Use an old analog cellular phone to verify the integrity of the cellular phone network in your local area.

A neat thing about old cellular phones is that when they power on they switch to a local roaming network.  On Motorola handsets, the yellow "Roam" LED will start to flash, even if the phone is not activated.  If for whatever reason, say ladyada is on a jamming spree, the phone will then begin to light the red "NoSvc" LED.

If we tap the "NoSvc" LED and add a simple digital logic counter and 555-timer circuit, you can trip a relay to activate a buzzer or a pager whenever the cellular phone is out of service or jammed.  The counter circuit waits until the "NoSvc" LED lights pulses on-and-off eight times, then it activates a piezo buzzer for three seconds.

By adding an external duplexer and a little attenuation, you can increase the jammer's detection capability.

Block Diagram

Schematics

• Cellular Phone Jammer Detector - Counter Logic  (42k PNG)

Pictures

• Cellular Phone Jammer Detector - Picture 1  Outside case overview.  The phone's LED display is mounted on the front panel next to the buzzer.  A TNC jack is provided for the antenna.
• Cellular Phone Jammer Detector - Picture 2  Inside view.  The phone's internals are on the right and the counter logic is on the left.  The silver rectangle in the middle is the duplexer to act as an extra bandpass filter.  A seperate AC power supply is also provided.
• Cellular Phone Jammer Detector - Picture 3  Behind the front panel.
• Cellular Phone Jammer Detector - Picture 4  The cellular phone, a Motorola TeleTAC 250 in this case, has it's LED display mounted on the front panel via extension wires from the phone's 28-pin LED socket.  Also shown is the "NoSvc" LED wire (single white wire), it goes to the sixth pin from the right.
• Cellular Phone Jammer Detector - Picture 5  Another internal overview.
• Cellular Phone Jammer Detector - Picture 6  LED display showing a good signal strength and the yellow "Roam" LED flashing.
• Cellular Phone Jammer Detector - Picture 7  LED display of a jammed or disabled phone showing the red "NoSvc" LED flashing.