# Cell phone detector circuit

#### daniel_33

Apr 13, 2021
3
Hi guys, I found few circuits similar to that one but I can't find any proper explanation of working of such circuit other than very very general one. I'd like to know what exactly every element does. Could anyone help me with analysing this circuit ?

May 12, 2015
4,562

#### AnalogKid

Jun 10, 2015
2,740
Please post a link to the source of the circuit. As shown it will not do anything. Assuming that ANT.1 is an RF signal, that signal will be shunted to the power ground by C1 and shunted to the circuit reference by C2. The output at pin 1 will sit at approx 1.5 V. The voltage across the LED will be less than 1.0 V, not nearly enough to cause it to illuminate.

My guess is that the circuit is acting as a comparator or AM detector. The problem is that a 358 is a relatively slow and low-gain part. It can barely amplify audio, let alone RF.

If what is coming in ANT.1 already is amplitude detected and coming from a low-to-medium-impedance output stage of something and is a large enough signal, then this circuit will do something. Note that there are three *big* IFs, and all must be true.

BUT - given the number of cell phones everywhere, and the fact that all of them are transmitting almost constantly whether or not anyone is actually talking into them, what is it you are trying to achieve?

ak

#### daniel_33

Apr 13, 2021
3
Please post a link to the source of the circuit. As shown it will not do anything. Assuming that ANT.1 is an RF signal, that signal will be shunted to the power ground by C1 and shunted to the circuit reference by C2. The output at pin 1 will sit at approx 1.5 V. The voltage across the LED will be less than 1.0 V, not nearly enough to cause it to illuminate.

My guess is that the circuit is acting as a comparator or AM detector. The problem is that a 358 is a relatively slow and low-gain part. It can barely amplify audio, let alone RF.

If what is coming in ANT.1 already is amplitude detected and coming from a low-to-medium-impedance output stage of something and is a large enough signal, then this circuit will do something. Note that there are three *big* IFs, and all must be true.

BUT - given the number of cell phones everywhere, and the fact that all of them are transmitting almost constantly whether or not anyone is actually talking into them, what is it you are trying to achieve?

ak
Here's the link:https://www.electronicsforu.com/electronics-projects/mobile-phone-detector-using-lm358 , i'm trying to do this as a project for my classes, it's supposed to light up the diode when cell phone is dialing.

#### Alec_t

Jul 7, 2015
3,356
it's supposed to light up the diode when cell phone is dialing.
It just might do that, if it's very close to the phone and if C1 and C2 are both rubbish capacitors with a high enough equivalent series resistance.

#### PETERDECO

Dec 19, 2019
239
Puzzling to me is the 1uF to ground from the antenna. Wouldn't that cut the RF?

#### bertus

Moderator
Nov 8, 2019
2,920
Hello,

I approved the moderated post.
A RF circuit will not work well on a breadboard.
Also the 0.22 uF capacitor will basicaly short any RF signal.
I have great doubt if those circuit will work.

Bertus

#### AnalogKid

Jun 10, 2015
2,740
The text in the post #6 link explains the circuit in post #1. C2 and its lead inductances form a parallel resonant network, and the opamp is a current-to-voltage converter and, effectively, a half-wave rectifier.

I'm still struggling with an LM358 amplifying a 9.0-to- 2.4 GHz signal, but it appears to be all the rage.

ak

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,193
I'm still struggling with an LM358 amplifying a 9.0-to- 2.4 GHz signal, but it appears to be all the rage.
The LM358 per se is good for < 1 MHz only. I guess the RF is rectified and thus demodulated at the input protection circuit of the opamp or possibly by the input transistors themselves. The demodulated signal in the audio range is then amplified and used to drive the LED.
The effect is the same as can be observed when a smartphone lies next to a radio and the radio from time to time emits a loud hum when the smartphone contacts the network. At least that's what I think happens here.

#### bertus

Moderator
Nov 8, 2019
2,920
Hello,

This circuit might have a chance of working:
https://www.eeweb.com/cellphone-detector/
It does have a detection circuit before the opamp, but it is not selective.
It will react on any RF signal near the circuit.

Bertus

#### Ylli

Jun 19, 2018
386
These circuits depend on the input structure of the IC to rectify the incoming RF and shift the DC operating point of the amplifier. Not what they are designed to do, but I guess if the RF is strong enough it might work.

#### Audioguru

Sep 24, 2016
3,653
The circuit in this thread does not work because it was designed or copied by a kid in India who has not learned any electronics yet.

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,193
The circuit in this thread does not work because it was designed or copied by a kid in India who has not learned any electronics yet.
I would not straightforwardly deny the possibility of this circuit to do what it is supposed to do.
While the use of an opamp to detect a Ghz signal definitely is not good engineering, the circuit may work by using undocumented properties of the opamp as mentioned above. The risk involved is that different chips or different revisions of this chip may or may not work, depending on uncontrollable circumstances, as these undocumented properties are consequently neither tested nor guaranteed by the manufacturer.

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