long distance cell phone detector

Thread Starter

000SHREDDER000

Joined Apr 9, 2017
49
Hi guys I am alvin new here, can someone help to me to solve this circuit problem? I found this schematics on circuitdiagram website after building it many time it doesn't work correctly, circuit designer claims this circuit can detect cellphone waves from long distance but when I test it I didn't saw any detection led is always on I tried to make different designs with same components but none of them worked some designs only detect human body!!! from near distance, so what do you think about this circuit and how can I fix it? tank you:thinker:


Here is schematic link:
http://www.circuitdiagram.org/images/long-range-cell-phone-detector.gif
 

#12

Joined Nov 30, 2010
18,223
That circuit is useless.
The op-amp gain has declined to (1) one at about 3 MHz and the lowest cell phone frequency is 700 MHz.
The gain resistors set no limit on the op-amp, it has no frequency filters, and the input impedance is so high (1 Terraohm) it would pick up geese flying overhead in the vast open tundra of the arctic circle. It is obvious that it won't shut off.

OK. I exaggerated, but you get the idea.. Without something to tune out the power line frequency and radio stations in a city, this "detector" is going to detect constantly.
 

Attachments

Last edited:

ian field

Joined Oct 27, 2012
6,539
Hi guys I am alvin new here, can someone help to me to solve this circuit problem? I found this schematics on circuitdiagram website after building it many time it doesn't work correctly, circuit designer claims this circuit can detect cellphone waves from long distance but when I test it I didn't saw any detection led is always on I tried to make different designs with same components but none of them worked some designs only detect human body!!! from near distance, so what do you think about this circuit and how can I fix it? tank you:thinker:


Here is schematic link:
http://www.circuitdiagram.org/images/long-range-cell-phone-detector.gif
Elektor magazine published one many years ago, but I couldn't find it last time I looked. Theirs had an appropriately sized wire dipole shunted by a germanium detector (you can probably get away with Shottky-barrier). They used an LM358 for the amplifier.

Such devices have a fair bit in common with a typical microwave oven leakage detector.
 

Thread Starter

000SHREDDER000

Joined Apr 9, 2017
49
That circuit is useless.
The op-amp gain has declined to (1) one at about 3 MHz and the lowest cell phone frequency is 700 MHz.
The gain resistors set no limit on the op-amp, it has no frequency filters, and the input impedance is so high (1 Terraohm) it would pick up geese flying overhead in the vast open tundra of the arctic circle. It is obvious that it won't shut off.

OK. I exaggerated, but you get the idea.. Without something to tune out the power line frequency and radio stations in a city, this "detector" is going to detect constantly.
Tank you #12, I am not an electronic expert, can you tell me how can I fix it exactly or i must build another circuit, I didn't find long distance(about 10 meters) UHF detector on the web.
 

#12

Joined Nov 30, 2010
18,223
can you tell me how can I fix it exactly
No. I am not that good at RF circuits, but I can see an amplifier with it's gain limited only by it's bandwidth and with incredibly high input impedance (10^12 ohms) as being so easy to excite that it will amplify almost anything. You could build this circuit on a clean piece of glass and it would detect your hand passing over it. So, for starters, it must be built in a completely shielded metal box.

I suspect that the size of the antenna and the way the circuit is built on a circuit board can provide limitations on its frequency range, but those things do not show up in a schematic. Each diode has about 1 pf of capacitance and the input of the op-amp has a capacitance of 4 pf. These capacitors form an AC voltage divider, but there are no resistors or inductors working with then to form a frequency filter unless you consider the (also not obvious) inductance of circuit board traces. This technology is beyond my abilities.
 

#12

Joined Nov 30, 2010
18,223
No man you are good,
I'm good in a couple of fields, but not this one. Electronics is so big that nobody can know it all, so everybody has specialties. Maybe somebody else can help. Wait for the earth to turn. This site has good members all around the planet.
 

bertus

Joined Apr 5, 2008
21,603
Hello,

From the first counter : SAB6456 is used as 1:256 predivider. Its upper limit is 1300MHz.
The second counter uses a 74HC393 as input.
That chip has a practicle max input frequency of 100 Mhz.

Both circuit are frequency counters, that will be used to determine the used frequency of a circuit connected to the input.

Bertus
 

bertus

Joined Apr 5, 2008
21,603
Hello,

The schematic in the first link does not match the description.
It shows some kind of amplifier.

The second link shows a circuit to determine the frequency of a tuned circuit.
In the old days it was called a grid-dipper.

Bertus
 

ian field

Joined Oct 27, 2012
6,539
Hello,

From the first counter : SAB6456 is used as 1:256 predivider. Its upper limit is 1300MHz.
The second counter uses a 74HC393 as input.
That chip has a practicle max input frequency of 100 Mhz.

Both circuit are frequency counters, that will be used to determine the used frequency of a circuit connected to the input.

Bertus
The SAB part sounds like the sort of thing that can turn up in UHF tuners for devices with a frequency display.

The frequency band is near the right ball-park - it may have a front end amplifier stage, or application examples might describe one added to it in a practical circuit.

The consumer equipment prescalers are often pressed into service in DIY test bench frequency counters.
 

Alec_t

Joined Sep 17, 2013
12,261
The '1458 opamp used in your post #17 linked circuit has a unity gain bandwidth of only 1MHz. I don't see how it can possibly operate up to even 50MHz, yet alone 500GHz as claimed.
 

ian field

Joined Oct 27, 2012
6,539
The '1458 opamp used in your post #17 linked circuit has a unity gain bandwidth of only 1MHz. I don't see how it can possibly operate up to even 50MHz, yet alone 500GHz as claimed.
Near as I can tell; the diodes are supposed to detect some form of lower frequency envelope - but as #12 pointed out; the circuit is crap.
 
Top