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Electrical Engineering Technology
Department of Applied Engineering & Technology
California University of Pennsylvania
EET 370: Instrumentation I
Spring 2015
DMM Lab
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Table of Contents
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-5
Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-6
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-11
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
List of Figures
Figure 1 Block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 2 (R2 open circuit Voltage vs frequency) . . . . . . . . . . . . . . . . . . . . .
Figure 3 (R2=142.5k circuit Voltage vs frequency) . . . . . . . . . . . . . . . . . . .
Figure 4 (R2=100k circuit Voltage vs frequency . . . . . . . . . . . . . . . . . . . .
Figure 5 Display and driver circuit . . . . . . . . . . . . . . . .
. . . . . . . . .
Figure 6 astable 555 timer (1) . . . . . . . . . . . . . . . . . . . . . . .
Figure 7 One Shot 555 Timer (2) . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 8 One Shot 555 Timer (3) . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 9 One shot 555 Variable Rx for Ohmmeter . . . . . . . . . . . . . . . . . . .
Figure 10 Timing diagram for VCO and timer 3 . . . . . . . . . . . . . . . . . .
List of tables
Table 1
Resistance results
Table 2
Voltage results
. . . . . . . . . .. . . . . . . . . . . . . . . . . . . ..
. . . . . . . . . .. . . . . . . . . . . . . . . . . . . .. . .
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Introduction
This lab allowed students to create a digital multimeter that could measure voltage and resistance. The
design of the multimeter consisted of a voltage controlled oscillator, timers and seven segment display.
Design and theory of the multimeter was implemented producing results as a voltage and ohmmeter.
The outcomes of this lab include design, implementation, troubleshooting and formal analysis.
Figure 1. Block diagram This Block diagram displays the different phases of the DMM
Individual stages
Voltage Controlled Oscillator (VCO) phase (4046 IC CMOS)
The principle behind the VCO is that the VCO accepts a reference voltage and a corresponding frequency
are produced on the output. The center frequency was determined by using the data sheet for the 4046
to achieve a center frequency of 100 kHz with a reference voltage of 5 volts. The 5 volts reference is half
of the source voltage labeled as VDD. The design steps included to start with R2 to be an open circuit
and apply 5 volts for Vref. Then R1 and C1 were decided upon from the specification sheet on the 4046
to achieve the desired center frequency. Adjustments were made until the center frequency was
approximately 100 kHz. After that further calibration was done with R2 to fix the equation of the line.
Figure 2 in the appendix displays Vref versus frequency and the equation of the line for R2 as an open
circuit. As seen the y intercept is at -22. The goal was to have a y intercept close to 0. By adjusting the R2
and plotting the points again the y intercept was adjusted to .15 with a 142.5k value of resistance (figure
3). This allowed a good basis to the linear approach of the design.
Timing Phase (555 timers)
*Timer circuits and values can be found in Appendix under figures
(1) The first timer used was an astable multivibrator that produced the clock. The clock time was
adjusted to be 1 Hz. This was established by using the standard circuit for an astable multivibrator and
adjusting values to achieve the desired pulse. A value for C was decided upon and R was found.
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𝑻=
𝟏
𝟏
=
= 𝟏 𝒔𝒆𝒄𝒐𝒏𝒅
𝑭 𝟏𝑯𝒛
𝒕𝒉 = 𝑻𝑳 = 𝟎. 𝟔𝟗𝟑𝑹𝑪
𝑹=
𝒕𝑯
𝟎. 𝟔𝟗𝟑𝑪
(2)The next 555 timer operated as a one shot timer. The pulse duration was 1 microsecond and
triggered off the negative edge of the clock. This one shot timer was used to control the reset pin on the
driver and LED display. A value for C was picked and R was solved for
𝒕𝒑𝒘 = 𝟏. 𝟏𝑹𝑪
𝒕𝒐𝒏 = 𝟏𝒖𝒔 = 𝟏. 𝟏𝑹𝑪
(3). The last timer was also a one shot timer. The pulse duration was determined to be 5 Milliseconds
based upon the center frequency and desired output on the display. Once the pulse width was
established the design included choosing correct values of resistors and capacitors to achieve the
desired pulse width. This signal was inverted due to the active low of the enable pin on the driver circuit.
The inverted consisted of a transistor circuit. The below equations were used to find the timer pulse
width (tpw). N equals desired output and fvco was the center frequency,
𝑵
𝒕𝒑𝒘 =
𝒇𝒗𝒄𝒐
𝒕𝒑𝒘 =
𝟓𝟎𝟎
= 𝟓𝒎𝒔
𝟏𝟎𝟎𝒌
Display
This stage consisted of three cascaded seven segment displays that would operate as a 3 digit display
ignoring the decimal point. These displays were used with a 4026 driver operating as a decade counter
and seven segment display driver circuit.
