Response of a Voltage Dividerwith a Photoresistor

ME 120 Noteshttp://me120.mme.pdx.edu

Gerald Recktenwald

Portland State University

Department of Mechanical and Materials Engineering

October 4, 2017

ME 120: DC Circuit Computations with Excel

The Goal

• Show how a photoresistor output can be converted to voltage

• Analyze a voltage divider

• Set up an Excel plot of voltage divider output as a function of resistance

ME 120: DC Circuit Computations with Excel page 1

Photoresistors or Photocells allow circuits to respond to light levels

photons

In an electrical circuitthe resistance dependson light intensity

ME 120: DC Circuit Computations with Excel page 2

Photoresistor in the Tinker Kit

ME 120: DC Circuit Computations with Excel page 3

Voltage Divider Circuit and Output Formula

Rp

Rc

Vs

Vout

I

Apply Ohm’s law for the entire circuit

Vs = IReq =⇒ I =Vs

Req(1)

and again for the voltage drop across the constant resistor

Vout = IRc (2)

Substitute the I from Equation (1) into Equation (2) to get

Vout = VsRc

Req

Or, since Req = Rp + Rc

Vout = VsRc

Rc + Rp

ME 120: DC Circuit Computations with Excel page 4

Information from the Sparkfun Web site

1. Go to the Sparfun web site, https://www.sparkfun.com/

2. Enter “photocell“ in the search box to

get the “mini photocell” product page:

https://www.sparkfun.com/products/9088

The photo to the right is from the

Sparkfun product page.

3. Click on the “Documents” tab

4. Get the datasheet from the link:

https://cdn.sparkfun.com/datasheets/Sensors/LightImaging/SEN-09088.pdf

ME 120: DC Circuit Computations with Excel page 5