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 Requirements1. 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 Requirements2. 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 Requirements3. 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:

11)3ln(21

CRf

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.