6.7 Lab Experiments

Lab Three

Important: be sure to show your name, handwritten or typed, visible in the background of all your submitted photos and videos.  You need to do this in order to get credit for your work. 

3.1 Build the following circuit, which will light up an LED when bright light shines on the CDS cell.  You need to choose four resistors in this circuit. Choose a value for Vref so that the LED does not glow in the presence of ambient room light; it should only glow when bright light (such as a flashlight or the light from a smartphone) shines directly onto the CDS cell at short distance.

3.2  Attach one of you breadboards to your assembled car chassis by removing the peel-off backing from the breadboard. Connect one of your car motors to the op amp output in parallel with the resistor & LED output circuit in the circuit from lab 3.1. Does the motor operate when you shine bright light on the CDS cell? If it does, try mechanically loading the motor by letting the car operate on the ground or on a carpet.  You should find the output voltage of the op amp drops to a low value owing to the large current pulled by the motor. The op amp can not supply this current. Build the following circuit,  where you drive your car’s motor using a TIP120 power transistor configured as a low-side switch.  (Note: The video above and the circuit shown below use a TIP31 transistor and a small hobby motor; you should use the TIP120 transistor supplied in your kit and one or both motors from your assembled car chassis.) All resistance values shown in this circuit are example values only. You should use the resistances you came up with for the previous problem.)

Important: For parts 3.1 and 3.2,  the LED and/or motor must remain off during the presence of ambient, or background room light, and they turn on only in the presence of strong light on the CDS cell.  You may not cover the CDS cell during these experiments; instead, the reference voltage for the comparator circuit needs to be set appropriately so that the LED & motor remain off during ambient light conditions.

What to submit:

• Submit two photographs of your circuit from part 3.1, one in ambient room light with the LED off and another with bright light shining on the CDS cell and the LED on.
• Submit a short video demonstrating part 3.2. Your video must demonstrate that the LED and motor from your car are off in the presence of ambient room light, and they turn on when a strong light shines directly on the CDS cell.
• As always, be sure that your name is visible in the background of both the photo and video.