Design a Voltmeter
Experiment 19
Goal
• Design a circuit that displays the value of
unknown voltage between certain voltage
ranges.
– The unknown voltage can be between +9V and -9V
– Only voltages between 0V and +8V should cause the
display on the 10 segment LED display to change.
– A discrete red LED should be lit when the voltage is
less than 0V.
– A discrete green LED should be lit when the voltage is
greater than +8V
When the unknown voltage, V, is:
– V < 0 V, a red LED should be lit, no segments on the LED display
should be illuminated.
– 0 V ≤ V < 1 V, one LED segments should be lit
– 1 V ≤ V < 2 V, two LED segments should be lit
– 2 V ≤ V < 3 V, three LED segments should be lit
– 3 V ≤ V < 4 V, four LED segments should be lit
– 4 V ≤ V <5 V, five LED segments should be lit
– 5 V ≤ V <6 V, six LED segments should be lit
– 6 V ≤ V <7 V, seven LED segments should be lit
– 7 V ≤ V <8 V, eight LED segments should be lit
– V > 8 V, a green LED should be lit along with eight LED segments
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.
Example: DC Sweep of V2
Output voltage of the LM324
(voltage marker) will change from
V- to V+ when V2 equals the
voltage at the - input pin (4.5V).
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 under 10mA.
DC Sweep
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Sweep Variable: Voltage Source
Name: V2
Sweep Type: Linear
Start Value: -9
End Value: 9
Increment: 0.1
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 (supposed to be 1.5V less than V+).
– To measure exactly what the maximum voltage is,
disconnect the all components on the output of the LM
324, place a 1-10 kW resistor on the output pin, and apply
a volt on the + input terminal that is greater than the
voltage on the – input terminal and measure the voltage
between the output pin and ground.
• Use this voltage when you determine the resistor to use in series
with the LED to limit the LED current to 10mA.
LM 324 Quad 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
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.
BE CAREFUL: The 10 segment LED array is more sensitive than the discrete LEDs.
Discrete LEDs
10 Segment LED Bar
• A LED is located between
pairs of pins.
Side view
Top view
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.
Pre-Lab Report
A .pdf file containing:
– Calculations of the resistor values used to create a
voltage divider that will have node voltages used
as the reference signal for the voltage comparator.
– The reference signal will be one of the input voltages at the op
amp.
– A plot of the current through an LED as the
voltage of the input of a voltage comparator is
varied from -9V to 9V using PSpice.
– Select the comparator that is the one that switched the first
two LED segments on.
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
– Our red and green LEDs need between 2-2.5V.
Lab Report
• The report should include:
– A full schematic of the circuit designed and constructed.
– A description of:
• The circuit used to regulate the input voltage between -9V and +9V.
– This circuit will be used to create the unknown voltage.
• The voltage divider used to divide the 9V supply from the ANDY
board into the voltage steps described on Slide 3.
• The operation of the voltage comparators.
• How the current to the LED array and the discrete LEDs was limited.
• The circuit used to light the discrete red LED when the unknown
voltage is negative.
– Includes calculations and PSpice simulations.
– Explanation of any discrepancies in the operation of the
circuit.
– Do not include a validation checksheet mentioned in the
manual.
Limitations of Student PSpice
• 64 nodes
• 20 components
– Complex parts, such as some of the Op Amps,
count as more than one component.
You will not be able to simulate the operation of
your entire voltmeter circuit because of these
limitations. Break the circuit into subcircuits.
Debugging Your circuit
• The best way to determine why your circuit is not
functioning as expected is to separate out the subcircuits
and verify that they work as designed – i.e., measure node
voltages and branch currents and compare them with your
calculations and PSpice simulations.
– Start at the subcircuits at the beginning of the circuit.
• Voltage dividers that are used to define the unknown voltage and the
reference voltages for the comparators.
– Work your way to the LEDs.
• The best way is not to pester the OpEL GTAs until they
rebuild your circuit for you.
– Reasonable amount of assistance from the GTAs is acceptable.
• If you do not know what the voltages should be at certain points of
your design, this could impact your validation grade.