A photo transistor is similar in construction to an ordinary bipolar junction transistor. But while a normal bipolar transistor is controlled by applying current to the base, in a photo transistor the base is light-sensitive. Normally only the collector and emitter of a photo transistor are connected to a circuit, and the device is switched by shining light on the base. A photo transistor is useful for using light to control circuits that aren't electrically connected or for applications such as a porch light that turns on automatically when it gets dark.
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Connect the collector of the photo transistor to the supply voltage through a load resistor, called Rc. You will determine the value of the load resistor later, based on how you need the transistor to operate. Connect the transistor emitter to ground. In this configuration, called a common emitter configuration, the output of the transistor is available at the collector. The voltage at the collector goes high when light hits the transistor, and it goes low when the transistor is dark. Current is amplified at the collector by the transistor's gain value.
Connect the photo transistor in a common collector configuration, also called an emitter follower, by connecting the collector to supply voltage and the emitter to ground through a load resistor called Re. The output of the transistor is available at the emitter. A common collector photo transistor operates similarly to a common emitter configuration.
Use the transistor as a switch by choosing a value for the load resistor based on the maximum expected current when the transistor is exposed to light. Choose a value R for the resistor so that R times maximum current is less than the supply voltage. In this case, when the transistor is exposed to light it will be saturated, or fully on. When it is dark, the transistor will be off.
Use the transistor in "active" mode by choosing a load resistor value R such that R times the expected maximum current when lit is greater than supply voltage. The output level will be roughly proportional to the amount of light reaching the transistor.
Connect the base of the transistor to ground via a resistor to bias the transistor, setting a minimum light level required to activate it. High base resistor values stop small amounts of light from activating the transistor, making it more reliable as a digital switch. Not every model photo transistor makes the base connection available.
Tips and warnings
- A component called an optocoupler pairs an LED and a photo transistor in a light-sealed package, so that lighting the LED activates the transistor. This way, the voltage switched by the transistor is isolated from the control signal. This is functionally similar to how the starter relay in your car uses the small ignition current to switch the large current from the battery that starts the car, while keeping the two currents separate.
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