Automatic Holiday Light Display
Goal of Experiment
• Design an automatic light display in which a set of
blinking lights (LEDs) turns on as the amount of light
detected by the CdS photocell decreases below a
specified value.
• The set of blinking lights should turn off when the light
detected by the CdS photocell increases to the same
specified value.
• However, the set of lights should not turn on if there is
a transient in the light detected by the photocell (e.g.,
if someone or something momentarily blocks light
from reaching the photocell).
Design Requirements
1. To avoid operational issues that might arise as the
ambient room lighting changes, use a green LED as the
light source for the photocell. Use a trimpot to allow
the user to adjust the current flowing through the
green LED from 0mA to no more than 25mA, thereby
changing the amount of light shining on the photocell.
– You will have to experimentally determine the optimal
distance between the LED and photocell so that you have
some control over the photocell resistance as you change
the current through the LED.
– You may have to optically isolate the LED and photocell
from the rest of the circuit and/or room lighting. I.e., you
may have to cover up these two components so the only
light reaching the photocell is emitted by the green LED.
Design Requirements
2. When the resistance of the photocell has increased
to 200kW, the set of blinking lights should turn on
after a 1 second delay. If the light level on the
photocell increases (and the photocell resistance
drops below 200kW) during the 1 second delay, the
set of lights should not turn on.
Design Requirements
3. Given the response time of human vision, the length
of time that the lights are on should be no less than
50ms for each ‘blink’. Use a square wave generator,
also known as a relaxation oscillator (shown to the
right) to switch the LEDs on and off when the photocell
resistance is sufficiently large.
– The minimum design will use two red LEDs that blink on
and off at the same time.
– Extra credit will be given if three LEDs (one green, one red,
and one yellow) are turned off and on sequentially.
– For this design, the current through each LED should be a
maximum of 5mA. When calculating the current, assume
that all LEDs can be modeled as a 2V battery.
CdS Photocell
• Light sensitive resistor
http://www.advancedphotonix.com/ap_products/pdfs/PDV-P9203.pdf
CdS Photocell
http://www.advancedphotonix.com/ap_products/pdfs/PDV-P9203.pdf
Square Wave Generator:
Relaxation Oscillator
The duty cycle of the
output will be 50%
when R2 = R3.
The frequency of the
square wave output is
given by:
1
f 
2 ln( 3)  R1  C1
The delay in obtaining the square wave output is a result of
- the initial condition of the capacitor (IC = 0V),
- the amount of electronic noise in the circuit,
- the degree to which your op amp is nonideal, and
- the RC time constant of the charging circuit for the capacitor.
Modulated Square Wave Generator
• When the voltage of Vin
attached to R3 is 0V,
the square wave
generator operates as
designed.
• When absolute value of
Vin ≥ V+ (or Vin ≤ V-),
Vo is pinned to V+ (V-).
Independent Circuit Design
• You need to design a circuit that triggers the
square wave generator to oscillator when the
resistance of the CdS increases to 200kW, but
has a delay in the trigger of 1 second.
Report
• A .pdf file using the electronic assignment link on the
course Blackboard site.
• The report must include
– a circuit schematic,
– a description of the operation of the subcircuits that were
designed to meet the three design specifications listed in
the experimental procedure including appropriate
calculations,
– one or more plots from PSpice simulations of the circuit
that show that the design specifications were met
theoretically, and
– a short conclusion in which any deviations from the
expected results are explained.
Pspice Simulation
• The PSpice simulation may have to be done on
subsections of the circuit because of the
limitation on the number of nodes (64) and
components (20) in the student and demo
versions of OrCAD PSpice.