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Return to Previous Lesson: Brighten Up Your Day with LEDs and Switches

Lesson Overview:

Now we'll learn about using transistors!

Step 1: NPN Transistor

The NPN transistor is a current amplifier. Small current between the base and emitter is used to control a larger current between the collector and emitter.

In the circuits below we can see how the NPN transistor amplifies a signal.

Tip: use the NPN transistor to drive modules that require high current loads like the motor.

  1. Using your printed NPN circuit, try placing your fingers on the two pads where the resistor or switch used to be.
  2. Try placing some of the other non-conductive objects from Lesson 2 on the pads. Are they conductive enough now?
  3. Stuck? HINT: Did you know, the LED is probably dimly on when you aren’t touching the pads? Oil from your fingers and minerals in the paper are conductive enough to activate the NPN transistor and turn on the LED.

Step 2: Connecting Components

Now you'll use to conductive pen to complete the circuit.

  1. Select the conductive pen tool.
  2. Connect the positive(+) of the battery to one side of the LED . Connect the negative(-) of the battery to the emitter on the NPN transistor.
  3. Connect the other side of the LED to the collector on the NPN transistor.
  4. Connect the SPST switch between the base and collector of the NPN transistor.

Step 3: Simulating

When simulating the circuit you can see when the SPST switch is open the LED is off (or very dim). When the SPST switch is closed the LED is on.

  1. Simulate the circuit and flip the SPST switch.
  2. Print out your template and try it!

Step 4: Using a Resistor

Using a very large resistance, like 100,000 Ω (100 kΩ), in series with a LED only allows a very small current to flow and the LED is very dim. See what happens if we use that resistor to trigger the NPN transistor.

  1. Select the SPST switch and use the trash can icon in the upper right to delete it from the workplane.
  2. Use the Modules + tab to add a 2-Pin + Resistor module to the workplane.
  3. Place this module where the SPST switch used to be in the circuit.
  4. Set its resistance to 100 kΩ.
  5. Simulate. How does the brightness of the LED compare to when it was in series with the resistor?
  6. You can use the circuit you already printed to try this out with your kit.

Step 5: Another View of the Low Current

The large resistor connected to the base of the transistor makes the current going through it very low.

Let's add another LED to see just how low.

  1. Select the conductive ink trace between the resistor and the collector of the transistor.
  2. Click the trashcan icon to delete it.
  3. Add another BI-LED module using either the "Modules" button or copy and paste the one that already there.
  4. Place the new BI-LED somewhere above the resistor.
  5. Using the virtual conductive pen, connect the top of the resistor to one side of the new BI-LED.
  6. Connect the other side of the new BI-LED to the + terminal of the battery. (Be sure not to cross other ink traces).
  7. Simulate the circuit.

Step 6: Observe the Current

The new BI-LED that you added should appear either very dim or completely off. There is a very small current running through it. The current is not enough to light the LED, but it is enough to "turn on" the transistor.

  1. Select the conductive ink trace between the resistor and the collector of the transistor.
  2. Click the trashcan icon to delete it.
  3. Add another BI-LED module using either the "Modules" button or copy and paste the one that already there.
  4. Place the new BI-LED somewhere above the resistor.
  5. Using the virtual conductive pen, connect the top of the resistor to one side of the new BI-LED.
  6. Connect the other side of the new BI-LED to the + terminal of the battery. (Be sure not to cross other ink traces).
  7. Simulate the circuit.

Step 7: Touch Sensor

Remember the conductivity meter circuit? Your skin was not conductive enough (or too resistive) to allow the LED to light up. Now we’ll try using an NPN transistor to help us conduct.

  1. Using your printed NPN circuit, try placing your fingers on the two pads where the resistor or switch used to be.
  2. Try placing some of the other non-conductive objects from Lesson 2 on the pads. Are they conductive enough now?
  3. Stuck? HINT: Did you know, the LED is probably dimly on when you aren’t touching the pads? Oil from your fingers and minerals in the paper are conductive enough to activate the NPN transistor and turn on the LED.

Congratulations, you have completed this project!

Check out other great projects here.

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