LightSensortoswitchonaLightoranyDevice.docVIP

Engr003 Mission College Automatic Night Lite Introduction: In this lab an LED is automatically turned on when the ambient light level falls below a threshold. The threshold is determined by adjusting a variable resistor (potentiometer). The circuit contains a transistor that amplifies the current coming into its base. This current controls the LED. Parts: LDR or PhotoCell 10K?? -100K?? (1M ? Dark) (DigiKey p/n: PDV-P9007-ND) NPN 3904 1N400X Diode (optional) 100 K Pot 22 K?? 470 K?? Bright LED (3000-4500 mCd) 9V Battery and battery straps 22 gauge wire Solderless Breadboard The LDR: The Light Dependent Resistor (LDR) is made from a Cadmium-Sulfide compound (CdS). CdS is a semicondutor material. The resistance of the LDR changes as the light level changes. More light produces more electron – hole pairs in the material, making the material a better conductor in light. Resistance decreases (conductivity increases) as light levels increase because there are more charge carriers in the material. Note that R = 1/G (G is conductance) The LDR controls the voltage on the transistor’s base. As it gets darker in the room, the voltage on the base will increase until the transistor is forward biased. This results in high current flowing from the transistor’s collector to the base. This collector current flows to the LED and is responsible for turning the LED on. The Bipolar Transistor: The transistor in this lab is an NPN transistor. When the LDR’s resistance increases in low light, the voltage on the transistor’s base increases. When this occurs the base-emitter junction of the transistor becomes forward biased while the base-collector junction is reverse biased. This is called active-mode. In active-mode current flowing from the collector to emitter is about 100 times larger than the current flowing into the base of the transistor. Thus, the gain is roughly 100. Very small changes in current are used to control the LED. The Circuit: