Trying to make simple radio

Discussion in 'General Electronics Chat' started by tobyw, Mar 24, 2013.

  1. tobyw

    Thread Starter Member

    Mar 21, 2013
    37
    4
    Hi

    I have been trying to make a simple radio circuit. I don't think there is a schematic on the web I haven't seen now...but this is the one I am trying to get working now and here is the page explaining it.

    Whatever I do, nothing happens. I don't hear anything at all in my crystal earphone.

    I have tried with a coil I wound myself (100 turns of magnet wire on a ferrite core) and some coils from old radios, plus a coil I bought from Rapid.

    I've also tried with a number of different capacitors, all bought from Rapid. Like the coil I bought from them, the datasheet is completely unfathomable.

    So that leaves me not really sure how to connect the capacitor (one with 3 leads, one with 6) or the coil (mine has two leads, the others all have 4)

    The thing is - I'm not sure what I SHOULD be hearing when it 'works'. Even if there is no station nearby, or the tank circuit is the wrong frequency, should I not still hear at least some static like on a normal radio? At least that would be something. At the moment there's just nothing at all.

    And does anyone by any chance have an image they could show me of how to connect up the capacitor and the coil? It looks like there is two windings, but I don't get what they are..

    Sorry this is such a vague post, but I've read literally 40 different instructions, and watched all sorts of videos...but it never shows the connections clearly enough for me to translate to the components I have...
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
  3. Jaguarjoe

    Active Member

    Apr 7, 2010
    770
    90
    MILTFP41!!

    The article calls for a 64 ohm headset. The crystal earpiece you are using is much higher than that.

    Most small headsets are 32 ohms/earpiece. If you wire the 2 earpieces in series, you'll have 64 ohms.
     
  4. patricktoday

    Member

    Feb 12, 2013
    157
    42
    Are you referring to the variable capacitor? Because the others on the schematic should just have two leads each. Do you have a link to the cap or a part number? If you have a coil with just two leads, that can't be the one in the schematic because it's supposed to be tapped at some point which would require at least a third lead at the tap point. If you have any links or part numbers, please post them.
     
  5. tobyw

    Thread Starter Member

    Mar 21, 2013
    37
    4
    Hi PatrickToday - I have tried two capacitors from Rapid

    Miniature tuning capacitor

    and

    Miniature tuning capacitor

    It's probably my ignorance, but I can't make head nor tail of which way up the drawings are, or how the drawings relate to the actual object, and which pins are which - ant and osc I guess are antenna and oscillator..but which do I connect to where in the schematic? Is the oscillator connection the center tap you mentioned?

    The non home-wound coils I have all have 4 coloured leads. There seems to be a main winding, and then a smaller metal bar with a separate winding on it, 2 leads at the end of each winding. I am guessing this is what's called a secondary winding, but again I struggle to translate the physical object into the schematic.... I will try to post a picture tomorrow.

    So if I use the wrong coil, would that mean I just hear nothing? Or static?

    Thanks for the help
     
  6. Tesla23

    Active Member

    May 10, 2009
    318
    67
    If you do really have a crystal earphone then Jaguarjoe has pointed out a major problem, this circuit will not work with a crystal earphone. The supply current to the transistor passes through the earphone, so a magnetic one is essential (I wouldn't worry too much about the difference between 32 and 64 ohm at this stage). A crystal earphone will appear as an open circuit at DC so block the DC reaching the transistor.

    If you have a multimeter check the resistance of the earpiece, it should be in the tens of ohms. If it appears open circuit then you have a crystal earpiece (or a broken magnetic one).
     
  7. patricktoday

    Member

    Feb 12, 2013
    157
    42
    Looking at page 2 of the capacitor datasheet:
    http://www.rapidonline.com/pdf/12-0255.pdf

    In the top left diagram, it shows the part as if you laid it down on a table with the pins pointing upward to the ceiling and rotated to the bottom side (towards you). The middle pin must be common, the left is called "ANT" and the right is called "OSC." So there are basically two whole separate variable capacitors in this device. (I hope I'm getting this right, I don't know this part at all, just reading the datasheet :) ) If you look at the chart of page 3, it tells you what capacitance you should expect to see on the OSC and ANT when you rotate the dial to different levels.

    So the left and middle pin are one variable cap, the right and middle are the other. You could use either one in a circuit and if you were using, say, the left and middle for your capacitor, it wouldn't matter which direction you hooked up the pins. (Pin 1 to the top and 2 to the bottom or the opposite would equally work.) HOWEVER, the circuit calls for a cap with a size of 0 to 500pF and this one only goes from 0 to 141.6pF.

    It's too small; but to hook it up you'd hook up the middle pin and the pin on the right to the top and the bottom of your antenna coil, and leave the left pin disconnected. If you by chance have a multimeter that reads capacitance, you could measure this.

    I'm really not too sure about the antenna coil. The diagram looks like it is one winding with so many turns on the top and so many turns on the bottom with a center tap -- or a tap more to one end -- but just a single wire coil. Where did you get the specs on the one you made? Perhaps someone else has an idea on this?

    And, unfortunately, I don't know if there should be static or not. I was checking out an antique radio for a friend a few months ago and I was wondering the same thing.
     
    tobyw likes this.
  8. vk6zgo

    Active Member

    Jul 21, 2012
    677
    85
    Yeah,it won't work with the crystal earpiece,for the reasons stated by the previous posters.

    You could try replacing the earpiece with a similar value of resistor to the recommended earpiece,then connect the crystal earpiece
    across the resistor,using a capacitor in series with one leg of the earpiece .-----a fairly big cap,0.1 uF or above.

