Out of Range Alarm

I want to have a RF transmitter and a receiver, that would set an alarm (sound) off when they're away from each other a certain distance (5 meters range). The transmitter has to be light (5"X5"X3") if it possible to place it in a base unit where if someone would steel it the alarm would goes off, powered by a battery last for at-least 2-4 weeks. The Receiver to be powered through an adapter and have a battery for backup.

Don't you want the alarm attached to the receiver?
How big does this have to be?
What sort of battery life?
What range?
How quickly do you need to know?

...and what are the consequences of a false alarm?

It has to be small so it would be hidden within the unit.

Please can you define "small"?

A housebrick is small compared to a truck, but a Lego brick is massive compared to a watch.

I want to have a transmitter and a receiver that would set an alarm off when they're away from each other a certain distance. I need the alarm to be attached to the transmitter so that i can attached it to valuable stuff for example.


Read your own post back to yourself.

Have you provided any information on the transmitter and receiver that would help anyone to give a sensible reply ?

Is the transmitter, sound, light, Wifi, Bluetooth or some other method of transmission.

And do tell the forum why you want to do this.

It would be much simpler if your alarm can be at the receiver and the "certain distance" doesn't have to be very accurate or precise: you could use small commercial BLE beacons (e.g. iBeacon, Eddystone), receive the beacon signals with a bluetooth module+Arduino or RPi to get RSSI and set off an alarm when the strength drops below a "certain value."

There's also this RFID device, but it only reaches the low end of your range (5m). And it is subject to the same limitations: alarm at the receiver, not very accurate.

Perhaps a switch in the base of the transmitter that sends the signal to the receiver when the Tx is moved off the base.

Look into commercial anti-shoplifting tags. They're much smaller than what you describe so will fit very easily, and set off an alarm the moment someone tries to take it out of the door.

I want to have a RF transmitter and a receiver, that would set an alarm (sound) off when they're away from each other a certain distance (5 meters range).

This is not going to happen.

It is not possible to detect range due to the reducing signal strength with that sort of precision and reliability. There have been many other Threads about a similar requirement.

You need a system in which both the Tx and Rx know their positions and can communicate that so that the distance between them can be calculated. Outdoors, and for larger distances, one would use GPS. I believe that a very expensive indoor system is available - called Pozyx.

The devices that @wvmarie mentions in Reply #10 work AFAIK when the thief walks through receivers at the doorways - in other words at very close range.


Indeed. And I think you actually have to be in between, at least for the magnetic ones, as that's when you disturb the electromagnetic fields between the two loops.

A long-range RFID may do the job - it's not accurate, but you can still detect whether it's in or out of range. It would defeat the metallised bag trick which can be used to defeat with magnetic patches. OP probably doesn't care too much whether it's 2m or 5m or 8m - as long as the alarm goes off when it's taken too far from base.

The alarm itself in this case should of course be on the device that's taken, not on the base station, as it should point out the would-be thief (or unsuspecting customer that wants to show it to her friend on the other side of the shop).