Simple Switched Servo Driver

Posted on Feb 5, 2014

This design was created to overcome a problem I was having with the tailwheel doors of the P-51 Mustang. The problem was that the undercarriage sequence is complicated and would have required 2 independent sequencers. The closing sequence is: Main gear doors open, Tailwheel retracts, tailwheel doors close, main gear retracts, main gear doors clos

Simple Switched Servo Driver
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e. The complication is the main gear doors close again after the main gear extends, but the tailwheel doors remain open. Additionally the tailwheel takes 2 seconds to close (electronic servo slow) and the pneumatics for the main gear is restricted to slow their operation. This design simply drives a servo to close the tailwheel doors once the tailwheel has retracted. It runs from the servo battery supply, but has no signal connection to the RX. When the tailwheel is almost completely retracted the wheel (or leg) closes a micro-switch. This circuit causes the door servo to move to the other end of its travel, closing the doors. almost as soon as the tailwheel starts to move the micro-switch is released and the doors open (almost instantly). The signal that the RX sends to the servos (in most systems) is pulses between 1 ms and 2ms long at a rate of 50 Hz. This design produces pulses to drive the servo directly. The schematic and Vero Board layout are (click for full size): The circuit is quite simple to construct, having few components and no particular handling problems with the components. Firstly cut a piece of Vero board the correct size and make the cuts in the tracks as shown. Insert the wire links and solder in place. I normally use PCB pins for the external connections (much easier to work with later) and if used insert and solder these. Fit the resistors to the board and solder. The NE555 is fitted in a socket and...

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