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Electrical Engineering Technology Department of Applied Engineering & Technology California University of Pennsylvania EET 370: Instrumentation I Spring 2015 DMM Lab Submitted by: 1 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 . . . . . . . . . .. . . . . . . . . . . . . . . . . . . .. . . . . . . . . . .. . . . . . . . . . . . . . . . . . . .. . . 2 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. 3 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. 4 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. 5 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. 6 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. 7 Appendix Figure 2- (R2 open circuit Voltage vs frequency) Figure 3- (R2=142.5k circuit Voltage vs frequency) 8 Figure 4- (R2=100k circuit Voltage vs frequency) Figure 5- Display and driver circuit 9 Figure 6-astable 555 timer (1) Figure 7- One Shot 555 Timer (2) 10 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 11 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>. 12 Student signatures Sam Austin Jason Alex Name:_______________________________________Date:__________________ Name:_______________________________________Date:__________________ Name:_______________________________________Date:__________________ Name:_______________________________________Date:__________________ 13