Hmm, what you are trying to achieve has already been included in the Keeloq devices of Microchip.
That is what I use now instead of the old coding chips or soft-coded scheme.
But, the current flow is not the key factor in this case. It is the voltage level and the keeloq devices that is exactly what they measure.
More, current is a little more difficult to measure when you don't have linear consumption. Keyfobs are pulsing current out of the battery, so you have somehow to integrate the measurements and find the actual average current flow.
But I insist on that you need to measure the voltage level only and you will be more that good.
How will the result be indicated is another matter. Either locally on the LED as you stated or remotely by another indicator with battery state transmitted by the keyfob.
Ioannis
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