Probably most people, including our rules people, don’t realize that the timer’s job (giving throttle instructions to an ESC) is not a simple thing, electrically speaking. Note that the only connection between the timer and the ESC is through the three wires: ground, signal, and (if the ESC provides a BEC) +5 volts.
The only kind of a throttle instruction that an ESC will accept on the signal wire is one in which the signal voltage is at ground level for 90% or more of the time and, when it is not at ground level, it must be at a logic level of around 5 volts for between 0.001 second (throttle off) and 0.002 (maximum throttle). All the ESCs we use require a “throttle off” signal (pulse) before they will “initialize” and accept positive throttle requests; otherwise the motor simply doesn’t even start to run.
Some of the very first attempts at timers used discrete components (such as the 555 integrated circuit) but I believe all current timers use a microcontroller that is programmed to generate these pulse outputs. The possibility of a damaged microcontroller producing valid throttle signals is almost zero. With around 1400 or more of my timers now in circulation, I know of none that has done this. In addition, I know of none that has failed during a flight—but, if they have, would certainly just stop the motor with an invalid signal voltage.
It is possible to damage a timer’s microcontroller. For example, as has happened, if one connects the three-wires incorrectly (off center), it is possible to switch the ground and the 5-volt wires and this kills any microcontroller (except one specifically protected against this). Another possibility is static electricity. But such a damaged microcontroller is totally incapable of putting out a pulsed output; instead, in my experience, it will put out a steady voltage, usually zero.
The power requirements of a timer’s microcontroller are so small that heating is not a consideration; if it does get hot, it has been killed and will certainly not generate pulses on its output pin.
I have recently inspected three FM-9 timers that couldn’t be programmed and that wouldn’t initialize or run an ESC; they were found to be running the internal flight program correctly except that the output driver on the signal pin had been damaged so that the voltages were too low (well below logic levels). It is unclear how this occurs and to what extent the ESC is responsible. In any case, I now leave the signal pin in an “input” state for a second after the timer is first connected (and then change it to an output and provide the “throttle off” signal) and also change it to an “input” state some fifteen seconds after the motor is stopped, as an extra precautionary measure.
In sum, then, I believe that the possibility of any of my timers (or probably any others) re-starting the motor at the end of a normal flight is very close to zero. Even if the battery is disconnected and then re-connected, the timer will only put out a “throttle off” signal, at most. (I know of at least one instance in which an intermittent battery connection caused the timer to reset in flight and this stopped the motor prematurely, with no other ill effects.) If the plane “lands” somehow before the flight time is up, it is important to be able to use the timer to stop any continued motor run, to protect the ESC and the motor and bystanders, as has been noted for timers now in use, even though a battery disconnect will also do that.
Will