There’s a special type of satisfaction that comes from really understanding how something works at the end of a reverse engineering project. This grid above is the culmination of [Spencer’s] effort to reverse engineer the IR protocol of a Propel ExecuHeli indoor helicopter toy.
The first thing he looked at was the three different controller channels which can be selected to allow multiple helicopters to be used in the same area. [Spencer] was surprised that they all used the same carrier frequency. The secret must be in the coded packets so his next challenge was to figure out how the data was being transmitted via the Infrared signal. It turns out the packets are using pulse-length coding (we were unfamiliar with this protocol but you can read a bit more about it here). The last piece of the puzzle was to capture packets produced by each unique change of the control module. With each bit (except for bit 11) accounted for he can now format his own codes for a controller replacement. Perhaps he’s looking to make the helicopter autonomous?