Electronic – How to indicate that an IR LED is functioning without detecting its emissions

Rather than transmit the marker ID code from the marker to a central receiver, perhaps it would be simpler to transmit the marker ID code from a central transmitter to the marker.

The central transmitter (perhaps a 38 kHz IR transmitter or some wireless transmitter) would send, in effect, "Marker number 22, please turn on for the next 20 ms on my mark: NOW". (Ideally, while that marker is glowing for those 20 ms, the central transmitter is sending out the ID of the next marker to turn on).

Since that one central transmitter is controlling the timing, you won't have to deal with markers getting out-of-sync and accidentally transmitting at the same time.

Hopefully you can place that central transmitter close enough to the position tracker, so that if any marker can't see the commands sent by the central transmitter, that marker wouldn't be in the visual field of the position tracker anyway.

That also gives you the flexibility of using the data you get back from your position tracker to dynamically adapt which markers you select:

• If some markers seem to be motionless or extremely slow-moving, perhaps you only need to check on them once every 3 seconds or so.
• Perhaps you can check up on the latest positions of more than one marker at a time; something like "OK, marker number 22 and 23, please turn on for the next 20 ms on my mark: NOW".
• If some markers are not visible from this vantage point, perhaps you only need to check if any of them have re-entered the visual field once every 3 seconds or so.
• The time slots you free up with the above techniques could be used to track the remaining markers at a somewhat faster update rate than you could if you simply cycle through every marker in a fixed pattern.
• If, say, marker #22 is so close to marker #23 that receiver of #23 is blinded when marker #22 is active, you could shuffle the order you turn on the markers so that the "#23 please turn on" message happens a few slots before the "#22 please turn on" message.

As Olin points out, it's hard to tell what you're really asking. I'm going to assume that you want an indicator LED to light up whenever the optocoupler in ON. In that case, your circuit should look like the one below:

The "limit switch", when closed, should tied the INPUT signal to 3.3V and light up the indicator LED and the optocoupler's internal LED. The OUTPUT signal goes to an input of your PIC microcontroller.

The value of the resistors will depend on the optocoupler and the LED you choose to use.