This project dates back to 2010 when it was designed to limit the reverse speed of radio control model boats. Most speed controllers give full speed in both directions and while full speed may not be required for reverse use, often it is undesirable to have it available! The problem is that boats only have one pointy end - apply too much speed in reverse and the blunt end dips under the water and the boat is swamped and may even sink. This is particularly easy to do with high speed launches where even relatively little throttle yields a speed which is dangerous in the reverse direction.

This unit intercepts the throttle channel from the receiver and passes a processed speed demand signal to the ESC (Electronic Speed Controller). The forward speed demand remains unchanged, whereas any reverse speed demand is suitably scaled. A potentiometer allows the user to set the maximum reverse speed anywhere between 0 and 100%.

It was published as a DIY constructional article on the TECHNOBOTS web site, and is assembled on stripboard using components readily available from that supplier. Were I to revisit this project today, I'd incorporate some means of adjusting the neutral point (currently fixed at 1.5mSec) and use 3 pin headers for the input and output to ease assembly and tidy it up. It may be worth considering adding adjustment for the forward speed as well, such that asserting full throttle on the transmitter prevents the boat from travelling faster than its scale speed or to tame a fast launch to be more safely driven by novices or children. Finally I would also design an Isolation Milled PCB to build it on.