Project-5 The Transistor Switch

Objective

The objective of this project is to demonstrate how light can switch a transistor ON or OFF to control a device.

General Instructions

This project uses two breadboarded circuits. The circuit shown on the left side of Figure 4.9 is used to generate infrared light. Another circuit, shown on the right side of Figure 4.9, switches on a buzzer when the infrared light is blocked by an object.

Fig 4.9

 The infrared light in this project is generated by a light-emitting diode (LED). In an LED, a current runs through a PN junction that generates light. This same process occurs with all diodes. Infrared LEDs are simply diodes with a transparent case that enables the infrared light to show through. LEDs also have a PN junction made with semiconductor material that produces a large amount of infrared light. Figure 4.10 shows a typical LED and its schematic symbol, the symbol for a diode with arrows pointing outward, indicating that light is generated.

Fig 4.10

In this project, a photodiode detects the infrared light. When light strikes a PN junction in a photodiode (or any diode), a current is generated. Infrared photodiodes also have a transparent case and junction material that produces a large current when it absorbs infrared light. Figure 4.11 shows a typical photodiode and its schematic symbol consisting of the symbol for a diode with arrows pointing inward, indicating that light is absorbed.

Fig 4.11

When the circuits are set up, the buzzer sounds whenever the infrared light is blocked from the photodiode.

Fig 4.12

 Parts List

• One 9-volt battery.
• One 6-volt battery pack (4 AA batteries).
• Two battery snap connectors.
• One 100-ohm, 0.5-watt resistor.
• One 1 k , 0.25-watt resistor.
• One 10 k , 0.25-watt resistor.
• Two breadboards.
• Two terminal blocks.
• One piezoelectric buzzer with a minimum operating voltage of 3 volts DC. Using a buzzer with pins (such as part # SE9-2202AS by Shogyo International) enables you to insert the buzzer directly into the breadboard. If you use a buzzer with wire leads (such as part #PK-27N26WQ by Mallory), you need another terminal block.
• One infrared LED.
• One infrared photodiode.
• One PN2222 transistor. Figure 4.12 shows the pinout diagram for the PN2222.

Set up the circuits shown in Figure 4.9. If you have some experience in building circuits, this schematic (along with the previous parts list) should provide all the information you need to build the circuit. If you need a bit more help building the circuit, look at the photos of the completed circuit in the “Expected Results” section.

Carefully check your circuit against the diagram, especially the connection of the long and short leads to the LED and photodiode. The LED is connected so that it is forward-biased, whereas the photodiode is connected so that it is reverse-biased, as indicated by the direction of the schematic symbols in the circuit diagrams.

1.  Align the rounded top of the LED toward the rounded top of the photodiode with the circuit boards a few feet apart from each other. (If you use a typical LED and photodiode, you must bend their leads to align them.) Note that the rounded top of both the LED and photodiode shown in Figures 4.10 and 4.11 contain a lens to emit or absorb light. Some LEDs and photodiodes have lenses on the side, instead of on the top. If it isn't obvious where the lens is in your components, check the manufacturer's data sheet.
2.  Turn on the power switch. When the power switch is on, the buzzer should sound whenever the photodiode does not sense infrared light.
3. . Bring the circuits close enough together so that the buzzer shuts off.
4. Block the infrared light; the buzzer should turn on.

Expected Results 

Figure 4.13 shows the breadboarded buzzer circuit for this project.

Fig 4.13

Figure 4.14 shows the breadboarded LED circuit for this project.

Fig 4.14

Figure 4.15 shows the test setup for this project with the rounded top of the LED and photodiode aligned toward each other.

Fig 4.15

The photodiode is connected to the base of a transistor. Therefore, current generated by the photodiode turns the transistor ON. When the transistor is ON, VC is about 0 volts, turning off the buzzer. When the infrared light is blocked, the photodiode stops generating current, which turns OFF the transistor, increasing VC, which turns on the buzzer. These circuits work with the LED and photodiode about 7 inches apart. With more complicated photo detectors that have circuitry to amplify the detected signal, this technique can work over several feet. One common application of this technique is a buzzer that sounds when a shopper enters a store, blocking the light, setting off a sound, and alerting the sales staff.