This is where it gets confusing . Failsafes for robots dont have to failsafe! Well, at least in the sense that if they go faulty they dont. There purpose is to stop the robot when there is a loss of rx signal when all the other electrics are working correctly so failsafe is really a bad name for them.

The ultimate system would be as in pauls web site that would bypass everything and cut main power. You could simplify the RC detector circuit using multiple micros. I did make a RC to 0-5V circuit using opamps to drive a 4QD a while ago that worked on a pulse by pulse basis using an integrator, sample & hold circuit. It could be used to detect out of range pulses by 2 comparators on the outputs and could be modiifed to act as a true failsafe circuit but it would be complex.

Another point that didnt really affect big nipper but since we started with feathers noticed the delay in some failsafes / controllers. For Axes, Flippers and other fast weapons Ideally when we press the button on the Tx we want it to fire the weapon instantly. On the other hand if a single interfenece pulse appears this would also fire the weapon and no one wants that. So in practice the RC Relay will count a number of pulses from the Rx and if they all say fire then it will fire. This can add a significant delay by which time your opponent has moved out of the way. If you use a PCM Tx & Rx then it has its own error checking and failsafe (which they dont publish tech details of) which dont introduce much of a delay and you can reduce the number of pulses before the weapon activates. Its all trade offs and in some cases safety is reduced so we have to be careful.