# Lab 4: Design a Logarithmic Voltmeter

```Week 9: Design a Night Light
The experimental procedure
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
http://www.soton.ac.uk/~solids/ir_clip_image002.jpg
Night Vision Vs. In Day Light
Wavelength (nm)
Principles of Vision by M. Kalloniatis and C. Luu
http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=webvision&amp;part=ch24psych1
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
Voltage Comparator
• An operational amplifier (Op Amp) can be
used as a voltage comparator.
Voltage Comparator
• The output voltage (vo) 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 (vo) 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%.
This is NOT the circuit that you will construct.
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.
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 Rlimit 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.
http://www.national.com/ds/LM/LM124.pdf
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
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-W 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 15mA.
• Assume that the green LEDs need 2 V to turn on.
Pre-Lab Report
• The report should be written in Word with images inserted
appropriately. Equations and calculations should be
entered using the Equation Editor in Word. The file should
be saved as a .pdf before it is uploaded for submission on
Scholar.
• The list of what must be included in the report is:
– Measurement of the CdS resistance when illuminated and in the
dark.
– Calculations used to select values of R and Rlimit
– Circuit schematic
– PSpice simulation of the circuit that demonstrates that 15 mA
flows through the equivalent of the green LED (DBREAK and the
1.3 V source) when the voltage drop across the photocell is
equal to Vref
• Clarity of writing, writing style, and correctness of the
Post-Validation Report
• The report should be written in Word with images inserted
appropriately. Equations and calculations should be
entered using the Equation Editor in Word. The file should
be saved as a .pdf before it is uploaded for submission on
Scholar.
• This report should include the analysis, simulation, and
measurements that demonstrate the operation of the night
light.
– 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