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Simple tune motor driver for "Mag"loop antena.
pa0nhc 20150216

Click-right on a picture to download it.

This simple design is a variant of elsewhere published. It can power a little 12V/0,5A DC motor, for turning a air tuning capacitor. That motor must be able to turn forwards and backwards.

The system generates a square wave signal with constant amplitude, but changing duty cycle. As a result the motor torque is relative high at low RPM. C3 can be changed for another frequency.

With S3 or S4 the rotation direction of the motor is chosen, and the motor switched-on. It are tumbler switches which return automatically in the neutral position when not pushed. The motor is then short circuited, in order to stop it immediately. LED 1 or LED2 indicate the rotation direction.

If S3 and S4 are both activated, the motor is short circuited too.

REM: When a tuning capacitor is used which has end positions, end switches should interrupt a motor line there.

T1 can be any PNP TO220 or SOT93 power/switching transistor. Extra cooling is not needed, as the transistor always is driven in saturation.


This double sided PCB can also single sided be used, after soldering five wire bridges (green). The switches and components are on the top side, and soldered at the bottom side.

On a double sided PCB: BR, C9, IC1, LED2, D7 and the switches must be soldered on TOP AND BOTTOM planes. Some of their connections serve as "vias".

After soldering the connecting wires, the PCB can be screwed onto the inside of the lid of a small ABS box, using the switch ring nuts for fastening.

Tr1 and Tr2 must be connected with a 12V~ / 6VA mains transformer.
Rel1 and Rel2 must be connected with a band switch relay at the antenna.
M1 and M2 must be connected to the tune motor.


PCB mask (1200dpi).

Printing the B/W PCB mask on inkjet transparency :
Centered on A5, landscape, size 100%, COLOR, highest resolution / quality, photo, max. contrast, max density.

REM: the motor connections msut be isolated from mass etc.
Connect the mass of the PCB to safety earth (static charges on the antenna.


Drill template / top silk 600DPI.
Printing the drill template : size 100%.


To prevent HF signals on the drive cable to cause problems, put ferrite clamps on the drive wires inside the box, and at the motor. Connect a capacitor 0.15uF/100V across the motor connections.