Firstly hope this is in the right place (mods help a guy out if not, thanks!).
Secondly, working on a key fob project for a vehicle whereby I'm trying to mostly replicate the wireless authentication that smart key less entry and go systems have, where by they scan for registered keys and perform security checks on them without the need for user interaction. Already aware of security requirements, I'm trying to pick the best RF transceiver module for this task.
I've seen both the nRF24l01+ (and have been testing with) and heard about the RFM69 chipset which features a useful listen mode on it. From what I've read, to conserve battery power and thus extend the life, I have to make sure the MCU is sleeping in its low power mode and turn off the transmitters. Now what I want to know is which module might be best suited for this application?
For the nRF i'd need to periodically wake it up to listen for commands, but this does use a lot of power even if short bursts and greatly reduces battery life where as the RFM chipset seems to have a listen mode that does this already but I wasn't sure on how much power it used.
The other option I considered was having a periodic 125KHz pulse emitted by the car which a dedicated IC would detect and cause an interrupt on the MCU, keeping power usage to an absolute minimum but the IC i have is an AMS low power wake up chip. This method would yeild best results but I'm not sure how to use the AMS chip when its talking about LC tank circuits and also having to design the ping device for the vehicle too.
The battery I want to use is a 3.7v LiPo 300mAh pack with an integrated charger. Whilst the charging is convenient, I'd also like a respectable life out of it.
I presume the in-car device will be the listener and it will be powered from the car battery so, while it should be as economical as possible, battery life is unlikely to be a problem. And if that is correct the transmitter device only needs to be ON when the user actually presses the button so, again, I don't see a battery duration problem.
I only have experience of nRF24s and my applications (radio control) have not needed low power or sleep modes.
Robin2:
I presume the in-car device will be the listener and it will be powered from the car battery so, while it should be as economical as possible, battery life is unlikely to be a problem. And if that is correct the transmitter device only needs to be ON when the user actually presses the button so, again, I don't see a battery duration problem.
I only have experience of nRF24s and my applications (radio control) have not needed low power or sleep modes.
jremington:
Since the fob receiver has to be on most of the time, battery life will be quite a problem.
Currently the fob consists of a SSD13306 OLED, Atmega328P, LiPo cell and charger. It also has the NRF chipset onboard but not in use. The unit is always asleep and powers off everything and will only turn on when the user has pressed a button to give a display before shutting off again. The part I want to happen is that upon placing your hand on the handle or similar which the car can sense, the car will transmit a signal to wake up the keyfob so it can autheticate with it without a user using the keyfob its self (else theyd just use the buttons to unlock which defeats the purpose). So, really i just need to know whether i'm stuck with limited battery life by ocasionally sniffing the air for signals or whether i can use a chip or solution to wake up the keyfob and keep it asleep until the car wishes to wake it or the user presses a button.