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RF Wakeup - RFID without the ID

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timpa04

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I'm creating a "keyless" entry system using an nRF24L01+ and an AVR on both sides.
The base units power consumption isn't a concern, however the keys power consumption is.

The key will be powered by a CR2032 battery, and I would like to have an average consumption <14 µA.
I've played around with duty cycles, but I can't get the consumption low enough since every message takes about
25 ms from wakeup until the message bounces back.

The only solution I can think of is having another RF module attached which would handle nothing but wakeup.
It would be perfect if there is something like a passive receiver with a range of 1m-3m which would send an IRQ
when it receives a signal.

Do you have any suggestions?
 

The average power consumption is mainly a matter of duty cycle. Your description isn't informative in this regard. A more detailed explanation of the intended system operation would be helpful.
 

ON consumption: 17mA
OFF consumption: 4 µA
Lowest on-time / transmitted message: 25ms

If my calculations are correct, the average consumption would be around 88µA if the key
sleeps for 5s between messages (5s is a long time, I would like it to be around 2s).
The lifetime on a 230mA battery would realistically be around 75 days?
400 days is a minimum!
 

My first doubt is about 25 ms on-time. nRF24L01 datasheet specifies 1.5 ms startup time from power down, so there should be a way to design the communication more effectively.

The general question is if 2.4 GHz is the suitable band for your application. A purely passive receiver which 3 m range isn't easily achievable with permitted 2.4 GHz power levels. Particularly the receiver might be triggrered by other equipment using the band
 

I've had the same doubts about the message round-time, haven't found a solution yet.
However, even if I was able to cut down the on-time, it would have to come down to 4ms or pref lower which might be impossible.

Do I have to limit myself to 2.4 GHz? How about using the 2.4 GHz band for the handshaking coupled with another receiver (using another band) to wake the main system up?
 
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