Connect with us

Variable inductor instead of varible capacitor in FM crystal set

Discussion in 'General Electronics Discussion' started by dietermoreno, Apr 30, 2013.

Scroll to continue with content
  1. dietermoreno

    dietermoreno

    238
    0
    Dec 30, 2012
    Variable inductor instead of varible capacitor in FM crystal set.

    So I don't have a varicap, so I wondered if I could modify my circuit to use a variable inductor instead and use a fixed capacitor. Variable inductors are extremely easy to build, but variable capacitors are hard to build (especially at a small enough size for VHF).

    I thought if I make the coils smaller I can use more coils so I can have better selectivity with taps on the coils.

    The original schematic uses a varible capacitor and has 4 coils wound around a Sharpie marker and then the coils are removed from the Sharpie and they slightly expand.

    My new schematic uses a fixed capacitor and has 16 coils tightly wound and tightly spaced around a bic pen, which is about half the diameter of a Sharpie.

    Also, if I made the center inductor lead variable as well, the bandwidth could be adjusted for optimal reception, with the most distance apart from coil #8 to coil #1 for the greatest bandwidth, and with the least distance apart from coil #8 to coil #1 for the lowest bandwidth. Decreasing the bandwidth of the slope detection can help to receive farther away stations 15 miles away and increasing the bandwidth of the slope detection will provide the optimal sound quality to receive the local station 5 miles away.

    The power supply of the transistor is not currently shown because it is implied and it is not important to understand using a variable inductor instead of a varible capacitor .

    File attached. Please tell me if it will work or if I should give up and get a varible capacitor .

    Thanks!
     

    Attached Files:

    Last edited by a moderator: Apr 30, 2013
  2. davenn

    davenn Moderator

    13,833
    1,950
    Sep 5, 2009
    Don't call them varicaps, varicaps are a type of diode

    why mess around with something thats not likely to work ?
    just use a variable capacitor

    Dave
     
  3. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    652
    May 8, 2012
    That won't work because Amazon boxes contain high levels of Unobtainium. I suggest a different box.

    Chris
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,496
    2,837
    Jan 21, 2010
    And Q1 looks like a triode valve with a directly heated cathode set up as a rectifier

    gnarly!
     
  5. dietermoreno

    dietermoreno

    238
    0
    Dec 30, 2012
    Well transistor or triode valve, same thing pretty much as far as the circuit schematic is concerned. but it says in fine print that it is a transistor.

    Yes that is intentional that the transistor is set up to rectify to achieve better results than a diode since a diode is a lossy passive device. Also it can be modded to regen receiver.




    Oh, I always thought that "varicap" stood for "variable capacitor".

    Well the simplest crystal set doesn't use a variable capacitor and uses a tapped coil with a fixed capacitor instead, so why not use the identical circuit as AM crystal radio just with an extra inductor lead in the center of the inductor, smaller coil, less coil turns, smaller capacitor, and different type of antenna?




    So are you saying that the box is too large making the antenna too big for VHF? Is 6 inch x 12 inch loop too big for VHF?
     
    Last edited: May 1, 2013
  6. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    652
    May 8, 2012
    I haven't read all 124 posts that dietermoreno has currently accumulated but I'm beginning to get suspicious that they all have one goal... Rattle our chain! Think about it. He invents component names, terms and doesn't have the time to Google a transistor symbol!

    dietermoreno has completed high school and college. He isn't a youngster that needs to be handled with kid gloves.

    Think about it,
    Chris
     
  7. dietermoreno

    dietermoreno

    238
    0
    Dec 30, 2012
    Oh so here it is.

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    Opps. I'm sorry for my idiocy.

    I didn't know it would **** you off that much, when the title of the thread is refering to the tuning, not the rectification.

    I guess electronics types get ****y over such minor details.

    Maybee I will get that fussy when I learn more about electronics.



    No I don't type these posts just to mess with you. I wouldn't intentionally destroy two amplifiers just to mess with you. I really am that dumb. I told you I'm not an engineering major and the physics classes I took were not very useful for electronics considering that the chapter on alternating current was skipped.


    I don't see a way to salvage this thread. I think if I think it will work I should try it and if it doesn't work then I see that it doesn't work and move on to variable capacitor. I think Steve should lock this thread.
     
    Last edited: May 1, 2013
  8. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    652
    May 8, 2012
    OK, I'm going to take your word for it. It's just that reading you're posts is much like watching Mr. Magoo stumbling around aimlessly!

