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

The purpose of this project was to create a useful yet easy to build Logic Analyzer that would provide much of the functionality of a real Logic Analyzer. The biggest drawback that the computer has is its sampling rate. The program was written on a 75 MHz Pentium computer which provided accurate sampling rates up to 10 kHz. Although this doesn’t match the speed of real Logic Analyzers, this should be sufficient speed to be useful for many projects. source

This little circuit will help you to scan the line of a 220V live wire in the wall. The FET gate is connected to a capacitive metal sensor (which is usually a simple metal plate). When you hold the sensor close to the wall, the LED will indicate the current flow around where it detects the wire. Works on 9VDC.source