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i have successfully implimented below circuit for active cell phone detection but i dont seem to understand the working of this circuit,if someone could explain it would be a great help.Thank you.

enter image description here

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  • \$\begingroup\$ I think they work as huge gain amplifiers. So antenna picks some electricity, charges c2 a little which than is amplified and produces some voltage difference which lights the led. \$\endgroup\$ – Darko Mar 22 '16 at 16:22
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    \$\begingroup\$ It's not specifically a cell phone detector - it will likely detect any reasonably sized electric field of almost any reasonable frequency thus making it an electric field detector. Is this what you wanted to build? \$\endgroup\$ – Andy aka Mar 22 '16 at 17:33
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    \$\begingroup\$ The LM358 has a unity-gain bandwidth of only about 0.7 MHz, so in this case it's not really used to amplify the GHz signal from the antenna. See this answer for further speculation and a potentially "real answer" to this question. \$\endgroup\$ – uhoh Aug 5 '17 at 7:02
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enter image description here

As Andy Aka has pointed out this is an alternating electric field detector. I'm no expert on radio but this circuit is easy enough to understand.

  • U1-A is wired as an inverting amplifier. R1 and R2 bias the + input to half-supply voltage (BAT+ / 2).
  • R3 is the amplifier feedback resistor but the usual inverting mode input resistor is missing. Therefore the input antenna will provide alternating current to the virtual earth of the inverting input. For every nA of current flowing towards the inverting input the output will decrease by \$ 1~nA \cdot 6.8~M\Omega = 6.8~mV\$ (to keep the inverting input at the same potential as the non-inverting input).
  • U1-B is configured as a comparitor. The inverting input is held at a reference voltage by sensitivity adjust pot R5. In setup this would be adjusted downwards until LED D1 turns off. (For LED off, pin 7 must be high so inverting input must be lower than non-inverting input.)
  • When the antenna signal is strong enough U1-A output will oscillate about the half-supply point and when it goes below the threshold set by R5 the LED will begin to light.
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  • \$\begingroup\$ that was quiet an interesting explanation thank you for the explanation now i know how it works... \$\endgroup\$ – thisisbhavin Mar 23 '16 at 16:01
  • \$\begingroup\$ I'm glad it helped. If no better answer turns up and you're happy that this one answers your question then please click the "accept" icon. \$\endgroup\$ – Transistor Mar 23 '16 at 18:23
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Non-inverting amplifier (Op-Amp 1), is used for amplify signal from antenna, as to make suitable to drive LED and buzzer op-amp (Op-Amp 2) with only positive supply is used. Upon change in input of driver op-amp, it'll be reflected on LED and buzzer.

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    \$\begingroup\$ The LM358 has a unity-gain bandwidth of only about 0.7 MHz, so in this case it's not really used to amplify the GHz signal from the antenna. See this answer for further speculation. \$\endgroup\$ – uhoh Aug 5 '17 at 7:01

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