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Simple motor driver for tuning a Magnetic loop antenna.
Redesigned and tested by pa0nhc 20180405
Under construction.
PCBs will become available.

(Copyright) The use, copy and modification of all info on this site is only permitted for non-commercial purposes and thereby explicitly mentioning my radio amateur call sign "PA0NHC" as the original writer / designer / photographer /publisher.

This manually operated little box drives a small 12V / 0.5Adc motor and loop tuning capacitor.
The tune motor must be able to run forwards and backwards by changing the polarity of its supply voltages
If end stops will be used, they should change the motor polarity and thereby its rotation direction.


       Small and very useful.


New PCB.


Downloads :


New schematic

This system is digital, but does not generate radio noises. The LM/SE/NE555 multi vibrator circuit is designed according to data at the manufacturers data sheet. By changing the duty-cycle of the square wave output, ON-ON switch S1 sets the tuning motor speed to low or high. The low motor speed can be adjusted with P1.

PNP power switching transistor Q1 (BD912) delivers the needed output current. It is driven into full saturation, hence its power dissipation is very low, abt. 1/2(2.5 A  x 1 V) = 1.25W max. It needs therefore no cooling. If a pure DC current should drive the motor, in very slow motion the motor could need for instance 2.41A @ 1.5 V. The dissipation in Q1 would then be abt. ((12.5 - 1.5)V x 2.41A) = 26.5 W (!). Together with the  mains transformer and the rectifier, Q1 should have to dissipate a lot of heath. Q1 should need a big cooling profile, the mains transformer should be 30VA, and the cabinet should therefore be bigger and more expensive.

In this version, a 12V 10VA transformer works well. Cheaper and cooler. Even bridge rectifier Br is a type with low voltage drop and low power loss. As the transformer still warms up, even when not loaded, i suggest drilling a few 8mm ventilation holes into the right hand side and the front side of the  cabinet. Check that from the outside of the cabinet no 230Vac contacts can be toughed.

The drawback of driving a motor with a square wave voltage is, that the max. output duty cycle is abt. 45%. The motor speed is therefore lower than feeding it with pure 12Vdc power.
BUT : If the motor should be connected directly to 12Vdc, its takes 2,2A, and needs a 30VA transformer and a 3A bridge rectifier.

Harmonics of the square wave output could generate radio interference. The output voltage is therefore effectively filtered with a low pass PI filter, consisting of 3 turns into a small Hi-u (Ui=10.000) ferrite core FR2, and C6 / C7. As an extra, directly behind the DIN output plug, the screened multi wire output cable is wound 3 times through a big hi-u ferrite core.

In the mains input, the three mains cord wires are also wound three times through a big hi-u ferrite ring core. In practice, no noise etc. is heard with my 10m circumference loop antenna in a city environment with S7 background noise. But, even if some noise should be noticed when S5 is "on", S5 in "off" position kills all electronics and possible noises. The +12V power still remains available for the anti-condensation heather and band switch relay in antenna tune box.

The motor only runs as long as switch S3 or S4 is pushed. These switches define the square wave voltage polarity presented to the motor, hence its rotation direction. When not pushed, both switches return to their default position, both motor lines will be grounded, and the motor lines cannot radiate noises. And : the short-circuiting of the motor will act like an effective motor brake. The motor stops immediately, without overshooting the reached tuning position.

With ON-ON switch S2, an external relay can be activated via output bus pin4, for instance for band switching. On pin2 is always +12V present for a 5W anti-condensation heather in the tune box. A 230Vac mains on-off switch  is not implemented, as the anti-condensation heather should 24/7 be powered. 

REM: To keep the heather in the antenna tune cabinet powered, the mains cable should be plugged into a wall-outlet, which is always powered. 

        MOTOR LINES : 
Depending on the rotation direction of the motor, each motor line is grounded or contains a positive voltage.
Both motor lines between tuner and antenna MUST therefore be floating, in other words : be fully insulated from mass etc.

            Construction.
If a band switch is not needed, then omit R7, LED1, S2 and their holes.

Drill all holes into the cabinet. Use the drill template for accurate switch and LED positioning.
Then solder all components to the PCB, except the switches, and the LEDs
Put the switches and the LEDs loosely into their holes in the PCB. Do not solder yet.
>>> Check the correct position of the LEDs, their flat side on the rim must coincide with that on the PCB.
>>> Be sure, with S3 and S4 in their default position, both do connect motor outputs M1 and M2 to ground. Measure it.
Put the cabinet lid over the switches. The switch positions are now correctly spaced.
Solder the PCB switch connections. All switch bodies should have the same height. 
Adjust the switch nuts for correct distance between PCB and cabinet lid and fasten them.
Press the LEDs into their holes in the lid. Their rim should be flush with the inside of the lid.
Solder and cut the LED wires.

            Output.
All wires should have ample length, in order to be able to lay the lid upside-down near the open box.

Solder C7 directly onto its pins on the DIN bus.
Solder all (different colored thin) wires onto the DIN bus.
Screw the DIN bus into the cabinet.
Twist the wires.
Wind the wire bundle 3 times through the 6mm hole of Hi-Ui core FR2.
Cut the wires to length.
Solder the wires into the PCB.

            Power.
Solder two wires at the fuse holder for connection to the PCB and transformer Tr.
Push the fuse holder into the cabinet.
Glue the transformer upside-down onto the bottom of the cabinet. Use fast hardening two component glue. Wait for hardening of the glue (see manufacturers info).
Solder one transformer 12Vac output pin to PCB "Tr1".
Solder one fuse wire to PCB "Tr2".
Solder the other fuse wire to the still free transformer 12Vac output connection.

Screw the strain relief into the cabinet.
Strip enough outside insulation from the mains cord, to be able to wind the three remaining insulated mains voltage wires several times through the hole of ring core FR1.
Push the mains cord through the strain relief and fasten the cord.
Wind the three wires at least 2 times through the 8.5mm hole of FR1.
Cut the wires at length, and (stated colors below are valid for Europe / The Netherlands)  :
    solder the yellow/green safety ground wire to PCB "Rel-".
    solder the brown 230Vac wire to the one transformer 230Vac input pin.
    solder the blue 230Vac wire to the other still free transformer 230Vac input pin.

Stick thin felt pads onto the bottom outside near the corners.