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Rand
Rand Dual Pak Elevator Rate Decoder Circuit Description
By Jay Mendoza
December, 2009
The receiver output pulse from an open collector PNP type transistor, is fed to the base of Q1 and Q5 of the Rand rudder actuator servo, and to a 100 ohm resistor which pulls down the base of Q1, turning it on, and holding Q5 off with no signal applied. In this condition the Q1 turns on Q2 and Q3, which energize the actuator motor to run in the direction which gives low throttle; fail safe mode.
When the receiver outputs a pulse to the input of the switcher, a low going pulse causes the actuator to behave as described above. When the pulse goes positive however, Q1 is turned off, along with Q2 and Q3. At the same time Q5 is turned on, it turns on Q4 and Q6 causing the actuator’s motor to spin the opposite direction. Thus the switcher responds to high and low pulses from the receiver by reversing the polarity of the 3.6-volt supply across the motor, which causes it to alternately reverse direction.
The incoming pulse is isolated and buffered by Q1, which doubles to act as an emitter follower to drive the differentiator circuit at the input of the rate decoder. A .47uf and 8.2 k resistor differentiate the pulse. The positive pulse now appears as a short positive going spike at pin 1 of U1A, a two input dual NOR gate RTL type device. The differentiated positive spike triggers the one shot multi vibrator composed of U1A & B and associated components Ct and Rt. When the positive spike appears at pin 1, pin 7 goes low and discharges Ct, making pin 5 go low, and pin 6 goes high. Pin 5 will gradually ramp back up to 3.6 volts as Ct is charged via Rt. The time it takes for Ct to charge controls the time pin 6 remains high and holds pin 2 high, which keeps the output pin 7 low. Once Ct has charged, pin 6 goes low, driving the NOR gate input pin 2 low and switching pin 7 high. At 12 PPS, Ct and Rt are chosen to give a pulse that is about 42 milliseconds in duration. If the next incoming trigger pulse is at a rate of 12 PPS, which is about 82 milliseconds, the pulse width will be about 50/50 on to off ratio at 12PPS.
As the pulse rate increases, the output duration of the one-shot’s pulse remains constant, thus the time after the one-shot has fired becomes shorter, making the positive half of the pulse width decrease with the rate. When the pulse rate goes below 12 PPS, conversely the period after the one shot has fired is longer, which now makes the positive half of the pulse’s width longer. Therefore the incoming pulse triggers a one shot, which in turn outputs a negative pulse of fixed duration. The period of the positive half of the pulse is determined by the rate at which the one shot is triggered. As described above, if the incoming trigger pulse’s rate or interval is double the time of the one shot’s pulse duration, the resulting pulse output from the decoder is on 50% of the time, and off 50% of the time. As the incoming pulse rate changes, the pulse width ratio of the decoder shifts to cause the actuator servo to drive more in one direction than the other.
Ct can be made about 4.7uf and a 5K pot can be used with a 4.7K in series for Rt to make the actuator decoder adjustable for neutral at different neutral pulse rates from about 10 to 16 PPS.
