Remote control and infrared light

At the weekend, I received an unexpected envelope from an unidentified sender.  In it were some LEDs labelled 940nm and 850nm (i.e. different infrared wavelengths).  I have a strong suspicion about the identity of the sender, and I'm pretty sure they will read this - thank you!

The infrared LEDs allow a nice combination of two of our recent activities which the small boy has enjoyed, namely electric play dough and using our infrared camera (baby monitor).  I helped the small boy to make a very simple series circuit with the 940nm infrared LED (which I described to him as the 'new LED') and one of the red LEDs that he's played with before.  He remembered that he needed to put them in the play dough the right way round, checked for the longer 'leg' and put both LEDs in correctly. However, when the circuit was completed he could only see the red one lit.  He was confused and took the infrared one out again, and put it back, and only the red one lit again.  I asked him whether the electricity was able to flow around the circuit, and after a long pause, which I didn't think was going to result in an answer, he told me it must be a complete circuit because the red light lit.  He was right, but he was clearly puzzled about why the new LED didn't seem to be doing anything.

Infrared and red LED in a play dough circuit

I did a bit of prompting to get him to think about what the other LED could be doing.  I gave the answer away when I suggested that we might need the baby monitor (which was sitting on the sofa rather than in his brother's room), and he told me it was infrared.  We needed to test his hypothesis, so we switched the lights off and the baby monitor turned onto its infrared mode.  We had a look at the play dough and LEDs with the infrared camera, and could see both LEDs looked bright.  The red LED clearly emits some light in the range of wavelengths the camera detects in its infrared mode, so both appear lit on the infrared camera.  The small boy had a quick look, and then rapidly lost interest.

The infrared LED (left) and red LED (right) seen in infrared

I'd been going to suggest some further experimentation with the other infrared wavelength, but as his interest had waned, I decided to try something else.  I asked him to get the remote control for our CD player instead, and to have a look at the end of it.  He spotted the LED, and I asked him whether he'd ever seen a light come out of it, to which he said he hadn't.  He then proceeded to press some buttons and check... but no light.  At least not a visible one.  I suggested he had a look in the infrared camera, and - after a bit of faffing to get the remote in the camera picture at an angle where he could both press the buttons and see what was going on - he was delighted to see it flash.

Small boy and his flashing remote control, seen in infrared

He tried another button and saw several flashes.  He then proceeded to try every single button on the remote control in turn to see what pattern of flashes it produced, and then went back to some of them again.  There's a short video clip below of one of the buttons being pressed, although it's without sound (as the littlest picked that moment to howl).  We never did go back to the other LED in the mystery package, maybe another day...

When Daddy came home from work, the first thing the small boy said was that he wanted to show him the infrared LED!  Sadly for the sender of the mystery package, this wasn't their LED, but the CD player remote control.  One day I'll let him know that the TV remote control also has an infrared LED, and we can compare the pattern of flashes for the different buttons!