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).

CdS Photocell Light sensitive resistor

CdS Photocell

Design Requirements 1.Design a voltage divider with a fixed resistor is in series with the CdS photocell. – The resistance of a CdS photocell decreases when the active area is illuminated with an increasing intensity of visible light. 2. The change in the voltage across the CdS photocell should be used to trigger the operation of the oscillator that provides power to the LEDs. – You should set the reference voltage on a voltage comparator after determining the the voltage across either the CdS photocell or the fixed resistor in the voltage divider when the photocell is not illuminated.

Design Requirements 3. When the resistance of the photocell has increased to whatever your threshold it, the set of blinking lights should turn on after a 1 second delay. If the light level on the photocell increases (and the voltage across the photocell decreases) during the 1 second delay, the set of lights should not turn on.

Design Requirements 4. 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 to switch the LEDs on and off when the photocell resistance is sufficiently large. – The minimum design will use a red LEDs that blink on and off while a green LED blinks off and on. – 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.

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:

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 some value when in the dark (you need to measure this), 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.