• I'm currently co-teaching a Technician License HAM Radio class and made some experiments for illustration of subject matter that involve also mc in general and Espruino in particular. This experiment is about resonance of an LC circuit.

    To show how an LC filters out of the 'wave salad' / 'statics' the frequency / band by resonance a signal, I excite an LC circuit, detect a signal and measure its strength as a function of the exciting frequency. I hoped to cover the AM Radio Band - about 500...1600KHz (EDIT: and eventually I got there, post #4). L works with 0..470pF in the AM Radio Band and receives audibly AM stations with Ge diode as simple detector. Since I could (EDIT: initially) only get up to a bit more than 262.144KHz, I increased the tuning capacitor to a 22nF to experience resonances within that range at ~ 37, 52, 88 and 255 KHz, ordered by increasing signal strength..

    Attached shots and clips as follows (clips are large and take noticeable time to download):

    Shot 1:

    • setup with Espruino:
      • PICO
      • IDE built-in graphical testing / monitoring w/ configuration f_vs_smV
      • B7 PWM 50% duty cycle 3.3V exciting L1 over 330R w/ f=32..262[KHz]
      • multi-tap L2 w/ fixed capacitor of set of 1nF..94nF for different resonances
      • detector w/ diode w/ 'bucket' capacitor and load (drain) on L2 hot end
      • A0 reading detected signal strength w/ s=noisy_0..250[mV]
      • digital read pins to detect button presses for operator interface (OI)
      • analog read pin to detect variable slider resistor voltage divide (OI)
      • speaker on PWM pin 7 octavae lower in audio range than exciting frequence
    • frequency control two ways:
      • toggled by pressing red and green button simultaneously:
      • red freq decrease / green freq increase button w/ speed controlled by slider
      • slider only covering frequency range
    • yellow button to produce extra reads after frequency stabilization because general signal read is triggered only on on frequency change and signal strength change by relative value (set threshold)

    Shot 2 and f_vs_signal clip 1:

    • ...262..32...262... frequency sweep shown in @JumJum 's testing / monitor
    • resonances as 'Golden Gate bridge pylons and Bridge Deck hangers'
    • schema in character graphics (in shot only)

    Clip 2:

    • Espruino console w/ frequency [Hz] and signal strength logged (int [mV])
    • Showing resonances on 'signal value bumps'

    Interestingly, most of the code written is to handle the buttons and slider 'UI' and come up with the next frequency to apply. The code to get the excitement going is only a few lines 'tall'. - published at a later point.

    PS: The experiment if for Quality and not Quantity... even though the values - quantities - are pretty close to the calculated values.


    5 Attachments

  • Shots w/ detector detail and enlarged graph (Thank you, @JumJum !).


    2 Attachments

    • setup_detector_view.jpg
    • f_vs_signal.jpg
  • Wow, that's fantastic!

    That's great! I'd never thought of using it for frequency sweeps while also reading back the result!

  • With a do-over - don't know where I went astray before - I was able to get easily up to 2MHz, see shot on my very old / out of calibration 20MHz Hitachi Oszi. I even can get up to 8MHz - not smoothly, but in the end with jumps. So I will shoot some new graphs with 500KHz...2MHz MW AM Radio Band - same L, but band fitting variable capacitor for tuning. Stay tuned.


    1 Attachment

    • Oszi2MHz_picoPinB7.jpg
  • Great! Yes, the PWM uses a counter with a prescaler, so as you approach the maximum frequency the value in the counter/prescaler reaches zero, which means far less accuracy :(

  • ...but good enough for my purposes.

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Espruino controlling LC resonance experiment in HAM Radio class

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