03/01 - Lab 4: Dusk to Dawn Light

Lab 4 - Dusk-to-Dawn Light

In this lab, we created a light-sensitive LED using a photocell, bipolar junction transistor, and a resistor. If all works out, the LED will be unlit if there is light shining on the photocell. When the photocell is covered up (or the room gets dark), the LED should turn on, similar to a night light for kids. We started by running a few calculations through our circuit diagram in order to determine the correct orientation and values of the resistors.

Calculated values of Vb for photocell resistances of 5 kOhms and 20 kOhms.

We found that the resistance of the photocell under light is 11 kOhms, and it is 62 kOhms when covered up. We reran calculations to find Vb for the actual circuit, which we found to be 2.6V. When we measured the actual voltage across the photocell, it turned out to be 2.5V.

Actual set up on the breadboard.

In this video, you can see the light turn on when the photocell is covered and off again when the finger is lifted.

In-Class Examples:

1) In the beginning of class, we were shown a demonstration involving running electrical current through a hot dog. LEDs were placed in various configurations along the hot dog, and we were asked to guess how the hot dog would react to the applied voltage and which lights would turn on. write our responses on a white board.

2) Find current io and voltage vo in the circuit shown:

5) Find vo and io in the circuit:

4) Find the equivalent Resistance of the following network:

5) To determine the voltage across each resistor substitute the expressions derived for the current into Ohm’s law. Write the voltage across each of the resistors.

6) An automotive pressure sensor has an output between 0-12V and will be interfaced to the TI MSP430 that has a 3.3V ADC and can sink a maximum of 10mA. Model the sensor as an ideal DC source and draw schematic for the appropriate interface circuit.