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LM358: MOBILE PHONE DETECTOR USING LM358 IC

ANTONY JOHN
Prodigy 90 points
Part Number: LM358

The circuit works by detecting electrical disturbances caused by RF signals from mobile phones followed by using LM358 IC to amplify it to give output through an LED. Can someone explain how each component in the circuit works and what design equations were used to calculate the values for each component in the circuit(especially the resistors and capacitors)?

  • 0
    TI__Mastermind 22391 points

    Hello Antony,

    This does not appear to be a TI design, just a design that uses the LM358. 
    If you had any questions about the LM358 specifically, or about a specific TI design, I would be more than happy to answer.

    There is a video explanation for this circuit here https://www.seattleu.edu/scieng/ece/student-resources/laboratory-resources/cellphone/?redirect=true 

    Best,
    Jerry

  • 0
    TI__Intellectual 1130 points

    Hello,

    Please see this link to where it was designed.

    The section that describes this circuit in the link doesn't make much sense to me. I will take a stab at it and perhaps someone can give you a better answer.

    U1-A is used as TIA where R1 & R2 create a voltage divider for a reference voltage to the TIA. The current picked up by the antenna is amplified by R3 - 6.8M resistor. But since the LM358 is Single Supply and the equation for a TIA is Vout = -I_in * R3 then the voltage will decrease on the output as current flows, hence the need for the 4.5V reference as it will provide headroom to swing to ground. C1 is used to minimize any voltage ripples and high frequency noise, as the frequency increases it will become a short to ground. C2 creates a "delay" in the amplifier so it doesn't become unstable when the RF is detected otherwise the amplifier can oscillate, this kills its bandwidth. U1-B is pretty much a comparator since there is no feedback. If the voltage at Pin5 is lower than Pin6 the output of U1-B will go to zero and opposite is true. If the output of U1-B is high (Pin5 > Pin6) then there is almost no potential across the D1 and it wont light up. But when RF is detected, the voltage on Pin5 < Pin6 and the a potential is created since the output of U1-B goes low and therefore the LED will light up. C3 is used in the same way as C1. 

    There are no "specific" equations to go by. 

  • 0 in reply to Jerry Madalvanos
    Prodigy 90 points

    I had already referred that website, but couldn't get any clear cut answers. Appreciate the effort though. Try to give a point of view. Thank you.

  • 0 in reply to Fadi Matloob
    Prodigy 90 points

    Thank you for your answer, it gave me some more insight to the circuit. Can you tell me more about what the transimpedance amplifier does in the circuit and does U1-B work as a voltage follower or as a comparator as you mentioned. The thing is that, i was of the assumption that U1-A was acting like a comparator and U1-B amplifies whats coming as output from U1-A so that the gain gets multiplied and the circuits sensitivity increases to detect the disturbance caused by RF signals from mobile phones and R5 is adjusted to prevent circuit from giving output to background RF sources. So your remarks made it a bit confusing.And as far as equations go, you can specify any equations which would help me design this circuit down from the values of the resistors and capacitors and the choice of IC. Appreciate the effort.

  • 0 in reply to ANTONY JOHN
    TI__Guru*** 140106 points

    Antony,

    U1-A is a nearly (or actually unstable) unity gain amplifier. U1-B is a comparator.  My suggestion is to do as follows.

    1) Study the DC solution of this circuit. Assume all caps are open and there is no antenna. Stay here until fully understood. Try TI-TINA

    2) Study the AC performance of this circuit. TI-Tina will help here. 

    3) Take note the mobile phone frequencies compared to LM358 specs.

    4) Study effect of EMI on op amp inputs and what that does to Vio (think about the DC gain choice)

    5) Study what other characteristics come with a phone transmission, Is it AM or pulsed (square wave 0%,100% AM) ?

    6) Research how the capacitor acts at mobile phone frequencies.

    7) Research how antenna length works with the capacitor impedance.

    8) Build the thing and play with the component values and antenna length.

    Learn by reading and doing.

  • 0 in reply to ANTONY JOHN
    TI__Intellectual 1130 points

    The TIA works by converting current to a voltage. U1-B is a comparator. There is no feedback on U1-B so that's a another reason why it can be a follower. It seems like you may benefit from taking a look at TI Precision labs. The OP AMP precision lab videos should help your understanding of these different configuration. 

    Cheers!

  • 0 in reply to Fadi Matloob
    Prodigy 90 points

    Thank you for your help.

  • 0 in reply to Ron Michallick
    Prodigy 90 points

    Will try to do as said. Thank you for responding.