Instructions for Design Part 1
Consider the Wheatstone bridge above. In Wheatstone Bridge Thevenin Equivalent Problem, I
had mentioned that this circuit can be used for measuring changes in variable resistors. In this
assignment you will be asked to design a resistive sensor circuit that will use a photoresistor to
detect when the lights are off in a room.
Our goal is to design this circuit to meet the following design specifications.
1. Vout = 0 when all lights in the room are on (photoresistor is not covered)
2. Vout > 0 when all the lights in the room are off (photoresistor is completely covered)
3. Vout = your calculated value when all lights in the room are off
Derive the equation for Vout in terms of Vs, R1, R2, R3, and R4.
We have to select the resistors (R1, R2, R3, R4) to be meet these specifications. All resistors must
be chosen from this Parts List. One of these should be chosen to be a photoresistor. What is the
resistance of the photoresistor in a fully lit room versus a completely dark room? You can find
this in the photoresistor link. Use this information along with your expression for Vout to decide
where to place the photoresistor and how to choose the values of the other resistors to meet the
There is also plenty of information online about how to use a Wheatstone bridge for
sensing. Your textbook also has a section on the Wheatstone bridge which I encourage you to
Sketch your design and perform hand calculations and simulations to verify that it meets design
In simulation, use a potentiometer (variable resistor) to model the photoresistor. Make sure this
potentiometer has the same resistance ranges as this photoresistor.
Instructions for Design Part 2
We will now explore using the design in part 1 as a night light circuit.
Wire the LED above (in simulation) to Vout. I selected this LED because it has the closest
specifications to the LED you will use in your lab. Observe if the LED turns off and on when
the lights in the room are on and off.
Let’s explore this behavior using Thevenin’s theorem.
We will use Thevenin’s equivalent circuit theorem to determine the maximum power that can be
delivered to an arbitrary load resistor for your design. Then we will compare that to the power
required by the LED to operate.
Derive the Thevenin equivalent of your wheatstone bridge design for the case with lights
off. Sketch this equivalent circuit model. Calculate the maximum power that can be delivered
to a load resistor.
Demonstrate your Thevenin equivalent circuits and maximum power calculations in MultiSim.
What is the operating power to turn on the LED based on it’s specifications?
How does that compare to your maximum power delivered?
Can you suggest any potential and reasonable re-designs of the circuit? Explain how this redesign could help performance (you do not have to demonstrate this re-design, just suggest a
reasonable approach). If you suggest modifying or adding components, then be sure to only use
components from your Parts List,
These solutions may offer step-by-step problem-solving explanations or good writing examples that include modern styles of formatting and construction
of bibliographies out of text citations and references. Students may use these solutions for personal skill-building and practice.
Unethical use is strictly forbidden.