Simple RC Signal Input
Have you ever thought of how to convert an RC signal to something useful? Its not a terribly easy signal to deal with, as it is a square wave pulse that varies between 1mS and 2mS every 20 mS. Also, most radio receiver outputs are true TTL, which means that they have a fairly large resistor that pulls the output up. This means very little drive current.
The most basic of systems follows.
If the RC output signal from a receiver is used to charge a capacitor for the time of the signal pulse, and then a larger resistor discharges the capacitor whilst there is no signal pulse, then we get a saw tooth waveform like that in Fig 2 & 3.
The circuit that produces this waveform is shown in Fig 1. The capacitor C1 is charged by the signal pulse through R1 and D1 when the signal is high, and discharged by R2 when the signal is low. D1 prevents the receiver from discharging the capacitor during this period.
With this combination, a short pulse (1mS) produces the waveform at the end of the time on Fig. 2. A long pulse from the receiver (2 mS) produces the waveform at the end of Fig 3. It can be seen from comparing the 2 waveforms, that there is a small voltage range between the 2 waveforms i.e. the highest voltage in Fig 2 is ~ 1.086V, and the lowest voltage (after the signal has settled) is ~1.093V. In practice this gap is larger, but can be increased by increasing the size of C1, or the relative sizes of R1 and R2.
If we now add a comparator, with the negative input adjusted to a value between the 2 waveforms, then the output at A Fig 4 will be :-
- Zero when signal is 1 mS, because positive input will always be below the negative input.
- V2 (5V) when signal is 2 mS, because positive input will always be above the negative input.
- A varying pulse width modulation (PWM) at all points in between, with the frequency of modulation being the same as the input signal, usually 50 Hz.
The output can then be used to drive mosfets to produce a soft start switch.
Note: - This is a low frequency PWM, and is not suitable for driving a motor output at variable speeds. Low frequency PWM causes heat build up in motors and batteries, damages gearing and burns commutators if used excessively. It is however a very simple way of turning a motor on and off and is quite suitable for use in a model aircraft.
Some years ago I incorporated this exact circuit into such a device, which I called the RC Air Switch (not very original ehh?) This device includes a BEC, low voltage cut out and safety on, to prevent motor fire up when the device is first turned on, regardless of the throttle position.
Author
Ian Armstrong