Battery-operated equipment often
requires a low-voltage monitor to warn
the user when the battery voltage is too
low or to perform other functions, such
as power-source switching or device
shutdown. Many specialized ICs are available
to satisfy this need, but most target
applications that use fairly low-voltage
lithium or NiCd batteries. Thus, they’re not
directly compatible with applications using
lead-acid batteries, which may go as high as
14 V during charge.
If you need only a simple logic-level
change in response to a low-voltage condition,
you don’t have to overpower the design
with sophisticated battery-management
circuits. A comparator and a few additional
parts can do the job instead.
The figure shows a monitor designed for
inexpensive circuits powered directly from
the battery, without an intervening voltage
regulator chip. The monitor requires only a comparator, a voltage
reference, four resistors, and a capacitor. When the battery
voltage is greater than the design threshold, the comparator
output is high. When the battery voltage drops below the
threshold, the output goes low and remains low until reset by
recycling the applied power.
The comparator, U1, is an open-collector (open-drain) type,
and the reference is an LM136-2.5. When the battery is connected
to the circuit, the reference voltage at pin 2 of U1 will
rise rapidly to 2.5 V and remain there. The capacitor, C1, will
pull U1’s pin 3 above the pin 2 voltage and the comparator output
will go high.
As the capacitor takes on a charge, the voltage at pin 2 will
assume a value determined only by the voltage divider comprised
of R2, R3, and R4. This relationship can be used to set
the battery voltage at the comparator trip point:
