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I've just got my Nrf24l01 but then I found out that the working signal range for the module is 100m and above. Our real intention for the module is to let the nrf24l01(Receiver) out of range for about 5m in order to activate the alarm.

The purpose of our project is for anti-theft alarm which sounds the alarm if the module is out of range. We only need a short distance in order for the alarm to work since our research project is only for personal use.

We have an idea which we want to clarify if it is possible, it works by having the (receiver) to reach a fixed distance value in the coding of the arduino and when the (receiver) goes beyond that value, it will activate the alarm or beep. We want to ask if our idea is possible and if yes, can you help us how to code the arduino?.

I don't have any background in making a code for arduino but I know how to program an arduino using a ready-made code from the internet.

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    \$\begingroup\$ Power based distance is crude, but the nRF24 cannot even tell you signal strength. You might consider a BLE chip, which is an evolution of the similar radio technology and does have a signal strength readout, which you will probably have to apply some filtering and averaging to. Look at the ranging stuff for BLE beacons - it is crude but people keep trying to use it. \$\endgroup\$ – Chris Stratton Feb 12 '19 at 14:09
  • \$\begingroup\$ If you make your receiver less sensitive or your transmitter weaker you can make it barely work at 5m range. However using link failure to determine range is kind of like gambling, so many other things can affect a system that is right at the point of sensitivity. Adjacent metal, human, electrical noise source etc. will all affect it more than the separating distance. \$\endgroup\$ – KalleMP Feb 14 '19 at 20:18
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No, not a fixed distance, but you can certainly reduced the expectable range, simply by reducing the output power of the transmitter, or by decreasing the sensitivity of the receiver.

These systems have no way of knowing distance – they can just "hear" or "hear not" the transmitter. How "loud" a transmitter is at the receiver depends on how loud it transmitted, or what happened to the signal from the transmitter to the receiver.

So, you need to tone down what the receiver can sense.

This can be either done by configuring the involved chips to use less power to transmit, or by using worse antennas, for example.

You'll find out that it's extremely difficult to achieve a reliable range by adjusting power – effects of random orientation, obstacles (e.g. humans) between transmitter and receiver and so on are much much larger than a few meters of difference in distance.

So, you might find out that it's conceptually impossible to define an "alert" radius without accepting a high number of false alarms or being too late to alarm.

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The way I did this was to use pre-made boards that use a circuit board j-pole antenna, lowest power setting and the highest speed. Once triggered they would ping-pong each other once every 2 seconds until the primary failed to receive 5 pings, at which point it would sound an alarm. This averaged out to be about 10 to 15m, which was plenty accurate for my application.

You could also use something like a box made from hardware cloth to reduce received power.

Ops, just noticed OLD question, but the answer still applies.

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Due to the relatively short wavelength compared to wall size and reflector/absorbers like metal and wall construction material, it is impossible to avoid Ricean Fading where signals vary by 10 dB or more if you attenuate the signal, this can reduce the range but the standard deviation on range then can be as much as the short range for alarm Loss of Signal or LOS.

Even if you use 10m to 20m band RF signals, you can reduce this affect but you still have non-isotopic antenna loss patterns, which can span 30dB range. so sorry but design concept will fail.

With a Return Loss bridge I made a motion sensor and short and long range telemetry design. short range was with no antenna, long range of more than 500 miles line of sight to satellites, when I discovered these properties in 1977 while designing an aerospace unwrapping dipole antenna for spinning rocket telemetry. the motion sensor worked only in short range with 2m bands using the modulation of reflections or echos called return loss from body reflections of the transmitted signal anywhere in the lab with a directional dipole antenna.

I later learned that alarm companies developed option sensors for hallways and interior space by picking up the modulation of echos in a directional coupler or splitter to an RF detection diode.

So your concept fails the properties of RF, physics and EE antenna theory.

But keep trying. IT often takes 10 to 100 good ideas to get 1 great one.

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