Week 7: Design a Night Light New Experiment It is not in the lab manual

Goal Design a circuit that cause a green LED to turn on when the intensity of light on a CdS photocell is below a certain value.

Eye Response

Night Vision Vs. In Day Light Principles of Vision by M. Kalloniatis and C. Luu Wavelength (nm)

CdS Photocell Light sensitive resistor

CdS Photocell The experience last semester was that the CdS photocells didn’t meet the datasheet specifications very well. You will need to measure the dark and illuminated resistance of your photocell to complete your design.

Voltage Comparator An operational amplifier (Op Amp) can be used as a voltage comparator.

Voltage Comparator The output voltage (v o ) of the Op Amp switches to some positive voltage (less than V+) when the input voltage on the v + (or INPUT + ) pin is greater than the voltage on the v - (or INPUT - ) pin. The output voltage (v o ) of the Op Amp switches to some negative voltage (~ V _ ) when the input voltage on the v + (or INPUT + ) pin is less than the voltage on the v - (or INPUT - ) pin.

Modeling the CdS photocell in Pspice Use R_var for the CdS photocell to demonstrate the operation of the circuit as the light intensity is varied. – The minimum value of R_var is the resistance of the CdS photocell measured when it is illuminated. – The maximum value of R_var is the resistance of the CdS photocell measured when it is in the dark. Use a resistor when you want to simulate the operation of your circuit for a particular light intensity. – Assume that the light intensity on the CdS photocell has decreased by 75%.

Example: DC Sweep of Vref Output voltage of the LM324 (voltage marker) will change from V- to V+ when Vref equals the voltage at the - input pin (5V). LED The light emitting diode (discrete as well as any one of the LEDs in the 10 segment LED display) is modeled as a battery in series with Dbreak. A resistor is needed in series to limit the current to 10mA. This is NOT the circuit that you will construct.

DC Sweep Sweep Variable: Voltage Source Name: V2 Sweep Type: Linear Start Value: 0V End Value: 9V Increment: ?

Plot from DC Sweep There should be a sharp transition as the output of the op amp switches. However, I used an increment of 0.5V. PSpice interpolated between the calculated values for the output voltage of the op amp and the current through R limit at Vref = 5V and Vref = 5.5V.

Nonideal Op Amps The output voltage of an ideal Op Amp is either V+ (VPOS) or V- (VNEG). The output voltage of a real Op Amp, such as the LM 324, is not quite 9V. – According to the LM324 datasheet, the maximum output voltage will be 1.5V less than V+, the positive voltage supply powering the op amp.

LM 324 Quad Op Amp

The Notch The recessed “U” on the DIP (dual inline package) package should be matched with the image when looking down at the package after it has been inserted into the breadboard.

Light Emitting Diode (LED) A nonlinear component that ‘looks’ like a battery when it is on and an open when it is off.

Discrete LEDs

Anode vs. Cathode Switch your DMM to the diode symbol. Place the red probe into the V-  plug and the black probe into the COM plug. Place your probes across the diode. – If the result is a very small number, then your red probe is contacting the anode and the black probe is contacting the cathode of the diode. – If the result is an overload (overflow) condition, then the red probe is contacting the cathode and the black probe is contacting the anode of the diode.

Simulating a LED in PSpice No LED part in the student PSpice so we use a series combination of parts. – Dbreak (diode breakout part) Allows current to flow when the voltage on the anode is 0.7V higher than the voltage on the cathode. – Vdc Set to the difference in the voltage needed on the anode to turn the LED on – The green LEDs need at least 2V to turn on.

What do you have to design? The method used to trigger the change in the output of the Op Amp when the resistance of the CdS photocell increases beyond a specific value. – You will define this resistance based upon your measurements of the resistance of the CdS photocell. The resistance used to limit the current through the green LED when the output of the Op Amp changes. – The current through the LEDs should be designed to be 10mA. Assume that the green LEDs need 2V to turn on.

Post-Validation Report There should be some discussion about the performance of the circuit with some explanation provided on any deviations in operation from that expected from the results obtained from your analysis or modeling.