Remote Motor Controller

Cryptic letters on the right side are codes for Japanese/Chinese characters.

You want to turn the variable capacitor of the antenna tuner near the antenna from the inside of your shack. You might then use a stepping motor attached to the variable capacitor and turn it with a digital controller. I actually did the same with a PIC micro controller before. It worked fine and had no problem, but what can one do with an analog circuit with a DC motor? Oh yes, I like the analog better than the digital circuits. First of all, the motor needs to be equipped with a gear box to cope with the critical tuning of the capacitor.

Then a DPDT switch can do the job as is shown in the next figure, but....



You easily run past the optimum point and have to go back and forth many times until you finally get the antenna tuned. You would be irritated with such an iteration.

Here is a good idea to do better with a remote turning knob attached on the axle of another DC motor (a common toy motor is OK). Remember that a DC motor generates DC current when its axle is turned. The polarity of the current is reversed if the turning direction of the knob is reversed. If you turn the knob the faster, then the higher the generated voltage is. Therefore, It could control the geared motor directly if they are mutually connected as is shown in the following figure.



OK, the generated current is too feeble to turn the other motor. Then, follow the standard practice. A DC amplifier can make it stronger. In the next figure, an NJM2073 from New Japan Radio Co., Ltd. is used for this purpose. It is a dual audio power amplifier IC for BTL output.

This simple circuit works very comfortablly and is best for the tuner of a common wire antenna as well as for a magnetic loop antenna, of which tuning is especially critical.

If this Japanese IC is hard to find out there, then you could also try it with an MC34119P or two LM386's. In those cases, the connection of the controling motor may be somewhat different from the following diagram. I leave you to think it over for your fun. Refer to the data sheet of NJM2073 for the hint.




MINOWA, Makoto
Last revised 2002-07-06.