I am trying to build an AVR based, RF service calling system. It works like this: someone at the transmitter presses a button and the guy at the receiver get notified with the ID of the transmitter (imagine using this in a cafe to call the waitress, so there will be multiple transmitters with separate ID’s and 1 to 2 separate but identical receivers). Transmitter will have 2 buttons: one to call waitress, and another to get the bill. So essentially this will be one-way communication, and I guess it will be via serial to transmit different information – i.e. ID, call type (?). A range of 100m is more than practical.
I have been searching around on the subject for a while, and I am still not too sure which direction to take, as far as which RF band to use, and whether or not there requires a microcontroller on the transmitter side, which I want to avoid if possible for simplicity and battery saving (on the receiver side I will be using an AVR – with Arduino code-, because I need to drive some 7-segment LED’s to show the ID). My main concerns are reliability and cost.
I hope to be able to run the transmitter out of batteries (e.g. 3V coin batteries or AAA batteries) for about a year. Receivers will be connected to wall though.
I would be obliged if someone could offer some advice on the followings:
-
As far as the RF band, my search for components at some local shops shows me that I can buy (build) the following transmitter/receiver modules: 2.4GHz (NRF24L01), 433MHz and 315MHz. I wanted to use the NRF24L01 with an AVR (can program for the AVR to sleep when not active to save battery, and communicate with the AVR at the receiver side via serial) for the simplicity and the wealth of info on the net, but the issue is that the cost is 4 fold compared to the other two. The other 2 modules are Chinese with limited documentation, and I am not too sure yet how to build the transmitter around them (with or without uC), but they are very cheap.
-
In terms of reliability and noise/interference, which band is advisable?
-
In the best scenario in my imagination, if it is easy to use a 433MHz or 315MHz module (for the price), preferably without a microcontroller, that will be great. Is this easy to achieve?
I have seen some cheap garage door remotes based on these bands, with a few buttons on them and a coin battery that run for a year. They are very similar to my transmitter. I just don’t have some working schematic so I don’t know how they work, how they are supposed to be “programmed” to have different ID’s and to communicate with the AVR at the receiver side.
I hope to receive advice on which direction to take. Thanks in advance.
Dave
Best Answer
I did a job at 434MHz that used tiny 0.25mW transmitters and a tiny PIC. Battery life was good - over a year because the transmitter consumed virtually nothing for about 1 minute then woke up, transmitted its small payload and went back to sleep (about 50 ms up-time). There were about a hundred transmitters all monitoring the temperature of individual freezers - basically they were "alarms" to warn of impending defrost should one of the freezers become faulty. One central receiver.
Conceptually I don't see a difference - you have a bunch of transmitters all of which can randomly talk to a central receiver. Transmitter current consumption when transmitting was about 30mA (from memory - it was in the early 90s so my memory is not that great on this).
I got the transmitter modules from a UK company called Radiometrix - they were FM 434MHz devices.
Because of the potential for transmit collisions I would also recommend a display that all customers can see that tells them their request is being processed.