Simplest Radio Receiver help

Discussion in 'Wireless & RF Design' started by BLOBY, Apr 10, 2011.

  1. BLOBY

    Thread Starter Member

    Mar 10, 2011
    37
    1
    Hi guys,

    Im trying to build a simple radio receiver circuit (in a breadboard!) which would light up a small bulb or something.. There are few questions about it which I tried to explain in parts...:D

    Please see my attachmeant below...(I know transistors are not well biased.. but it would work isnt it? the beta value of both of my transistors is between 140-160)

    Part 1:
    What happened was when i touched the base part of the transistor Q1, the multimeter read about 7.6 volts.. before i touched it, the reading remained at zero volts... I suspected that this happened due to my body itself acting as an antenna and picking up signals from air and this amplified and showed the reading.. plz tell me if my thinking is correct..

    Part 2:
    Assuming that my part 1 assumption is correct, I would like to know the designing of a filter or something so that only particular frequency passes.. Plz guide me on this too.. See my attachment number 2 which is my idea of the filter (bandpass filter)..
    Actually.. I even tried this in my circuit and the problem was the reading in the multimeter was still around 7.6 after some fluctuations.. but it stayed there at 7.6... :confused:... Is my filter working? or is the inductor acting like an antenna or something?


    Plz tell me if any info provided by me is inadequate... Thanks in advance...
     
  2. BLOBY

    Thread Starter Member

    Mar 10, 2011
    37
    1
    My circuit is basically a 2 stage amplifier.. I really wanna get my doubts cleared.. Plz help...
     
  3. ckrounch

    New Member

    Apr 26, 2010
    1
    0
    Hi Bloby,

    You're certainly right. But chances are you're only picking up 50Hz or 60Hz (depends in which country you live) low frequency electric field. Especially if you're in a building. The voltage you get at these frequencies are high enough to force your transistors to act as rectifiers, which changes the voltage you read on your voltmeter.

    Some good idea here. The 100Ω resistor is useless though. But what is the frequency you want to receive ? For frequencies in the MW LW range, your antenna must be... tens or hundreds meters high ! You'd better consider using a ferrite antenna in place of your inductance.

    And I doubt you will be able to receive anything except very strong emitters with this schematics. Also, consider having your transistors well biased. Try simulating this circuit with a simulator. QUCS is free and quite easy to work with.

    Oh, and, by the way, I don't understand very well what you're actually trying to do. Just lit a DEL when there's a radio station nearby ?

    Charles.
     
  4. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Are you using a protoboard, something like this?

    [​IMG]

    If you are it is the problem. Breadboarding RF requires some special techniques.
     
  5. bertus

    Administrator

    Apr 5, 2008
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  6. BLOBY

    Thread Starter Member

    Mar 10, 2011
    37
    1
    Thanks for the replies..

    I just glanced through the manhatten style prototyping and I think it is better than the breadboard I am using..

    Coming to my receiver part,
    When I placed a 1 ft wire near the base of Q1( the place where i touched earlier) the reading of multimeter was around 4.5 volts and it fluctuated as I twisted and bended my so called antenna of 1 ft wire..
    To be frank.. I am trying to learn more about radio waves and stuff by doing this project.. My initial objective is this...

    If the receiver is set up with a proper filter somehow, the initial multimeter reading should be 0 volts (actually it is 0v in my breadboard circuit if i dont touch it) and this part is interesting... When I create a "spark" by quickly turning the mains switch on and off, the reading should increase.. I have read that the spark produced thus will emit radio waves of almost all frequencies.. so, the purpose of filter is to distill out all other frequencies which might interfere.. The receiver should be at around 4 metres from the switch i mess around with... So plz guide me on the improvements i could do to this circuit.... and also plz mention the disadvantages of using a breadboard on a project like this... :)
     
  7. bertus

    Administrator

    Apr 5, 2008
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  8. beenthere

    Retired Moderator

    Apr 20, 2004
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  9. BLOBY

    Thread Starter Member

    Mar 10, 2011
    37
    1
    Ok.. to make things clear, my receiver is not FM or AM but just a detector of em waves which is produced by spark from the mains switch... detector means that multimeter connected should show some high reading like 3 -4 volts ONLY when I create a spark but the problem i am facing is the reading is about 5 - 6 volts even when i am not creating any spark.. Now i need some help to prevent this.. my receiver circuit is in the attachment... I got some new questions on this..

    1) ckrounch mentioned that i am picking up 50 hz ac mains supply..... I mean this is a very low frequency signal and to pick it up i need a long antenna isnt it? or am i wrong?

    2) I wound inductors not from magnet wire but from plastic coated wires!! i hav attached the pic of it too.. I just want to know if the plastic coating is different from magnet wires in any way.. I know that magnet wire coils are tightly packed and give more inductance for a given space but other than that no difference isnt it? Even the inductors i wound are pretty well wound..Does the plastic coating conduct electricity or something causing failed performance of my filter?

    3) Finally, I am doing this circuit on breadboard before making permanent circuit using manhatten style or using a pcb or something, i just wanted to know that is there a possibility of one line of a breadboard ( the line with 5 holes which acts as a single node) to come in contact with adjacent line and thus shorting my circuits??? you know if i push a lead too deep inside or something..

    The main objective is that is I should get a 3-4 volts (or even lesser) reading only when i create a spark from the mains switch and all other time the multimeter reading should remain at zero.. or maybe like 200 mV.. but not much.. But now, I am reading 5 - 6 volts even without doing anything...

    thanx for the help...:)
     
  10. BLOBY

    Thread Starter Member

    Mar 10, 2011
    37
    1
    Help soon... Plz.. waiting for 2 days
     
  11. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Have you started breadboarding yet? There is another technique out there, called dead bug prototyping, that works well. Google for the technique, I am not on my computer and don't have the reference handy.

    The bread board I showed in post #4 (you never replied to my post, you want people to respond you need to respond) is very good for low frequencies. It can handle low frequency RF, though I would not recommend it. It was not a shorting wire.

    Dead bug uses super glue and a metal plate (or PCB). When using a PCB it can be mixed with manhatten prototyping, the two are very compatible.

    Be aware though, metal cased transistors have the case as collector (usually), so there is a potential for shorting.
     
  12. radiohead

    Active Member

    May 28, 2009
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