TDA7000 FM Radio project

Discussion in 'Wireless & RF Design' started by Outdoors546, Apr 10, 2016.

  1. Outdoors546

    Thread Starter New Member

    Apr 10, 2016
    4
    0
    Hey guys, Im new here decided to join in search of some help. Im currently taking a coarse in college called electronic projects, I opted to build a FM radio circuit with an audio amplifier. This is very important its either pass or fail if I can get this working in time by the end of the semester, so I will even go as far as to offer a cash reward(paypal) to the person or persons who helps me get this working in time.

    Ive got the circuit breadboard but Im having problems getting it to pickup an FM signal. Its being powered by a single 9V battery and ive measured around 7V on the main rail via multi-meter. The audio amplifier circuit portion seems to work just fine and i get a sound out of the 8ohm speaker that sounds as if it should. (The same noise as turning two way radios on without having any data transmit) The 10k trimmer pot increases and decreases the volume as it should.

    Upon first construction of the circuit, I had put the TDA7000 in the wrong way. (Pin 1 was on the opposite end where it should be) Upon power on the chip got pretty hot. This likely could be my problem(I may have hosed myself), I have a 2nd TDA7000 on its way to test if this is the issue. For the turning of stations Im using a 3.5 turn adjustable coil as shown in the link. Whenever I adjust this manually it does not pickup any signal. I have got online and searched around and found another schematic that shows having a 100k variable pot, 100k resistor, and MV2105 varicap diode with a 1NF capacitor off pins #6. (shown here-http://electronics-diy.com/TDA7000_FM_Receiver.php) Is this really needed? I have decided to order these parts anyhow to see if it will help it get a signal, afterall Im at the point of trying anything before time runs out. Any help is greatly appreciated!

    I decided to follow the schematic as show below from this site : http://electronics-diy.com/fm-radio-with-tda7000.php
    [​IMG]

    My actual circuit as it is on a breadboard:
    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]
     
  2. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
    1,393
    First thing is to find out why the battery voltage is sagging. My experience is that the LM386 performs poorly with a weak battery. Get a brand new Duracell, hook it up and measure the voltage; if it's noticeably less than 9V, something is wired wrong.
     
  3. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    Making an RF circuit on a breadboard is a bad idea.
    Circuits upto 1 Mhz will work on a breadboard, above that there will be problems.
    The connection lines of the breadboard will make capacitors that will ruin the RF signal.

    Better use an other method to make the receiver.

    Bertus
     
  4. Outdoors546

    Thread Starter New Member

    Apr 10, 2016
    4
    0
    The battery measures 7.86V out of the circuit, the main rail is reading 7.7V with the battery connected. Its a new Duracell. Correct me if Im wrong but I dont think a new battery measures a full 9V?

    The link also shows a supply voltage of 4.5-9V
     
  5. MrChips

    Moderator

    Oct 2, 2009
    12,440
    3,361
    bertus beats me to it.

    Throw out the breadboard and find another way of wiring the circuit.
    There is too much capacitance in that breadboard.
     
  6. Outdoors546

    Thread Starter New Member

    Apr 10, 2016
    4
    0
    I have also heard this upon doing research, Im trying to test this circuit before making my own PCB and soldering in the components which still has to be done. What other method could I use to test?
     
  7. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,983
    745
    I would say you have blown the chip up putting it in the wrong way, i would make the circuit using the varicap diode, instead of the coil, use veroboard instead.
     
  8. Outdoors546

    Thread Starter New Member

    Apr 10, 2016
    4
    0
    Very likely, as I said I will test the new one when it comes. Also, theres a coil from pin #5 regardless, its just one link shows using a vericap diode with an extra capacitor and 100k pot and 100k resistor and the other link(which I followed) leaves these out.

    I do believe the 100k pot and 100k resistor are used for fine tuning FM frequency.
     
  9. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,983
    745
    Yes the varicap diode behaves like a capacitor when a voltage is applied to it, the smaller the voltage across it, the smaller the capacitance, the pot is used to vary its voltage, .
     
  10. jjw

    Member

    Dec 24, 2013
    173
    31
    Smaller voltage -> higher capacitance.
     
  11. SLK001

    Well-Known Member

    Nov 29, 2011
    817
    227
    Sorry, but you are incorrect. A new 9V battery will give 9V or slightly more.

    You have almost zero chance of getting your circuit working on a breadboard. They are fine for low freq stuff, but at 100MHz stray capacitances will kill you. If you want to prototype this kind of circuit, the very least you should use is perfboard on a 0.1" by 0.1" grid. Keep the leads as short as possible and you might have some success.
     
  12. bertus

    Administrator

    Apr 5, 2008
    15,647
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    Hello,

    L1 and C1 will make the oscillator tuning circuit (VCO in the schematic).
    Changing the value of either will change the frequency.
    Normaly a variable capacitor is used, but a variable inductor will also work.

    TDA7000_schematic_with_internals.png

    Bertus
     
  13. Art

    Distinguished Member

    Sep 10, 2007
    785
    61
    Not only off the breadboard, but in little real estate, and cut down tracks on the protoboard.
    The inductors are easy to ruin. I tried a breadboard first, then my second attempt on protoboard worked.

    If you did get it working on a breadboard, you’d have incorrect inductor values for the finished board.

    [​IMG]
     
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