Hello,,

I made this circuit:
http://www.electroniq.net/radio-frequency/cell-phone-detector-circuit-diagram.html

which it is about a cell phone detector ,, quite simble but the strange thing is that it wont work unless I connect the voltemeter on the Op-Amp at both pins 7 and 3 when I remove the voltemeter the LED stop blinking when the cell phone dialling.

Any explanation?

 

Hello,,

I made this circuit:
<snip>
it wont work unless I connect the voltemeter on the Op-Amp at both pins 7 and 3 when I remove the voltemeter the LED stop blinking when the cell phone dialling.

Any explanation?

The circuit in your link doesn't make much sense to me. The CA3130 is spec'd at 15MHz which is usually the -3dB roll off point. This is many orders below the cell phone band. The output of the chip feeds directly into the base of an NPN with nothing in the Emitter but a LED to limit excessive base current. The capacitor values at the input seem absurdly large at cell frequencies too! These are just a few objectionable things. For sure they're more.

Regarding your tests... It doesn't require much to detect a cell phone dialing out or when it echos from an incoming call. Just about any AM radio will pickup the packet envelope (which is low freq) when right next to it. In fact my radio starts pop-pop-popping about 2 to 3 seconds before my phone starts ringing. I've always assumed that what I'm hearing is my phone sending an AK back to the cell tower, confirming receipt of the call.

 

You're turning your meter into an antenna, one that works better than the designed one. The tiny impedance of the meter is not likely to be changing your circuit, but you could test that by putting a 1MΩ or 10MΩ resistor where you are placing your leads. But again, my hunch is the antenna effect.

 

<snip>my hunch is the antenna effect.

May I second that?

Edit: To this day I still say that something went terribly wrong when we advanced from VTVMs to DVMs. The young design engineers of the period must have looked at the old VTVM and said "What the hell is this 1MΩ resistor doing in this probe? That's why a keep a second probe with a 100KΩ inside. It's high enough to provide test lead isolation without causing a substantial drop in voltage reading.

 

The circuit in your link doesn't make much sense to me.

You have a point and I didn't consider to build it until I saw a video about it :
http://electroschematics.com/1035/mobile-bug-detector-sniffer/

You're turning your meter into an antenna, one that works better than the designed one. The tiny impedance of the meter is not likely to be changing your circuit, but you could test that by putting a 1MΩ or 10MΩ resistor where you are placing your leads. But again, my hunch is the antenna effect. Yesterday 05:46 AM

you are totally right I put a 1.5MΩ in series with one lead and the circuit is wont work again.

So how to solve this problem? How could I test the OP-AMP in the circuit to figure if it is not defective?

 

I think the cell phone is so close to the circuit its high power causes the inputs of the opamp to be severely overloaded then it gives an output.

 

I canceled (C1) and it works just perfect reconsisdring the distance between the cell phone and the circuit...

Thnaks for you all.

Regards

 

FYI. That circuit shows a "Buzzer" connected to the output of the 555. If it's actually a buzzer, which is electromechanical, and not a Piezo, then you need a protection diode across it before it pops your 555.