RF detector falling off for some reason

I am trying to build a cell phone detector. I found several schematics online, but I liked the one below best because it has an easy output for the Arduino so I can make fancy readouts and what not.

Because I am a noob with no formal training, I figured I'd better measure the output from Pin 6 before hooking it up to my Arduino and frying it. So I set it up on my breadboard and powered it up with my 11.4V battery.

It worked, sort of. About four feet away, I get zero volts on the output pin. When I move it in towards the antenna, the voltage comes up- for about a half second, then it drops off back to zero. The closer I get the phone to the antenna, the higher the voltage- but it still drops off back to zero if the phone doesn't move. When I move it, the voltage comes back up, but quickly goes down.

Clearly something is draining the signal. What do I need to do to make the voltage constant? I want the voltage to increase the closer the phone is to the antenna and stay there so I can give distance information with the arduino. Any ideas?? Thanks.

Cellphone detector.jpg

Okay, I've been doing a little reading. Apparently a normal cell phone only sends its "Hello" packets to the cell tower periodically, so maybe I'm not picking up anything because it is on standby. A smart phone with a running ap might broadcst constantly, but I'm using my wife's blackberry for testing, and it's not doing anything. I guess I need to make a call and try it..

Make a call, send a SMS/MMS, or navigate a browser.

This doc has nice RF detector on page 110.

Made a call. Nothing... :frowning:

It will not work, of course. You have to add a detector in front of the op-apm (a UHF diode for example) to get a DC voltage related to the input AC UHF amplitude..

When I move it in towards the antenna, the voltage comes up- for about a half second, then it drops off back to zero.

Moreover, the C3 is blocking any AC UHF signal to ground, so what you detect is just static field (almost DC one) coming from your hand (as the opamp has got CMOS inputs) - therefore you see some voltage coming up/down when moving your hand towards/backwards near the antenna. It will show you "something" with your mobile "switched off" as well. It detects the static fields only (ie. like a "storm detector" - btw, I think it is a "storm detector" :slight_smile: ).

Mind you want to detect 0.8-1.8GHz signals when detecting mobile phones transmissions, not the static fields.

Okay. makes sense. So how do I fix it? Do you have a suggestion for a op-amp?? (I dont' know what that is, btw...)

Thanks for the reply!

Oh, by the way, this is not the original schematic. I cut out the part that used a transistor to trigger a 555 timer. The 555 is supposed to beep a buzzer when a cell phone is near.

Here is the original:

That circuit is pretty much nonsense. However, you can make a rather insensitive cell phone detector (which actually does work) based on the ancient crystal radio technique, which involves a tuned loop, a detector diode and an LED. See this link: The Creative Science Centre - by Dr Jonathan P. Hare

The diode and the dimensions of the antenna are critical and it is difficult to impossible to get the OA91. I happened to have an OA91 in my junk box, but I got better results using microwave detector diodes from eBay (HSMS286K). They are not easy to work with because of the tiny packages with extremely short leads, but they work better than the OA91 diode. See attached. The LED lights up brightly near a microwave oven!



Your last image, :astonished: OMG.

Thanks for the link and info, jremington.

LarryD, my image or his? This circuit is all over the internet. I take it that it is complete crap?? lol

I take it that it is complete crap?? lol

A very large fraction of what you find on the internet is crap, and yes, that burning smell is coming from the tips of my fingers!

Here is an UHF detector for example. The 2 leads "to voltmeter" might be connected to your op-amp's input in order to amplify the voltage, and from the opamp's output you may go to arduino's ADC to measure it. The experts here may help you with designing the proper connection from "to voltmeter" leads to your op-amp input :slight_smile:
PS: the antenna "sides" must be a little bit bigger to cover your favorite cell phone band (a lower frequency than the 2.4GHz there).
Have fun with microwaves!

The LED on the microwave.

May be link was corrupted, here I attached a schematics:

?????.pdf (41 KB)