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All things pertaining to wireless and RF links
By rauld
I have made a wireless motion sensor to let me know if my dog is in the back yard. I have a receiver in the kitchen that lights up when the dog passes by the motion detector. I would like increase it for 4 areas of the house but I only see 315 and 433 MHz transmitters. Are there more frequencies available or am I going about this the wrong way.

Thanks, Raul
By rauld
I thought you couldn't have duplicate frequencies. So I thought I couldn't have 2 sensors operating on 433 at the same time. For example like RC clubs don't want 2 planes on the same frequency flying at the same time. Have I got this totally wrong?
By jremington
Duplicate frequencies are only a problem if a transmitter is continuously transmitting. For sensor data or event notification, only a very short message containing sender ID and the necessary data is needed. Furthermore, having separate frequencies does not guarantee communication. Radio is inherently unreliable.

Remote sensors for home weather stations use a single frequency and transmit data every minute or so, on staggered schedules. If there is an occasional overlap between two transmissions, you lose a couple of readings, but who cares?

For more reliable single frequency transmissions, you can use bidirectional modules (transceivers) with send/acknowledge on each end. NRF24L01 modules are very cheap and can do this, but the programming is more complicated.
By rauld
Thanks, I will look into 24L01. What is the range if you know? I'm probably looking at 100 meters for the farthest sensor.

Thanks, Raul
By jremington
The range depends on the module power, frequency and antenna. The range for the 315/433 MHz modules is generally larger than for higher frequency modules. I get 300 meters line of site range with the cheapest 433 MHz modules, and a good balanced dipole antenna.

For the NRF24L01 (2.4 GHz) you would need the high power modules to reach 100 meters.
By rauld
Ok, so since the signal is only going from the sensor to the board (which is simply turning on a led), and the signals won't overlap because the dog can't be in 2 places at once, I would be better off with the 433 MHz modules.

By stevech
Higher frequency = weaker signal at receiver. Laws of physics.

Two frequencies within the band can be used if you have two receivers.
Better, protocols like Radiohead (freeware) will detect and correct collisions in time via retransmissions (each sent message gets an ACK message from the receiving end). Common practice in wireless.

2.4GHz is MUCH more lossy due to much higher frequency. The Nordic NRF214L01 has a big disadvantage of that 2.4GHz, compounded by it being very low power, like 0.002 watts (2mW) as I recall.