Many applications call for measuring ac lines or high dc voltages. One common technique uses a large voltage divider followed by a buffer.A third solution yields high-accuracy, high-voltage measurements (Fig. 1). The integrator (OP177) supplies negative feedback around the difference amplifier (AD629), forcing its output to stay at 0 V. source

This is a simple circuit used to measure the current from a solar panel. It has very low power loss for currents in the 0-10A range. It also works as a general purpose DC current meter. The circuit can be used on either the positive or negative side of a DC circuit. source

This circuit use to measurement the different wavelengths of a pair of LEDs to stimulus for a rudimentary spectral analysis during quality-control sorting operations. By measuring the reflectance of each LED which can provide information about the spectral response of the object under test. The circuit’s operation is akin to the human eye’s using three receptors to deduce information about the entire visible spectrum.source

This circuit is a simple visible light or IR detector for testing any equipment which outputs light. The circuit can be also used for measuring ambient light levels. The circuit is based on one photodiode which is used in photocell mode so that the photodiode generates a voltage and current signal when it get light.The circuit is best used when you connect it directly to the input of your oscilloscope. Then set the input to the DC inout setting.source

This is a frequency counter based on PIC16F84 for 100Hz-2.5GHz, using LCD 16 chara.x 2 lines ,TCXO 12.8MHz(1ppm ) and PIC16F84. The Voltage(0-510mV) of the input signal is also indicated on the LCD , therefore tuning of the coil can be conducted while checking the frequency. Furnished with IF offset-function, this can be applied for the frequncy display of the home brewed transeiver. source