Throttle Quadrant with Electric Trim
In some airplanes, the trim wheel can be either rotated manually, or via up/down trim switches on the flight yoke. Since I'm flying many different planes, I have made a modification to the original Throttle Quadrant trim wheel:

I have added a geared DC motor to the trim wheel to rotate it forward or backward, controlled by the trim switches on the yoke. Since it needs to be possible to rotate it by hand as well, an electric clutch has been added to decouple the trim wheel from the DC motor drive when the motor is not activated.

The electrical circuit is very simple: The electric clutch magnetizing coil has no polarity, so both DC motor and electric clutch coil are connected in parallel (both are 24VDC).  The motor and coil are connected in H-bridge configuration. Below diagram shows the circuit, which I build from scrap components. The transistors are 60V/4A darlington types, so require little driving current. 

The elevator trim-up and trim down switches control the H-bridge: When both switches are off, both emitter followers Q1 and  Q2 are pulled high. There is no voltage delivered to the motor and coil. 
When Trim-up switch S1 is closed, transitor Q3 will pull down the base of Q1. D1 will become forward biased, and ensures that Q1 will be off when Q3 pulls the left bridge low. One side of the motor & clutch coil is pulled to ground, while the other side is still pulled high, and the motor turns in one direction, and the clutch coil is energized. When S1 is released, Q1 will pull high again, and the motor stops. Closing S2 will pull low the other side of the bridge, and the motor runs in the other direction. If both switches are pressed, both bridge halves go low, and the motor / clutch will not be activated. Diodes D3 ~ D6 need to be added to avoid voltage spikes across the transistors due to inductive load. The power supply of the circuit is taken from a standard 24V transformer with diode bridge and smoothing capacitors.  

Electric trim circuit, build on small perf board. The transistors do not get hot, so no heatsink is needed. 

For the mechanical solution, I have made use of junk components from old copy machine.

One of the key-parts is the electric clutch: I made this from a spring clutch: I removed the spring and glued the soft-iron cylinder on the gear wheel.

The clutch has two gear wheels: the main gear drives the trim rotary encoder which sends trim-up/down commands to FS. The main gear is fixed to the inner shaft, brass bearing and trim wheel. The clutch gear is only fixed to the iron cylinder, and is driven by the DC motor.

In un-powered state, the clutch gear will freely rotate, it will have no mechanical connection to the inner shaft. When the clutch coil is powered, the magnetic field will make the iron cylinder stick to the iron ring on the brass bearing. Now all gears are fixed together mechanically (with sufficient torque), and when the DC motor drives the clutch gear, the inner shaft, trim wheel and main gear will rotate together, and the trim rotary will send out continuous pulses to FS. The rotation direction of the DC motor determines trim-up or trim down action.

The clutch assembly is fixed to the Throttle Quadrant side board. An extra bracket with brass bearing supports the inner shaft.
The DC motor is a small 24V/90 RPM motor, with a 30 teeth gear wheel. It drives the 45 teeth clutch gear. The main gear has 60 teeth, and drives the 30 teeth rotary encoder. When the motor is active, it rotates the trim wheel with about 67 RPM. The trim rotary will rotate with about 134 rpm, about 2.2 rotations per second. with 12 pulses per rotation, you get about 27 pulses per second for trim-up / trim down.  Since both motor and clutch are activated together, there is no backlash: as soon as the drive is disabled. the trim wheel stops. In this way, trim can be adjusted accurately.


Google Sketchup file of the throttle quadrant with electric trim can be downloaded here. In this design I have made use of layers and groups, so you can check out each separate part individually.

A video of the electric trim action can be seen here.

Note: I noticed that Firefox will not always play the file correctly. If this happens, please download the file to your PC first and play from there.