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Complete operation as a voltage meter
The operation as a voltmeter can be best explained with Vref at 5 volts on the VCO. Vref was also Vx or
the desired voltage to be measured. In the case of 5 volts there should be a 500 on the display. The last
timer pulse width was determined by knowing the center frequency and the desired output of the
display. Described mathematically as N=tpw *f where N is equal to the display of 500 in the case of 5
volts and the VCO was set to 100 KHz. The timer pulse width (TPW) was determined to be 5ms (timer 3).
When the enable pin is active for 5ms the VCO was oscillating and created a count on the drivers until
the reset pin reset the count of the cascaded displays. Figure 10 shows the timing diagram for timer 3
and the VCO to graphically display the count. The frequency on the VCO due to the voltage that was
being measured determined the output on the display defined by the mathematical equation above.
Other voltages can be measured due to the linear relationship between the input voltage and resultant
frequency produced by the VCO.
Complete operation as an ohmmeter
In order to operate as an ohm meter the following adjustments were made. Vref for the VCO was now
to be set at 5 volts. Timer three now contained the resistor to be measured (Rx). For operating from
0…1k a 500 ohm resistor was used to calibrate the ohmmeter. This is similar to the voltmeter where half
of the supply (Vref) was used to calibrate to the correct center frequency for the VCO. The 500 ohm
resistor was then used to determine the capacitor value of the last timer. This would then set the pulse
width for the enable pin on the driver circuit. The equation to do this was
capacitor =
N was again used to represent the desired value on the display. To calibrate the ohmmeter for the
range of 0…1k the equation was evaluated using the following values.
𝑵
(
)
𝒇𝒗𝒄𝒐
𝟏. 𝟏𝑹𝑿
N=500; fvco = 100 kHz; Rx=500
The same operating principle as the voltmeter now applies. Rx can now be replaced by any value in
range. The timer (3) pulse width now changes as Rx changes and fvco still equals 100 KHz. As Rx was
adjusted the enable pin time active also changed and allowed the drivers to count to the desired output.
Results
The results below display measured resistance and voltages measured with another DMM versus the
values found with the designed DMM.
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resistance value
DMM measured
103
106
267
271
466
470
512
514
677
682
823
826
995
998
Table 1. Resistance results
Power supply
LED display
8V
800
7V
708
6V
608
5V
511
4V
402
3V
315
2V
210
1V
100
Table 2. Voltage results
Results continued:
The maximum voltage to be measured was 8 volts because the frequency of the VCO did not vary much
after 8 volts. The minimum and maximum frequencies for the VCO were 18 kHz and 160 KHZ
respectively. The center frequency was 103 kHz.
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Concluding remarks:
With the successful design of both a voltmeter and ohmmeter, the objective of this lab was met. The
DMM that was built was within a reasonable accuracy due to careful tuning of the VCO. If this lab were
to be done a second time, more focus would be put on accuracy and mathematical modeling. Overall,
the objective to develop a Digital multimeter from digital electronics was a success. Additional features
to add to this design consist of measuring capacitance using similar design techniques from the voltage
and ohmmeter design. The voltmeter can also be varied to scale to different ranges using operational
amplifiers to scale Vref for the VCO. The student outcomes discussed earlier were also obtained.
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Appendix
Figure 2- (R2 open circuit Voltage vs frequency)
Figure 3- (R2=142.5k circuit Voltage vs frequency)
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Figure 4- (R2=100k circuit Voltage vs frequency)
Figure 5- Display and driver circuit
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Figure 6-astable 555 timer (1)
Figure 7- One Shot 555 Timer (2)
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Figure 8- One Shot 555 Timer (3)
Figure 9- One shot 555 Variable Rx for Ohmmeter
Figure 10 timing diagram for VCO and timer 3
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References
Salim, Ghassan. "Ghassan Salim - CalU." Ghassan Salim - CalU. Web. 16 Mar. 2015.
<http://www.profsalim.com/>.
Rev. 5 — 18 November 2011, and Product Data She. HEF4046B (n.d.): n. pag. Web.<
http://www.nxp.com/documents/data_sheet/HEF4046B.pdf>
"4026." IC Pinout Diagram. N.p., n.d. Web. 17 Mar. 2015.
<http://www.elektropage.com/default.asp?tid=456>.
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Student signatures
Sam
Austin
Jason
Alex
Name:_______________________________________Date:__________________
Name:_______________________________________Date:__________________
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Name:_______________________________________Date:__________________
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