    Are you mechanically proficient? Do you have a workshop? I'm asking because both variable capacitors and variable inductors can be fabricated. In fact I wouldn't be surprised if Google returned hits for home made L & C.

    Are you aware that we have an electronics tutorial section? I highly recommend that you spend some time in that section and start at page 1.

    Chris
     
  9. dietermoreno

    dietermoreno

    238
    0
    Dec 30, 2012
    Oh, I found a hit!

    I think I just wasn't using the right search term.

    I was using "how to make variable capacitor" as the search term, and it didn't have what I had in mind.

    Then I tried using "how to make air variable capacitor" and that one word changed everything.

    It looks no more complicated to build than a variable inductor as long as I can find some good metal for the plates. Aluminum foil has been used to make home made capacitors before, but only for the toliet paper roll capacitors and not for air variable capacitors. I have a toliet paper roll capacitor that I made and then I thought it is too big for VHF and it is probably not selective enough. Aluminum foil can not be used for the plates of the air variable capacitor because a sheet metal that can mechanically support itself is needed.

    I suppose another trip to the second hand store might find me some sheet metal that is the right size for this. The tutorial I was reading said that the sheet metal should be thin enough to cut with any scicors and drill holes in with any power drill. So then I already have all the tools I need (scicours and power drill).

    I would still need to find a metal rod that can fit bolts on it for the stator. Maybe I can find a good sized metal rod piece of junk at the second hand store as well.
     
  10. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    652
    May 8, 2012
    You can make an air variable cap from aluminum or tin can stock. The latter will take solder if needed.

    Chris
     
  11. dietermoreno

    dietermoreno

    238
    0
    Dec 30, 2012
    I like this website:http://myplace.frontier.com/~bwalker1945/CrystalRadio.html

    [​IMG]

    [​IMG]

    He even recorded the sound for you to hear how good it works: http://myplace.frontier.com/~bwalker1945/crystalradio1.mp3
    Sounds as good as a store bought transistor radio! I am used to hearing staticy booming barely intelligible voices on the AM crystal set audio recordings on YouTube.

    This guy who made the website wants to make everything as cheap as possible using commonly available materials found everywhere so that you don't have to buy everything.



    I found a good site for building air variable capacitors: http://www.eham.net/articles/5217

    [​IMG]

    [​IMG]

    [​IMG]



    This is an example YouTube video I am refering to that sounds staticy and has all of the stations mushing together:

    The same that the stations picked up by my guitar were all mushed together and staticy, just his stations received have much greater RF gain from the phenomenon of resonance which increases RF gain at the resonant RF frequency before demodulation, compared to my guitar was just acting as a random wire antenna, rather than a tuning coil (although guitar pickups do pick up RF, but its just an antenna, nothing more nothing less, any stations "tuned" to by selecting more pickups is just controlling the gain received to block out stronger stations since adding more pickups is adding more antennas, in my video; actually I was using the volume pot on the guitar for this type of "tuning" as I found this to be far more effective than changing the pickups selected, as less pickups selected just made it sound quieter):

    Disregard the schematic on the video thumbnail. It is non-sensical now that I have more understanding of radio receiving theory and practice (using a guitar for radio receiving in itself is non-sensical).

    Actually, what I was doing for "tuning", I found that actually it is called a "trim pot", and it helps to block out stronger stations from interfering with weaker stations. The standard crystal sets don't have a trim pot, but if I use an amplified PC speaker with a volume knob to steal the transistor I can also use the volume pot of the amplified PC speaker as the trim pot the same that I was using the volume pot on my guitar.

    Through out that video I was very slowly turning the volume pot on my guitar from max volume to min volume, and at min volume towards the end of the video you can even hear the only classical station in Chicago that is 40 miles away from my house, and its FM! Presumably the different "lengths" of the 2 antennas (2 guitar pickups) was enough to act as slope detection. This was recorded at 3AM on a clear night, which helped things.

    The circuit I showed above on the poor man's electronics site uses a fixed capacitor instead of a trim pot, but I think I can make the circuit better using a pot instead of a fixed cap.





    Actually, I read that modern cheap transistor radios like clock radios don't even use sheet metal for the air variable capacitor blades because sheet metal would be too bulky and too expensive. Instead they use aluminum foil separated by plastic.

    So I think I could avoid having to search for sheet metal if I take some old plastic loyalty cards in my wallet and super glue sheets of aluminum foil on top with no nut spacing in between the plates because now the dielectric material is plastic instead of air. Then I would drill holes through all of the plates together so that the holes are aligned.

