I want to build a simple detector for cellphone signals nearby and I implemented the following circuit on breadboard:
Here is my breadboard wiring:
I've checked the wiring 10 times and seems there is no error in wiring.
The only difference with the original circuit is that I used BC547 instead of 2N3904 and 68 Ohm instead of 82 Ohm for the resistor R3. For loop antenna I used a alligator wire.
The circuit doesn't work when even I turn on off a cellphone at extreme proximity.
It only worked when I cut the loop and inserted a transformer primary winding in series and turn on a drilling machine very near the circuit.
How can I adjust this circuit to make it more sensitive to electromagnetic pick up?
Best Answer
This circuit is a bistable flip-flop. It's tried to get triggered with the voltage that the loop happens to catch. The transistors, which are general purpose switching transistors do not rectify cellphone's GHz-range signals. Using high capacitance and inductance breadboard finishes the result. GHz-range signals allow only few millimeter long wires except when they are specially designed transmission lines.
You need properly designed antenna for your cellphone band, and a crystal detector for that frequency. Then you must in practice test how much DC voltage you can get from the detector, when you tune the antenna. Finally you need a comparator circuit which gets triggered when the inputted DC is high enough. Anything after the diode detector can be on a breadboard, but the rf parts cannot.
I cannot give any proper schematic nor component values because I should do the same practical tests myself to be sure. Search for "Cellphone detector".
One example: https://www.electronicshub.org/cell-phone-detector-circuit/
It at least uses diode detector.
ADD: Detecting radio signals has been under interest about 150 years. Many practical devices has been developed. One example from year 1901:
https://patents.google.com/patent/US755840
Unwanted cellphone detection by audio systems should be familiar for everyone. Obviously some common amplifier ICs can rectify cellphone signals and audio circuitry can accidentally be proper antenna. Unfortunately no design cannot be based on knowlwdge that sparse. If you have some well working simple IC based audio amplifier, connect a few centimeter wire (=antenna) directly to the input pin of the IC and check, what you get when you have a transmitting cellphone. It's well possible that you find something usable.