BFO Metal Detector – Student Project

(* BFO: “Beat Frequency Oscillator” a technical term describing the most common operating method used by metal detectors.)

Here are the technical details of the metal detector our students made during the winter vacation.

Continue reading at Kids’ Resource Center……

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19 Responses to BFO Metal Detector – Student Project

  1. Amman Hawari says:

    I’ve tried to construct the metal detector using your method and followed your diagram and instructions. I ended up with a device that is mute. No sound was coming out from the head phones. I do not know what went wrong. Your help is highly appreciated. Thank you.

    • graemek99 says:

      The first thing to establish is whether or not the LM386 audio I.C. is working. You can do that quite easily by touching pin 2 with your finger. You should hear some buzzing as it picks up local power line noise. It will be louder if you hold some metal thing such as a screwdriver blade in your hand, and touch that onto pin 2. Be careful not to short any pins together.
      Let me know the result and we can figure out what to do next.

      • Amman Hawari says:

        When I touch pin 2 , I hear a buzzing sound. I think it’s working.

      • graemek99 says:

        OK, that’s a good start. Now, let’s see if there’s any kind of a signal coming out of the search coil. Can you get hold of a small AM radio? If you tune it to a not-too-strong local station then put it directly onto the coil, you should hear something in the radio. Try turning the tuning adjustment on the detector and see if you can hear any whistling noises. I can hear that on mine. That’s because the frequency of the detector is about the same as the “Intermediate Frequency” of the AM radio.

        Let me know what you find. And, do you have any kind of test instruments there? A multimeter would be useful at least.

  2. Amman Hawari says:

    No sound comes out when I place the AM radio on the coil while tuning the detector. I have a multimeter that I can use.

    • graemek99 says:

      OK. That means either it isn’t working or it’s too far away from 455 KHz.
      Let’s check next to see if the voltages on the transistors are close to mine. These measurements were taken with a common digital multimeter. (negative lead to ground, 0v.)
      Q1; the oscillator with the ceramic filter has these voltages:
      Collector = 5.0 v
      Base = 4.09 v
      Emitter = 4.05 v

      Q2; the search coil oscillator has these voltages:
      Collector = 5.0 v
      Base = 1.7 ~ 2.29 v (across the range of adjustment of the tuning pot)
      Emitter = 2.04 ~ 2.51 v

      And by the way, does your multimeter have a frequency counter function? Mine has one that goes up to 2.0 MHz. It was very useful in getting this working.

      • Amman Hawari says:

        I am not getting any considerable reading on both. I am sorry I do not have a frequency counter.

      • graemek99 says:

        OK. Did you understand where to put the multimeter probes? Do you get 5.0 volts on the collectors of both transistors? And base, emitters? What readings do you get exactly?
        This will probably tell us where the problem is.

  3. Amman Hawari says:

    Yes. The black probe on the board which have 0 voltage and the red probe on the collector, the base or the emitter of the transistors. The transistors are the one next to the 455 KHZ and the one that its base connected to RV1, Q1&Q2. I am getting 0V

  4. Amman Hawari says:

    To elaborate more, the base of Q1&Q2 is connected to R8(33K), C6(.001uF) and the 455 KHZ. The emitter is on the left when the flat face of the transistor is facing me. The collector is on the right and the base is in the middle.

  5. graemek99 says:

    OK, if you get 0 volts, it shouldn’t be too hard to track down the problem. Next we should check the 5 volt regulator, the 78L05. Do you have 9 v on its input and 5 v on the output?

    • Amman Hawari says:

      I think the problem is the 78L05 as I am not getting any voltage coming out of it. I will change it and report back to you. Thanks a million.

      • graemek99 says:

        Great!
        There are other things that could cause it too though. For example, something shorting the 5 volt supply to ground. Or the 78L05 might be connected backwards.

  6. Amman Hawari says:

    I have a problem with Q2. The readings are as follows:
    Base: 0.850V
    Collector: 0.336V
    Emitter: 5.03

  7. graemek99 says:

    That’s good. Did you find the trouble with the 5 volt supply?
    The voltages you have given indicate that the transistor is connected backwards, i.e., collector and emitter reversed.
    Unless of course you were simply looking at it wrongly when you took the measurements. That would probably mean that the circuit is not oscillating = something wrong with the wiring, the component values or the construction of the search coil.

  8. Amman Hawari says:

    The 5V supply was connected in place of Q2 :).
    I flipped both collector and emitter for Q1 and Q2. I am now getting the following:
    Q1:
    Base: 4.64V
    Collector: 5.04V
    Emetter: 4.07V
    Q2:
    Base: 1.222V
    Collector: 5.03V
    Emetter: 1.426V

    I can now hear sound on my head phones. But it is very weak. It can hardly be heard. I was able to adjust it so I can hear it only when the search coil is passing over a metal.

  9. graemek99 says:

    OK, some good progress.
    You should double-check your component values and wiring around the input of the LM386. Including C3, C4, D1, R6, R5, C2, C12 and RV2. And make sure pins 1 and 8 on the LM386 are joined together.

    • Amman Hawari says:

      Everything is working fine now. I would like to express my deep appreciation for all the effort and help you gave to us to make it work. Thank you for your time. I hope you and your team and students all the best.

  10. graemek99 says:

    Fabulous! You’re very welcome.
    I hope you have a lot of fun using it.

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