SIX RANGE LIGHT METER

Description: The circuit involves the switching of feedback resistors for an operational amplifier (op-amp) that is driven by a Radio Shack 276-115 selenium solar cell, resulting in a multirange linear light meter. A 1000-megohm resistor is utilized for the highest sensitivity, allowing the star Sirius to generate a photocurrent of approximately 25 pA when the solar cell is shielded from ambient light using a length of cardboard tubing. The power supply for the circuit is 9 V, and the meter has a range of 0-1 mA.

The described circuit functions as a precise light meter, utilizing an operational amplifier configured in a feedback arrangement to amplify the small photocurrent generated by the selenium solar cell. The choice of a 1000-megohm feedback resistor is critical for achieving high sensitivity, enabling the detection of extremely low light levels, such as the faint light emitted by distant stars like Sirius.

In this setup, the selenium solar cell acts as a photodetector, converting light into an electrical current. When exposed to light, the cell generates a photocurrent that is proportional to the intensity of the light. The operational amplifier amplifies this tiny current, allowing it to be measured accurately on the analog meter, which is calibrated to display readings in milliamps.

The switching mechanism for the feedback resistors allows for the adjustment of gain in the op-amp circuit, enabling the user to select different ranges for measuring light intensity. By switching to lower resistance values, the circuit can accommodate higher light levels without saturating the op-amp, while the 1000-megohm resistor setting is ideal for low-light measurements.

To ensure accurate measurements, the cardboard tubing serves as a light shield, blocking ambient light and allowing only the light from the target source, such as the star Sirius, to reach the solar cell. This design feature enhances the sensitivity and precision of the light meter.

The entire circuit operates from a standard 9 V power supply, providing sufficient voltage for the op-amp to function effectively. The output is displayed on a 0-1 mA analog meter, which provides a visual representation of the light intensity being measured. This configuration makes it suitable for various applications, including astronomy and low-light photography, where precise light measurements are essential.Switching of feedback resistors for opamp driven by Radio Shack 276-115 selenium solar cell gives multirange linear light meter. With 1000-megohm resistor for highest sensitivity, star Sirius will produce pho-tocurrent of about 25 pA when aolar cell is shielded from ambient light with length of cardboard tubing.

Supplies are 9 V, and meter is 0-1 mA.-F. M. M..