized to handle the big current peaks.

- Jan

Very cool, I’m definitely going to give these a try.

One problem though, I’m trying to build an order with some of the Vishay TSOP34156’s, but when I try to add them to my cart they come up as price “unknown” and total price “unknown”. Then anything I add to the cart after that is also messed up in the same way. New website bug maybe?



Thanks for notifying us about the bug, Adam. We’re looking into it now and should have it fixed shortly.

- Ben

Thanks for the note - I think those sensors should be working right now. Please give it another try and let us know if it you have any more problems with the cart!

Works fine now, rock on!


The IR detector/demodulators are great!

I want to use these to synchronize sonar transmissions, and a 100us variation means around 1.3 inches of sound travel, which isn’t great, but is acceptable in the worst case. I can factor the constant delay into my code, and in my bench tests I’m seeing a delay variation of more like 5us, or 0.07 inches of sound travel, forget about it!

I do need to seriously boost my LED output to get more range, but rather than overpowering weak LEDs in short bursts, I was thinking of getting some super-powerful LEDs (like these), having them part or full on most of the time (by PWMing them at a much higher frequency), and ‘winking’ them out at 56 KHz when I want to trigger the sonar (the LEDs are also serving as beacons to make an RC helicopter stand out in images taken by an IR-modified webcam). This approach seems fundamentally the same to me, is there a problem or downside I might not be seeing?


I’m not sure what the higher-frequency pulsing will do to the responsiveness of the IR detector. I suspect that it would go down some since I think these modules typically have some kind of automatic gain control very early on, so the presence of any extra light with the right wavelength will reduce sensitivity.

- Jan