Is your voltage regulator one of the newer "low-dropout" types (needs only
5.5volts in for a 5 volt output). If instead you have one of the old designs
(like the 7805 and LM340-05) the output sags if the input falls below about
7 volts. A green LED goes dark if it's leads see less than 2.1 Volts. I
guess if you connected a green LED (with a series resistor) from the input
side to the output side of your (old?) regulator it would go dark about the
time the regulator is dropping out. Although the current through the LED is
passed around the regulator, it shouldn't upset the regulation if the
minimum load is always more than the LED current.
The LED goes dark when the battery is low, instead of lighting up (the
opposite of the action you asked for), but it is a simple design (just an
LED and a resistor) that puts no additional drain on the battery.
From: Gray Chris [mailto:GrayC@...
A real cheap solution is to just make a comparitor using an op-amp or a
comparitor and compare the unregulated voltage to a resistor divider
reference setup using your regulated voltage.
Sent: Monday, July 31, 2000 3:43 PM
Subject: Low battery indicator?
I would like to add an LED to my PCB that will light when the
main battery is discharged to a preset level.
Lumex makes a low battery indicator (SSL-LX5099LBI-SRD) that
is in effect an adjustable-voltage indicator that lights when the
voltage drops below a preset level. Is there a better solution
available? If I were to use this indicator should it monitor the
unregulated voltage in or the regulated five volts (i.e. with the
detection voltage set to 4.95v)?
Any suggestions would be appreciated.