Session 3: Transistor Light Dimmer

Justification:

Transistors are often used as switches. The amount of voltage applied to the input will determine if the output is on, off, or somewhere in between. If we can understand the basics of how a transistor accomplishes this, we can build many different devices. In this session we will study how the transistor reacts to different inputs.

Science and Engineering Practices in the NGSS:

http://www.nextgenscience.org/sites/ngss/files/Appendix%20F%20%20Science%20and%20Engineering%20Practices%20in%20the%20NGSS%20-%20FINAL%20060513.pdf

1.  Defining problems (for engineering)

3.  Planning and carrying out investigations

4.  Analyzing and interpreting data

6.  Constructing explanations (for science)

7.  Engaging in argument from evidence

Problem Definition:

Control the intensity of an LED by manually changing the voltage at the input of a transistor.

Time Required: Approximately 30 minutes

Materials/Equipment:

1-2N7000 transistor: http://www.digikey.com/product-detail/en/2N7000/2N7000FS-ND/244278

2-3V disk batteries: http://www.digikey.com/product-search/en?vendor=0&keywords=P138-ND

Copper tape: http://www.digikey.com/product-search/en?lang=en&site=us&KeyWords=3M1181A-ND&x=11&y=11

1-Light Emitting Diode (options: Blue, Yellow, Surface):

http://www.digikey.com/product-detail/en/C503B-BCS-CV0Z0461/C503B-BCS-CV0Z0461-ND/1922944

http://www.digikey.com/product-detail/en/LTL-4273/160-1966-ND/3198546

http://www.digikey.com/product-search/en?vendor=0&keywords=516-1410-ND

1-4x5.5” card stock

Lead pencil

Digital Multimeter (DMM)

Background:

The human body has a resistance of between 1kΩ – 100kΩ depending on age, gender, size/weight, and a verity of other factors [1, 2]. Also the exact path through the body will drastically effect this number (e.g. if the current passes from the right hand to the left foot versus from one adjacent finger to the next). Let’s assume everyone here is around 1kΩ. This allows the body to interact with circuits – sometimes for good (Biomedical applications) and other for bad (Electric shock). Our application today is very safe and will not cause electric shock of any kind – you can safely touch any part of this circuit.

Pencil lead is a very poor electric conductor; but, it will still conduct some electricity. A strip 3.5 inches long by 3/16 inch wide provides approximately 50MΩ (50,000,000Ω).

Transistors act like a light switch, except rather than require a human to “flip the switch,” a transistor turns on/off by detecting an electrical signal. As shown in Figure 1, humans flip a light switch up and the switch makes a connection to the power source (left), or flip light switch down and the switch removes the connection (right ). Notice the human finger (input) is completely isolated from the electric circuit (output). We also know that some light switches allow for “partially on” states (not quite all the up or down), and these switches will allow the user to very the intensity of the light being controlled.

Figure 1: Human hand operating a light switch

One type of transistor, the MOSFET, is a “voltage-controlled, current-switch.” This means that voltage applied to the input effects the current allowed at the output. As with the human hand, the controlling voltage (input) is completely isolated from the controlled circuit (output). This concept is shown in Figure 2. On the left a voltage is applied to in the input of a transistor and a connection is made between the other 2 terminals (shown by the dotted line). On the right the voltage is removed from the transistor’s input and the 2 terminals are disconnected. In the middle a very small voltage is applied, a connection is made at the output, but it is very narrow and does not allow much current to flow.

Figure 2: Voltage controlled currents using a MOSFET

Procedure

Completing the circuit:

1.  The parameter of the circuit has been completed for you, Figure 3. Two 3V batteries are connected here to create a single 6V source. Notice, if the transistor was turned on, current would be allowed to flow from the positive side of the battery through the LED, through the transistor, to the negative side of the battery – a complete circuit.

2.  Place a 1 inch (approx.) copper tab on the input pin of the transistor (should be covering the pin and be oriented parallel to the top and bottom conductors. This will be used as a connection point, but right now should not touch anything other than the transistor.

3.  Using a pencil, completely shade in the box; the idea is to transfer as much lead as evenly as possible to the paper. Place 2 copper-tape tabs connecting the end of the box to the main conductor.

Initial circuit analysis:

4.  Initial observations:

  1. Can you get the LED to turn on/off using only your fingers?
  1. Can you get the LED to dim or brighten using only your fingers?
  1. Discuss with a partner and predict the reason why you are able to do this?


Advanced circuit analysis:

5.  Have your partner measure the voltage between the transistor’s input (positive) and the negative side of the battery (negative) while you control the circuit so that the LED is:

  1. Fully off: _____
  1. Dim: _____
  1. Bright: _____
  1. Fully on: _____

6.  Using the concepts discussed in the Background section above, explain why you are able to control the voltage at the input of the transistor. Discuss with a partner.

References:

[1] https://www.osha.gov/SLTC/etools/construction/electrical_incidents/eleccurrent.html

[2] http://en.wikipedia.org/wiki/Electric_shock

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