Have some fun tuning in radio stations with your RTL Software Defined Radio setup by using a real, physical tuning knob! You can make one with just a potentiometer and a Gemma M0, plus some screws to connect the two.
The Gemma M0 can act like a USB HID keyboard, so it’s great for sending key commands. Code it in CircuitPython to send keyboard shortcuts to CubicSDR software, or customize it for other uses!
Additionally, you'll need two M3x8mm screws and nuts to secure the potentiometer to the Gemma M0. (Alternatively, you can use alligator clip leads if you have some handy.)
Build the Tuner Knob
With some careful bending of the legs, you can get the the potentiometer to connect to three pads on the Gemma M0. You feed the center leg into pad A2 which will measure the analog voltage of the potentiometer, and then you’ll bend the outer legs to contact pins D2 and 3V.
In software we’ll set pin D2 to a ground level, so the potentiometer will be able to reference that ground and the 3V as the wiper turns and changes the resistance value output to the center leg.
First, bend the outer legs on an angle to widen their reach as shown.
Bend the center leg down just a bit so it will be able to dive down into A2.
Then, use the screws and nuts to secure the outer legs to their respective pads — the center leg will contact the A2 pad securely without the need for a screw.
Next, you can add the knob -- try turning the shaft fully to the left first and putting the knob on at the six o'clock position as shown.
Now, plug the USB cable into the Gemma M0 and your computer, and get ready to program it!
Code It
Follow this guide https://learn.adafruit.com/adafruit-gemma-m0/circuitpython to get started with coding the Gemma M0 in CircuitPython. Install the latest release version of CircuitPython on the board.
Also make sure you install the latest release version of the adafruit_hid library as shown here. Note: there will already be a previous version of the adafruit_hid library in the lib folder on your Gemma M0, but it may not be the version that goes with the latest release of CircuitPython that you just installed. So go ahead and replace the old library with the new! You can do this by downloading the latest release bundle, unzipping it, and then dragging just the adafruit_hid library to the Gemma M0’s CIRCUITPY/lib directory.
You may also want to install the Mu editor https://learn.adafruit.com/adafruit-gemma-m0/installing-mu-editor for your coding needs.
Once you can successfully code in Mu and upload to the board, return here.
The Gemma M0 Radio Tuning Knob software will read the voltage on pin A2. When the potentiometer it is at the midway point, the on-board LED will light up to indicate the center detent. In this position, no key commands will be sent.
In CubicSDR, pressing the bracket keys in the tuning window nudges the tuned frequency up or down, depending on which bracket is pressed.
Turn to the right and the Gemma M0 will send ‘]’ right bracket keys — the farther to the right you turn the knob the faster it will send them. Turning to the left does the same, but with the ‘[‘ left bracket.
Here’s the code for the Radio Tuning Knob. Go ahead and copy it, paste it into Mu, and then save it as code.py to your Gemma M0.
# SPDX-FileCopyrightText: 2018 John Edgar Park for Adafruit Industries
#
# SPDX-License-Identifier: MIT
# Gemma Radio Tuning Knob
# for fine tuning Software Defined Radio CubicSDR software
# 10k pot hooked to 3v, A2, and D2 acting as GND
import time
import board
import usb_hid
from adafruit_hid.keyboard import Keyboard
from adafruit_hid.keycode import Keycode
from analogio import AnalogIn
from digitalio import DigitalInOut, Direction
d2_ground = DigitalInOut(board.D2)
d2_ground.direction = Direction.OUTPUT
d2_ground.value = False
analog2in = AnalogIn(board.A2)
led = DigitalInOut(board.D13)
led.direction = Direction.OUTPUT
pot_max = 3.29
pot_min = 0.00
step = (pot_max - pot_min) / 10.0
last_knob = 0
def steps(x):
return round((x - pot_min) / step)
def getVoltage(pin):
return (pin.value * 3.3) / 65536
def spamKey(code):
knobkeys = [Keycode.RIGHT_BRACKET, Keycode.RIGHT_BRACKET,
Keycode.RIGHT_BRACKET, Keycode.RIGHT_BRACKET,
Keycode.RIGHT_BRACKET, Keycode.SPACE,
Keycode.LEFT_BRACKET, Keycode.LEFT_BRACKET,
Keycode.LEFT_BRACKET, Keycode.LEFT_BRACKET,
Keycode.LEFT_BRACKET]
spamRate = [0.01, 0.05, 0.125, 0.25, 0.5,
0.5, 0.5, 0.25, 0.125, 0.05, 0.01]
kbd = Keyboard(usb_hid.devices)
kbd.press(knobkeys[code]) # which keycode is entered
kbd.release_all()
time.sleep(spamRate[code]) # how fast the key is spammed
while True:
knob = (getVoltage(analog2in))
if steps(knob) == 5: # the center position is active
led.value = True
elif steps(knob) != 5:
led.value = False
spamKey(steps(knob))
Page last edited January 22, 2025
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