# Cell phone detector

#### parul77

Joined Apr 19, 2015
11
Hey,
I am doing a mini project on rf signal detection(detected by cell phones) using a 5 inch long antenna and IC CA 3130 op amp as a currrent to voltage regulator. The circuit diagram I found online is attached in this. Can anyone help me to understand why have we chosen the capacitor C1(0.22), it acts like a loop antenna. I just want to know how did we come to this value? and what other values will work?
Thank you

#### GopherT

Joined Nov 23, 2012
8,009
I would imagine C1 and R3 form a lag to keep the light on for a period (and the 555) even if it is on for just a brief moment. The values can be changed I think, as long as the RC time constant is similar to C1 and R3.

#### parul77

Joined Apr 19, 2015
11
Are you sure? R3 is a feedback resistor, and makes the inverting terminal high when the output goes high. Whereas the capacitor C1 is a mini loop antenna which detects the rf signal and gives it to the op amp which converts it to voltage..

#### parul77

Joined Apr 19, 2015
11
R1 and R2 act as voltage divider but I am again not sure how do we get the values of R1 and R2

#### Papabravo

Joined Feb 24, 2006
21,256
It is hard to imagine this circuit being able to distinguish a cell phone from any other source of RF energy. The voltage divider has a 23:1 ratio, so the input is near ground and any static discharge or other noise source will be seen on the input. What frequency range do the cellphones you are interested in use?

#### parul77

Joined Apr 19, 2015
11
I guess it will detect any form of RF energy. The freguency range is around 2.5Ghz, in the microwave region.

#### blocco a spirale

Joined Jun 18, 2008
1,546
Doesn't this kind of detector work by reacting to the modulated bursts of RF rather than the carrier itself i.e. the same chirping noise that we hear from audio equipment whenever there is a cell phone nearby?

#### parul77

Joined Apr 19, 2015
11
I guess so. I am just unsure about the design equations regarding this circuit.

#### blocco a spirale

Joined Jun 18, 2008
1,546
I don't think it is an exact science, in this case. You might just have to build it and experiment... which is probably what the designer did.

#### Alec_t

Joined Sep 17, 2013
14,370
R1 and R2 set the standing output voltage of the opamp. Provided that is below the point at which the LED begins to conduct I don't see their values being critical.
A cell phone should be easy to detect: it always has a teenager attached to it .

#### parul77

Joined Apr 19, 2015
11

#### Alec_t

Joined Sep 17, 2013
14,370
C2 is a typical decoupling cap value. Again, not critical.
I think C1 is just suck it and see. Depending on its physical size and construction it may have self-inductance which resonates with its capacitance in some way to make it suitable for use as an 'antenna' at the frequency of interest.

#### parul77

Joined Apr 19, 2015
11
Yes, thats true. Any other thoughts on this circuit?

#### #12

Joined Nov 30, 2010
18,224
A lot of capacitors fail in the Gigahertz region, so the standard Xc equation is probably useless for this application. In other words, C1 is not the capacitor you would expect from a school book point of view. More like an intentional fail at being a capacitor at the frequency of interest. Sure, it ruins the response to lower frequencies, and that is good, but it's probably not a good capacitor at the frequency you want, and that is good, too.

#### parul77

Joined Apr 19, 2015
11
Yeah, that does make sense. Thank you!

#### parul77

Joined Apr 19, 2015
11
Also this circuit is a current to voltage converter. Can anyone explain to me how that works? The C1 gives out a small current to the op amp and that is converted to voltage.

#### Alec_t

Joined Sep 17, 2013
14,370
We could argue all day as to whether the IC acts as a current to voltage converter or simply a voltage amplifier . The point is, any RF picked up by C1 will result in a small change in pin 2 voltage and the IC will try to cancel that out by virtue of negative feedback resistor R3.

#### parul77

Joined Apr 19, 2015
11
We could argue all day as to whether the IC acts as a current to voltage converter or simply a voltage amplifier . The point is, any RF picked up by C1 will result in a small change in pin 2 voltage and the IC will try to cancel that out by virtue of negative feedback resistor R3.

#### alfacliff

Joined Dec 13, 2013
2,458
the op amp is speced for amplification only to 10 mhz by the manufacturer. it will have no amplification at all at 2.4 ghz or even 100 mhz. the circuit might work if you put an rf detector ahead of the op amp. either germanium or schotkey doide would do, with proper care in building.