Low Battery Indicator

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JerryM909
 
Posts: 2
Joined: Fri Aug 31, 2007 6:08 pm

Low Battery Indicator

Post by JerryM909 »

I purchased the V1.2 kit, assembled it and it works just dandy.
I then did some math and ordered some SMT resistors for the Low Battery indicator. In particular, 249K and 113K plus a 200 ohm to limit LED current. I could not get the Low Battery LED to lite under any circumstances until I noticed that the anode of the LED was tied to the battery supply it was trying to monitor. With a low battery voltage of 1.8 volts there was not enough current to allow the LED to lite. The whole problem was solved when I removed the end of R3 (200 ohms) that connects to PIN 1 of the MAX756 and connected it to PIN 6 (5 Volt output). Actually I CAREFULLY soldered the 200 ohm resistor physically between X1 Pin 1 and the PCB pads at the junction of R3 & LED1. Now there is no resistor at R3. The low battery LED functions perfectly. Even with the battery voltage at 1.8 volts the output on pin 6 is still 5 volts which is enough to provide 25mA of current to lite the LED.
Substituting the batteries with a variable DC power supply, the LED comes on at 1.825 volts and off at any voltage greater than 1.825 volts.
BTW, I used a standard LED and it works so a SMT LED should work just as well.

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moses_410
 
Posts: 69
Joined: Sat Jan 06, 2007 5:00 pm

Post by moses_410 »

hey, could you post a picture or two? also what values did you use fro R1 and R2?
Thanks

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