Hands On Physics

Messing Around


This Sun Photometer does not use a standard photodiode for a light detector, it uses a green LED instead! LED stands for Light Emitting Diode. As you can see from its name, it is a diode, as is the photodiode. Although they emit light very nicely, LEDs have a very small response as light detectors. The tiny response of these diodes needs to be amplified. This activity shows how to do this with a simple electronic circuit that uses an important kind of chip, called an operational amplifier, or opamp. An opamp can be configured to increase the signal (current) from a diode and convert it to a measurable voltage. This detector and its amplifier circuit is the heart of this Sun Photometer

Questions to think about:


To amplify the signal from the diodes, you need to build a simple circuit on a breadboard.
For this, you will need:

Some tips to help you make a circuit that works:


Build the amplifier circuit illustrated below.

Figure M8
Detector Circuit

The opamp is the large triangle. Connect the two batteries in series as shown and define their common point as ground, symbolized as three parallel lines forming a triangle. Connect the three ground points together. Also connect the three ­p;9 V points and the two +9 V points.
Use a 1 Mohm resistor for R.

The schematic shows numbers for the pins (legs) of the opamp. The following illustration shows how to figure out which pin is which.

Figure M9

Look down on the top of the opamp. You will see a notch or round depression near one end. The pins are numbered in order counterclockwize from that point, starting with 1.

This circuit converts the current I that enters its input to a voltage, V, that appears at its output. The relation between the two is

V = -IR

If you arrange for the current to flow out of the circuit, then that is considered a negative current and the output voltage will be positive. Therefore, if the complete circuit is as shown, the voltage measured will increase as the light level increases.


Keep a journal as scientists do, even when messing around. Record all your questions and observations in a bound notebook. Don't erase, just cross out errors. Write enough so someone else could reproduce your experiments. Diagrams can save lots of words and help make your ideas clear. If you make measurements, record your data, with units. Tables are useful for repeated similar measurements. To make the logic of any calcuations clear, first record the formula you used, then show the numbers used to evaluate the formula, and finally your answer.

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