    You should then hear something,even if it is just a "click" when you apply power.:D

    But wait,there's more! The "OSC" section of the variable cap is intended for use with a Superheterodyne receiver & isn't used in your Simple radio.

    These little plastic caps are pretty hard to interpret,& it also seems that you have too low a value anyway.

    If you can find an old transistor radio which has Medium Frequency AM,it should have the correct value of capacitance.
    You should also be able to see which terminals go to the ferrite rod--these are the ones you should connect to your coil.

    The best variable capacitors for simple radios are those used in old valve (tube) type radios,as it is easy to see the connections,plus they have fairly large shafte which are easier to fit a knob to.

    Use a homemade coil,with a variable tap.
     
    Last edited: Mar 25, 2013
  9. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,979
    744
  10. tobyw

    Thread Starter Member

    Mar 21, 2013
    37
    4
    Thanks for the links. To save money though. is it not possible to add another capacitor in parallel to bring the whole capacitance up to requirement? I know it would reduce the adjustable range.. but it might let me find one station?

    Also - would the smallness of the capacitor mean I just hear absolutely nothing? Or would it not still produce static?
     
  11. tobyw

    Thread Starter Member

    Mar 21, 2013
    37
    4
    Thanks for this information. I didn't really understand about the difference between a crystal earpiece and a magnetic one, so it means I can at least eliminate that from my attempts to get the right components wired up.

    I have tried loads of different circuits with various coils, capacitors, a germanium diode and with radio ICs ( the 7642) BC548 transistors, and op amps. I have hooked up small magnetic speakers, larger magnetic speakers. I never hear anything at all. Do you really have to be this accurate about the components before you would even expect to hear a buzz or some static? It's like the whole circuit is dead, but I was hoping that I would start with interference and noise, and slowly 'tune' the circuit to get a broadcast...but so far, I can't even get a noise.
     
  12. Jaguarjoe

    Active Member

    Apr 7, 2010
    770
    90
    Can you scrap the variable capacitor and use an adjustable loop stick instead? You will only need a fixed cap for the tank. IIRC, when I was in grammar school, I used a "ferri-loopstick". a 220pf cap, and a 2N107 xistor.
     
    tobyw likes this.
  13. debe

    Well-Known Member

    Sep 21, 2010
    946
    184
    Couple of pictures of a basic crystal set using a loopstick aerial & var capacitor from a transistor AM tuner. It does work but a simple radio like this needs a large aerial & earth, unless there is a fairly close radio station.
     
    tobyw likes this.
  14. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,979
    744
    if the capacitance is the wrong value then the tuning frequency will be increased or decreased, lower capacitance is higher in frequency, and Viceversa.

    So if you use a smaller value in capacitance the Frequency will be on the Shortwave band instead of Medium wave,

    you can add extra fixed capacitance, but the amount variable limit will be reduced.


    What frequency scale are you looking to receive?
     
  15. tobyw

    Thread Starter Member

    Mar 21, 2013
    37
    4
    Any frequency would suit me - if I could just hear a load of static it would be better than I have achieved so far.

    I naively thought that if I took a coil, and made a tank circuit with a variable capacitor, connected a diode to an op amp and a speaker to the op amp - then I would at least hear some static (like when you are tuning a radio). I thought I would then be able to experiment with different coil windings and capacitors to see if I could find a station.

    Problem is - I don't hear anything. Completely dead. I'll try again tonight and take a photo of my circuit so maybe someone can tell me what I am doing wrong....

    Thanks :)
     
  16. tobyw

    Thread Starter Member

    Mar 21, 2013
    37
    4

    Thanks very much for these pictures.

    One thing I don't get is why the coil needs a tap - or where it should be?

    Also - can you make an aerial by wrapping some wire around a large circular form to avoid having 20 foot of wire hanging out the window?
     
    Last edited: Mar 25, 2013
  17. debe

    Well-Known Member

    Sep 21, 2010
    946
    184
    With simple recievers you will need a long wire antenna strung betwean 2 poles/or buildings/ building to tree. I live 60kms from nearest station & need about 15meters of wire strung up to recive any sort of signal. The tap is to match the signal to the circuit, if i conect the aerial to the top of the coil the circuit overloads with the strongest station & wont tune. (my first home built crystal set was over 50yrs ago & had same problems you are having, no internet then only a helpfull radio repairer who later become a life long freind & mentor.)
     
  18. takao21203

    Distinguished Member

    Apr 28, 2012
    3,577
    463
    I used weird capacitive tapped antennas to increase the gain.

    You'd for instance use a large wire drum, with a metal core (since it is not an antenna but really a spool of electric wire), and then also run a wire from the metal core, and test it either with earth, or to feed it back into a circuit.

    I used same for FM transmitter, there is a wire which is inserted into the antenna coil, and is feed back into the circuit.

    Otherwise it did not even work.

    Maybe some primitive kind of regeneration.

    Don't know if it works for your circuit, but you can try to earth it, or run with a wire to a secondary artificial earth.

    Main earth should be a water pipe or from live grid earthing.
     
  19. davebee

    Well-Known Member

    Oct 22, 2008
    539
    46
    I've gotten a crystal radio to work that doesn't even have the transistor, but it wouldn't work until it had a good earth ground as well as a long antenna wire. Maybe that would help your circuit.

    This kind of receiver can work with a wide range of parts, but you do need to get the receiving coil-capacitor pair tuned to be resonant at the frequency of a reasonably strong station. That depends on both the capacitance of the variable capacitor and the inductance of the coil.

    Yes, the receiving coil can also serve as the receiving antenna, but it will not receive as much RF energy as an extended long wire. If you have a strong local radio station, it might work fine, but for weaker stations, you will need that long wire.
     
Loading...