    Well also I would need to find two metal poles that can fit inside of holes drilled in plastic cards.

    [​IMG]
    http://solomonsmusic.net/FM_CrystalRadio.html

    Here I counted 8 total plates each about half the diameter of a thumb. So it needs to be that small for VHF, and apparently 8 total plates is a number of plates that works for good selectivity. So 3 loyalty plastic cards from my wallet with 4 same size semi circles that are the diameter of half a thumb and 4 same size rectangles the same length of the diameter of the semi circles should do. 4 plates would be stator plates and 4 plates would be rotor plates.

    I can't wait to build it and connect it to my Amazon box indoor loop antenna and connect the other end of it to be stuck inside my guitar amp hi-Z input jack and listen if I hear any stations.

    Actually, its so small that I will simply use two nails as the metal poles. I just have to be sure to pick nails that aren't rusty so that detection doesn't occur!

    In addition to the holes drilled for the 2 nails, I would also need to drill a hole in the center of the semi circles and at the same location of that hole I should use a hammered nail to open up a tiny semi circle inside of the rectangles the size of the rotor plate nail.

    Actually, since the plates are so small, simply nailing the hole using the nails that are to be the metal poles of the variable capacitor should work.

    I have many small nails that would work great for this, except they are all painted white, so they won't work because the paint would act as an insulator. I also have many small nails that are not painted white, so the unpainted nails should be used for this, except the painted nail should be used for the center nail that makes contact with my finger during the tuning so that body capacitance is not transfered.

    Ready to build it this weekend!

    20 minute loyalty card variable capacitor. This is good.
     
    Last edited: May 2, 2013
  12. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    652
    May 8, 2012
    Dave will be elated that the "G" in your crystal radio designs will be abandoned.

    FYI, a layer of white paint will not decouple your body capacitance from your home made cap. I'm referring to your white nails of course.

    Chris
     
  13. Raven Luni

    Raven Luni

    798
    8
    Oct 15, 2011
    :eek::eek::eek: .....not..........logical.........*brain explodes*
     
  14. dietermoreno

    dietermoreno

    238
    0
    Dec 30, 2012
    I was too excited to wait until the weekend, so I started building it this morning before school.

    So far I have all of the plates cut out from the plastic cards and I have completed nailing holes through the first rotor plate and through the first stator plate.

    I am using the semi circular plates as the stator plates and using the rectangular plates as the rotor plates. The rectangular plates pivot and are the adjustable ones. One of the nails on the bottom only secures the stators and the rotors can rotate freely out of that nail hole. Also, pulling the tuning nail in and out selects more or less rotors to be pivoted. I did not super glue on the aluminum foil yet; that is the last step.

    Its probably better for you to see pictures off it to see the geometry than for me to explain it. Images attached.

    Now I have to go to school now so that is all for today.
     

    Attached Files:

    Last edited: May 2, 2013
  15. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    652
    May 8, 2012
    Keep in mind that the aluminum foil on each plate must make physical contact with the nails.

    Chris
     
  16. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,496
    2,837
    Jan 21, 2010
    For low power use, it may be easier to build something that is created out of aluminium foil stuck between 2 layers of adhesive tape.

    By having 2 interleaved stacks of these and varying the amount they are interleaved, you'll get a variable capacitor. Because the plates are insulated, you free yourself from the main issue with the design above; that is, to keep the plates from shorting out.
     
  17. dietermoreno

    dietermoreno

    238
    0
    Dec 30, 2012
    I will use tiny machine washers on the nails in between each plate to make the contact from nails to aluminum foil.



    Yeah, but adhesive tape wouldn't provide mechanical support.

    The plastic loyalty cards are more for mechanical support than for dielectric properties. If all I needed was diaelectric properties I would have saved time and just used so paper.

    Oh, and the plates won't short because I would only be puting aluminum foil on one side of each plate so that the aluminum foil side of a plate is always touching the plastic side of a plate. I already thought of shorting and thus already thought that that is what I will do.
     
    Last edited: May 3, 2013
  18. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    652
    May 8, 2012
    That's very good thinking. ;)

    Chris
     
  19. dietermoreno

    dietermoreno

    238
    0
    Dec 30, 2012
    Would this be a good way to test quickly if the variable capacitor is working once I have finished building it before building the real circuit? (image attached)
     

    Attached Files:

  20. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    652
    May 8, 2012
    Do you own a DMM? Most have a Capacitance function.

    Chris
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-