{"id":1904,"date":"2017-01-19T15:46:24","date_gmt":"2017-01-19T18:46:24","guid":{"rendered":"http:\/\/blog.generaleccentric.net\/?p=1904"},"modified":"2017-01-20T11:54:00","modified_gmt":"2017-01-20T14:54:00","slug":"experiments-in-servo-actuated-radio-frequency-displays","status":"publish","type":"post","link":"http:\/\/blog.generaleccentric.net\/?p=1904","title":{"rendered":"Experiments in Servo-actuated Radio Frequency Displays"},"content":{"rendered":"

Over the past year I’ve been playing\u00a0with radios. I started by\u00a0building oscillators, then\u00a0amplifiers, and then radios.<\/p>\n

Regens<\/h2>\n
<\/a>
WBR<\/figcaption><\/figure>\n

Radios come in all different shapes and sizes, and this\u00a0also holds true for the circuits inside the radio. One of the earliest radio receiver techniques\u00a0is called \u201cregeneration\u201d.<\/a>\u00a0It was\u00a0devised by Edwin Armstrong<\/a> in 1912 when he was still a student at Columbia. This method routes part of the received\u00a0signal that emerges from an amplifying device back to the input to increase the gain of the circuit. The design was common up to the 1930’s when it was superseded by other more sophisticated methods. However, regen radios have lately had a bit of a comeback with the “internet of things”; your garage door opener receiver is likely a regenerative receiver, for example.<\/p>\n

So I’ve been building simple regen radios for various shortwave bands. Some work, some don’t. The latest one is called a \u201cWBR\u201d (Wheatstone Bridge Regen). This one works. I used designs from N1BYT<\/a>\u00a0(PDF file) and AA7EE<\/a>. This particular design is tuned by varying the\u00a0voltage from a ten-turn potentiometer. My radio tunes from around 9.4 to 10 MHz.<\/p>\n

<\/a>
Aliexpress 10-turn potentiometer with a reduction gear<\/figcaption><\/figure>\n

The 10-turn pot\u00a0results in smooth, very fine\u00a0tuning, but it’s difficult to know what frequency\u00a0in this range I’m tuned to. Many older radios have an\u00a0elaborate system of pulleys and strings that link the rotation of the tuning dial to an indicator. It is possible to buy a 10-turn pot with a reduction gear. They aren’t expensive, but the indicator goes from 0-9 or some arbitrary set of numbers, not the actual tuned frequencies.<\/p>\n

<\/a>
Micro-servo<\/figcaption><\/figure>\n

I’ve decided to see if I can\u00a0use a small servo motor (one that are used in radio controlled helicopters, planes and drones) to turn an indicator dial that accurately reflects the tuned frequency.<\/p>\n

Perceived and\/or Assumed Drawbacks<\/h2>\n

I did a cursory search online to see if other experimenters had tried this, and I found nothing. Am I such a divergent thinker that nobody else has thought of doing this? I certainly don’t think so. It’s just that nobody has thought to document their discoveries.<\/p>\n

At any rate, I’m assuming that there are several reasons why this technique hasn’t been adopted:<\/p>\n

    \n
  1. Servo noise may interfere with reception. An increasingly popular method of determining tuned frequency is to add a digital display. Some digital hash may leak into the radio circuits from the display’s digital pulses, but this\u00a0doesn’t stop experimenters and manufacturers from\u00a0employing this method in their radios by carefully shielding the digital section from the analog. However servo noise \u2014 that is, noise generated by the motor \u2014 may be louder than the digital noise that’s produced by the microcontroller and associated digital components, and this may drown out weak radio signals.<\/li>\n
  2. The servo motor is power-hungry, making it unsuitable for battery operation.<\/li>\n
  3. It may be difficult to consistently align the dial\u00a0markings to tuning.<\/li>\n
  4. Other unknown unknowns.<\/li>\n<\/ol>\n

    Follies<\/h2>\n

    My first thought was to control the servo using an Arduino. A quick sketch of the display circuit indicated that I only needed one analog input and one digital PWM output. I was able to breadboard a circuit and loaded in the “Knob” sketch that comes as an example with the Arduino IDE. Everything seemed to work well. But rather than use a larger Atmega328PU chip that has 28 pins\u201422 of which would be unconnected\u2014I decided to try\u00a0an ATtiny form factor. I had both an ATtiny45 and an ATtiny85 in the junk box.<\/p>\n

    <\/a>
    ATtiny programmer shield<\/figcaption><\/figure>\n

    I built a programming shield for the ATtinys, and managed to successfully flash the Arduino bootloader into them, but the standard Arduino servo library “Knob” sketch would not work on the ATtiny. I spent a day or so trying other libraries but it seems that there’s not been a lot of interest in overcoming this obstacle.<\/p>\n

    I went back to the junk box and found\u00a0an Arduino Mini Pro. It’s overkill but it’s\u00a0at least small enough to install in the radio, and inexpensive compared to an Arduino UNO. So, onto the breadboard it went: the sketch loaded and things were looking up.<\/p>\n

    <\/a>
    The circuit<\/figcaption><\/figure>\n

    In this radio,\u00a0the analog input comes from the wiper arm of the tuning pot, which goes from around 1 volt\u00a0to 8 volts; the tuning section of the radio uses 8v and the Arduino Mini Pro needs no more than 5v, so I had to build a 5v supply. This signal from the potentiometer\u00a0goes through a voltage divider to reduce\u00a0the maximum voltage to 5v to ensure that the analog input pin is not overloaded.<\/p>\n

    One thing that I found on the breadboarded circuit was that the servo motor would sometimes draw so much current as to reset the microcontroller. This problem was mitigated (somewhat) by adding a low pass filter composed of a small resistance and a large capacitance across the input of the 5v regulator.<\/p>\n

    Building Notes<\/h2>\n

    Time to build it into the radio and play with it\u2026<\/p>\n

    LeBlanc’s Law: Wiring errors are proportional to the density of components on the\u00a0circuit board.<\/p><\/blockquote>\n

    Jan 10<\/b><\/div>\n
    \n