A transistor is an electronic component that functions like a switch. Most transistors have three pins that are used for electrical connections: the base, the collector and the emitter. When a DC voltage is applied to the base through a resistor, a small amount of current is created. This "turns-on" the transistor and connects the collector to the emitter. When the DC voltage is removed from the base, the transistor is "turned-off" and the collector-emitter connection is broken. Because of the switch-like properties, a transistor is ideal for replacing analogue switches, such as toggle switches. The best way to understand the transistor as a replacement is to build a circuit which mimics the operation of a toggle switch.
- Skill level:
Things you need
- LM3046 transistor array IC
- LM3046 datasheet
- Electronics breadboard
- Jumper wires
- 7805 voltage regulator
- 7805 datasheet
- Nine-volt battery clip
- 1K resistor
- 330 ohm resistor
- LED (general-purpose, low-voltage)
- Nine-volt battery
Insert the LM3046 transistor array into the breadboard so that it is vertical and the notch is on top. Pin 1 of the LM3046 is to the left of the notch. The LM3046 should straddle the blank breadboard column that divides the rows into A-E and F-J sections.
Connect pin 13 of the LM3046 to the breadboard ground bus. The ground bus is a breadboard column labelled with a blue or black "-." Use a jumper wire for the connection. Pin 13 is labelled "Substrate" on the LM3046 datasheet. It provides isolation for the transistors in the IC.
Insert the 7805 voltage regulator into the breadboard so that the three pins are in separate rows. The 7805 should be vertical, similar to the LM3046.
Connect the 7805 pin labelled "GND" to the breadboard ground bus. Connect the 7805 "Input" pin to the breadboard power bus. The power bus is a breadboard column labelled with a red "+." Use jumpers for the connections and refer to the 7805 datasheet for the pin description. With the black, printed side of the 7805 facing you, pin 1 (left) is Input, pin 2 is GND and pin 3 is Output.
Connect the red wire of the battery clip to the breadboard power bus. Connect the black wire of the battery clip to the breadboard ground bus.
Locate the collector, emitter and base pins of transistor Q4, in the LM3046. The transistor symbol designates the three pins. The emitter is the pin at the point of the arrowhead, the ase is the pin near the flat line and the collector is the other pin. In the LM3046, the Q4 collector is pin 11, the emitter is pin 10 and the base is Pin 9. See "The Junction Transistor" in the Resources section for symbol information.
Connect one leg of the 1K resistor to pin 9 of the LM3046. Insert the other leg of the 1K resistor into an open area of the breadboard. Connect a jumper from the free leg of the 1K resistor to the 7805 Output pin. This connection provides five volts to the 1K resistor, which creates a small amount of current. That current operates the transistor and makes the connection between the collector and the emitter.
Connect one leg of the 330 Ohm resistor to pin 10 of the LM3046. Connect the other leg of the 330 Ohm resistor to the anode (longer leg) of the LED. Connect the LED cathode (shorter leg) to the breadboard ground bus.
Connect pin 10 of LM3046 to the 7805 Output pin. This connection provides five volts to the collector. When the transistor is on, five volts will travel through the emitter and will turn-on the LED.
Attach the nine-volt battery to the battery clip. The circuit will be "on" as soon as the battery is attached. The LED should be lit.
Remove the jumper-end that connects 1K resistor to the 7805 Output pin. The LED will turn off because the voltage that creates the current at the base of the transistor is disconnected. Connecting and disconnecting power at the base is how the transistor operates like a switch. It is similar to moving a toggle switch on and off.
Connect the jumper-end to the 7805 Output pin. The LED will light again. This demonstrates how the IC transistor can take the place of a two-position, on/off toggle switch.
Tips and warnings
- This circuit uses a single transistor for a two-position toggle switch. For a three-position toggle switch, two transistors would be required. A push button switch with a JK flip-flop can be used to control the transistor and can maintain the transistor in either the on or off position. A microcontroller can also be used to control the transistor.
- When replacing a toggle switch with a transistor IC, make sure that the IC ratings match the voltage and current ratings of the toggle switch. A lower-rated transistor IC will burn out and possibly damage the surrounding circuit in the device.
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