RC receiver hosts two independent switches
Tom Howell, US Army ARDEC, Picatinny Arsenal, Dover, NJ -- EDN, June 6, 1996
Typical inexpensive radio-control (RC) systems have servo-motor outputs, but no on/off switch outputs. You can add two independent RC switches to a system without making any transmitter or receiver modifications. The switch circuit of Figure 1 simply plugs in to two unused channel sockets of a commercial RC receiver in place of the servos. You operate each switch by pushing its corresponding transmitter-control stick to one end to switch it on and to the other end to switch it off. The position where the switch activates is adjustable over the full-stick travel by varying potentiometer R1 for switch 1 and R2 for switch 2.
You can use output transistors Q1 and Q2 to power lamps or latching relays, for example, from a separate power supply. You could also add optical couplers for high-voltage isolation. You can readily obtain the 4.8V required for circuit operation from the receiver's servo-output connector. If the receiver has a higher output voltage, simply add the resistor and zener diode denoted by the dashed line. Each channel of the circuit operates as follows (refer to Figure 2):
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The receiver's decoded 1- to 2-msec output connects to the inputs of IC1 and IC2. The retriggerable monostable IC1 generates a reference-clock pulse, whose width is a function of the R1C1 or R2C2 time constant. With the time constant set at 1.75 msec, advancing the transmitter's control stick from neutral (a 1.5-msec pulse-width output) to the 2-msec extreme limit causes the D output of flip-flop IC2 to assume logic one on the rising edge of the reference-clock pulse. The flip-flop's Q output goes high, and the output transistor turns on. IC2 holds this output until the arrival of the next rising edge of the reference clock.
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At this time (if the stick is still advanced), the D input of IC2 is again at logic one, its Q output is high, and the transistor remains on. If, however, the stick had returned to neutral, IC2's D output would be at logic zero, its Q output would go low, and the transistor would turn off. Use the Q output of IC2 instead of the Q output if you need a normally on switch at the transmitter stick's neutral position. You could also use the Q output if the desired extreme limit of the transmitter stick yields a 1-msec instead of 2-msec output and a servo-reversing switch is not available on the transmitter. (DI #1882)
