The failsafe system that we use on our models is not 100% reliable. If the voltage supply to the receiver is lost then failsafe will not occur because it needs the working RX to set controls to preselected positions. If power is lost to the RX through any reason, broken connections, faulty cell, on/off switch failed etc etc. and you had been flying at full throttle, the engine would remain at full throttle (unless there was a very strong return spring on the carburretta to swing the servo arm back).
The simple circuit here provides a solution, the relay is activated by voltage to the RX and will stop the engine if volts disappear. This problem occurred with me, the model had dual receivers so I could still control it but the throttle control was on the RX that had lost power. At full throttle it was much too fast to land so a controlled 'crash' was the only answer. If this was a single RX model serious consequences could have occurred.
The components are very cheap, the 5 volt relay should have coil resistance of around 100 ohms to minimise operating current and preferably change over contacts so that it can be used with battery ignition or CD ignition (details on diagram).
There is also an additional bonus for CD ignition engines, providing the RX is switched off you can prime your engine without fear of bruised fingers when turning it over.
PS. It is imperative that the FLYBACK diode is connected the correct way round. The silver band is the end connected to positive.
The diode can be the 1N4004 or any similar type as can the PROTECTION diode which is connected the other way round to the Flyback diode and will prevent the RX from being damaged if wrong polarity is applied.
