If your incadescent light need a battery, you can connect it directly to the batttery for which it is rated. The LEDs, on the other hand, should NOT be connected directly to a battery. If you try this, you will fry the LED. You need a resistor in series to limit the current. Try a 330 ohm resistor first. If the LED is too dim, then use a 100 ohm resistor.When your LED is operating, there is a voltage drop across it. Measure this voltage with your volt-ohmmeter. Record the color of the LED and the voltage drop for several LEDs. Do you see a pattern?
LED Light Source
A LED will only pass current in one direction. If you cannot light your LED, dry reversing its leads. Most LEDs have one lead longer than the other. The shorter lead should be connected to the negative terminal of the battery.
A photoresistor changes resistance depending on the light falling on it. To use it, simply attach its leads to a volt ohmmeter and set the meter to measure ohms. A photovoltaic generates voltage in response to light. To use it, simply attach its leads to a volt-ohmmeter and set the meter to measure voltage. The photodarlington, photodiode, and LEDs all act like gates or valves that allow current to pass through.. The amount of current in one direction is related the light level. For these, you will need a battery to supply the current and a way of measuring the resulting current. The correct way of connecting each device is shown below.
Diode and Photodiode
(The photodarlington, not shown, is quite like the phototransistor.) If your meter can measure current, you are in luck. Otherwise create a current meter by using your volt-ohmmeter to measure the voltage across a resistor the current goes through. Ohm's law in the form I = V/R will tell you the current I through the resistor R when you measure a volatage V across it. For instance, if you use a 1 Mohm resistor, a reading of .1 V across it tells you that .1 microamp current is going through it.
Once you have a detector, try it out on the various light sources. Be sure the detector "sees" only the light source and is not illuminated by room lighting or another source. You might do this by making your observations in a light-tight box, or by turning off room lights. Measure the response of all detectors at the same distance from each source. Then double the distance. This should result in one-quarter the amount of light falling on the detector. Does it give one-quarter the response? Look for trends. See what answers you can generate for the questions that began this section.