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detector circuit falling off too quickly

c131frdave

Oct 4, 2013
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Hello,

First, I am a complete noob, and have very little idea how most things electronic actually work. I know enough to blow up caps and ICs though... lol

Anyway, I wanted to build a RF detector, specifically a cell phone detector. I found several schematics, but I chose the one below because I want to connect an Arduino to it so I can do some fancy things with LEDs and what not..

I hooked up 12V to it and it works, sort of. I am measuring voltage out of pin 6. When I hold my phone about 4 feet away, I get zero volts. When I move it closer, the voltage rises- but just for a half second or so- it then falls off even if I keep the phone still. The closer I get the phone to the antenna, the higher the voltage, but still it falls off almost immediately back to zero.

Obviously what I want is the voltage to stay the same until I remove the cell phone. Why is this not working? Thanks!
 

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cl10Greg

Mar 20, 2014
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If your phone always transmitting an RF signal? Are you talking about during use or what are you trying to detect? What antenna are you using?
 

c131frdave

Oct 4, 2013
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Oh, and it's only outputting millivolts, so my meter is having a hard time keeping stable. When the phone is close, it registers about 34mv and then falls off almost immediately. I checked my wiring and it is correct to the schematic. Should I use a transistor or something?
 

c131frdave

Oct 4, 2013
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A cell phone always broadcasts to the cell tower. We just get charged for the time we talk or text, but it is always communicating back and forth. (I read that somewhere) If I understand it correctly, the phone will periodically send a "hello" packet to establish communication with the nearest tower. A smart phone might do that constantly if some ap is doing something or other.

So maybe you are right- maybe I'm testing while the phone is not doing anything and I'm just picking up RF from the electronics.
 
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cl10Greg

Mar 20, 2014
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What meter are you using and what settings? If the packet assumption is true, then it would be more of a pulse of dissipation. You almost need to have you Arduino log the signal over a period of time to get an idea of the spectrum. Also make sure your antenna is able to receive the signals and doesn't require any certain direction to receive the signal. So you are trying to read the GSM signals?
 

c131frdave

Oct 4, 2013
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Not sure what you mean by "what meter". The circuit I am using is posted in the first post. It is designed (according to the web page I got it from) to amplify signals in the 800-900mhz range, and 1800-1900 MHz range, which is the frequencies used in the US. My antenna is roughly "tuned" for 850/1850 (5 inch antenna, 20ga).

I did find out for sure that AT&T phones send "hello" packets to the tower every 5 minutes exactly for 2-3 seconds when the phone is idle. This is so that the phone will change towers if you are moving. I assume other US carriers do the same, but I can't confirm that.


EDIT: I reread your post. My meter is a RadioShack multimeter. I just have it set to read voltage.

I tested it again but this time placed a call. No voltage reading on the 6 pin at all. :(
 
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duke37

Jan 9, 2011
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I do not see how the circuit can work. The 3130 will have negligible gain at cellphone frequencies. Also the input is bypassed with a 100µF capacitor. You will need a detector at the input. Get a whisker from your cat.
 

davenn

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Sep 5, 2009
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^^ what duke37 said ^^

its a crap circuit and isnt designed to receive SHF frequencies that mobile phones work on

have a look through a few other sites and post links here so us guys can review them and offer advice on suitability

cheers
Dave
 

CDRIVE

Hauling 10' pipe on a Trek Shift3
May 8, 2012
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Where did you find that circuit? It looks like something you'd find on Instructables.com

Chris
 

KrisBlueNZ

Sadly passed away in 2015
Nov 28, 2011
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LOL Chris :)

Yes, you can find it on instructables.com: http://www.instructables.com/id/simple-mobile-detector-circuit/?ALLSTEPS

I found the same design on many web sites. The diagram and the accompanying text are copied verbatim, including a distinctive grammatical error. Just try this Google search: http://www.google.com/search?q="very+low+input+current+and+very+high+speed+of+performance"

Yes, plagiarism is alive and well on the Internets :)

That text explains how the circuit is supposed to work. One of the capacitors is mounted off the board to act as an antenna with the correct lead spacing.
 

duke37

Jan 9, 2011
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If the circuit does work, it will be due to the protection diodes rectifying the signal and the opamp amplifying this.
The 0.22µF capacitor will be effectively a short circuit at cellphone frequencies so the connctions to the amp will act as a loop aerial, hence the given dimensions.
 

CDRIVE

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If the circuit does work, it will be due to the protection diodes rectifying the signal and the opamp amplifying this.

Good call Duke. I've been scratching my trying to figure how the hell an OpAmp with an fT = 15MHz could possibly find any use in the 900MHz band. Yours is a logical & feasible explanation as any. ;)

Chris
 
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