Radio > EmComm/Emergency Power

low voltage alarm


so I got fancy LiFePo4 lithium batteries to run the radio, it is a 4 cell set, and I have 2 of them, a 4 AH and a 20AH set
set up the charging circuit, (that was easy)
but over discharge is a big worry, it can trash your fancy batteries in moments
some batteries have protection circuits built in, but I have had them mess up with the RF energy near the radio,
so I got my batteries with no protection at all (I cell balance separately )
I just watched the volt meter closely and tried not to mess up,
finally got tired of doing this and built a low voltage alarm

it has 2 alarms, one is an LED warning that you are getting close set at 12.90V,
the other is sound that is quite annoying that is set for 12.40V
it uses an LM393 and a 5.1V zener diode, so should be easy to find parts locally (if anyone else is going to build one)

I managed to get the standby current down to 2.12mA at my battery charging voltage of 13.35V
I pushed the trade off of voltage stability VS low power use as close as I could with the parts I had, and I am not far off from ideal.
from one day to the next it stays within one milivolt of accuracy even when dropping it some just to test,
cutting off the extra bolt length for the buzzer with a hack saw (vibration test) made it move about 4mV

I used what parts I had, so they are measured values on the resistors,
I did not go through all the extra work to check if this thing could be mass produced with standard parts and still work in a reasonable temp range and all that, (pretty sure it will though), but I did make sure the IC and zener could be replaced with any brand without issue.

having run out of most of my prototyping supplies, I just "point to point wired" it just like I use to do when I was a little kid with no money,
schematic and picture are attached

before I built this I looked on the web for low voltage alarms, and there is just not much out there,
and none that took low power and would work for the LiFePo4 and did not use a special chip that I have no way of ordering,
so I hope the schematic being out on the web helps someone.

Nice project Space.  Thanks for sharing. 


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