MY old friend - the 555 Timer

After discovering a fatal flaw in my manual override design I turned back to one of my favourite IC's for the solution. The problem was the in the event of the receiver loosing contact with the transmitter the override circuitry would stay in it's previous state (autopilot or manual mode). If it was stuck in manual mode the plane would fly off into the distance and we would have know way of recover (short of running after it).

So back to the 555 - this is an extremely versatile and accurate timing IC. It basically requires only a couple of caps and a resistor to set the timing requirements - and hey presto - we have a timeout function which will activate emergency procedures if the timeout period has been exhausted. A transistor is used to discharge the cap, as retriggering can only occur when the 555 has finished it's timing cycle - so this is artificial retriggering.
And finally my quote to Wade - "now our project must be complete - it has a 555". For some reason almost any electronic design we work on - a 555 makes it in somehow.