This circuit is used to measure the voltage on a 12V (nominal) lead acid rechargeable battery system. It was specifically designed for use in solar powered systems, but is general enough that it can be used for automotive or other 12V systems. Lead acid batteries normally spend their working lifetime in the voltage range of 11-15 Volts. This meter circuit was designed to show the voltage range of 10-15V on an analog meter movement, it can be used to show the battery charge state from empty to full. source

This circuit provides an audible and visual low voltage warning for 12V battery powered devices. When the battery voltage is above the set point (typically 11V), the circuit is idle. If the battery voltage should fall below the set point, the LED will light and the speaker will emit a periodic beeping sound to warn of the impending loss of power. The circuit was designed for monitoring solar systems, but it could also be useful for automotive and other 12V applications. source

This circuit uses LEDs to display logic states for high, low, rising pulse, and falling pulse, it is generally useful for debugging logic circuitry.This circuit can monitor TTL logic levels. TTL compatible CMOS logic families can also be monitored.This circuit can be built on perforated circuit board and wired by hand. The completed circuit can be assembled into a clear plastic box with the wires protruding out one end. The LEDs should be visible through the box. IC sockets should be used for easy chip replacement. source

This is a low power voltmeter circuit that can be used with alternative energy systems that run on 12 and 24 volt batteries. The voltmeter is an expanded scale type that indicates small voltage steps over the 10 to 16 volt range for 12 volt batteries and over the 22 to 32 volt range for 24 volt batteries. Power consumption can be as low as 14mw when operated from 12V and 160mw when operated from 24V.It is possible to set the meter to read equal steps across a variety of upper and lower voltages.source

This article describes details for constructing a 1-Wire Barometer which will work on a Dallas Semiconductor Corporation 1-Wire network. The goal of this design was to make a simple reliable 1-Wire barometer with reasonable pressure resolution and one which could be constructed without a specialized printed circuit board by a hobbyist.This design uses a Motorola MPX4115 Silicon Pressure Sensor, a Dallas Semiconductor DS2438 Smart Battery Monitor (to perform 1-Wire analog to digital conversion), an operational amplifier, a voltage regulator, a diode, and several resistors and capacitors.source