數(shù)字電壓表外文翻譯.pdf

Digital Voltmeter I Introduction Our goal here is to build a voltmeter We will do this using almost all digital circuitry rather than analog The circuit works like many other digital measurement circuits in that it converts the quantity to be measured into a time interval then measures that time Call the voltage we wish to measure Vin Say we charge a capacitor linearly with time that is at a constant charging current I starting at zero volts It will take a time t Vin dVin dt Vin C dQ dt Vin C I for the capacitor to charge up to Vin So if we can measure this time interval we can determine Vin To measure time in digital electronics we produce a high frequency pulse train and count the pulses occurring during the time interval to be measured The number of pulses counted is a measure of the time interval and hence of Vin With some suitable conversion and a display this makes a digital voltmeter DVM To realize this idea with actual circuit elements mostly integrated circuits or IC s we require a number of separate sections The complete circuit diagram for the eventual project is shown on the last page of this writeup don t let it frighten you On the second to last page is the same circuit with its functional sections outlined and labeled The sections needed for this project are A A pulse train generator In electronics this is called a clock We have already learned how to build this using the 555 timer chip This is the clock generation section B A way to monitor the voltage across the capacitor and generate signals when the charging starts at zero volts and when the voltage becomes equal to the voltage to be measured Vin I call this the section control and gating C A way to count the number of clock pulses that occur between the start and stop charging signals This is the counting section D A constant current capacitor charging circuit that can be turned on and off and reset to zero volts by the start and stop charging signals analog input transducer E A human readable way to display the number of pulses counted during this interval This is the latching and display section We will learn how to build each of these sections then connect them together into the functioning DVM II Section by Section Build Up So let us get started A Clock Generation Part A we already have learned to do in the previous lab Digital things though prefer to work at 0 or 5V Therefore either power the 555 from 5V instead of 10 or 15V see spec sheet or put pin three on a voltage divider to bring the output down to 5V This is the method chosen in the final schematic at the end B Basic Control and Gating To carry out the function of Part B we use a solution based on an integrated circuit called a comparator The entire purpose of this IC is to signal which of two voltages is larger One of the voltages will be Vin and the other will be the voltage across the capacitor The comparator has to do its job without drawing any appreciable current otherwise it will interfere with the constant current charging To achieve near zero input current we will use a LF355 FET input 1 op amp as our comparator We will discuss op amps in detail in class For now we can just learn that the op amp is a chip with two inputs labeled and with the voltages at these points called V and V and called the non inverting and inverting inputs It has just one output Of course like any IC it requires power and ground connections in this case two power supplies Vcc 10V and Vee 10V these are just conventional names for the power voltages If the voltage at the input is greater more positive than the voltage at the input V V the 355 will drive its output high near to Vcc If the opposite becomes true V V the output will go low near to Vee Just for this part of the lab we will use a couple of LED s to monitor the state of the op amp output The LED is symbolized by and is a Light Emitting Diode We discussed diodes generally in class They act as one way valves for current The arrow points in the direction conventional positive current is allowed to flow The shorter lead wire on an LED corresponds to the tip of the arrow For historical reasons this side is also called the cathode the other side of the device is called the anode When the diode is biased i e voltage is applied to conduct in this direction it is said to be forward biased If you forward bias these LED s with less than about 1 6 Volt little current flows and no light is produced On the other hand if you try to cause a larger forward voltage drop across a diode you will find that you won t be able to do it Any diode will always try to maintain a certain voltage drop of about 1 Volt across itself see below for details on the exact value The diode will keep its forward bias voltage drop constant by drawing more and more current from the power supply until either the supply sags or the diode burns up The exact value of the forward bias voltage drop depends on the material the diode is made of and nothing else Red LED s are made of gallium arsenide GaAs a man made material that does not exist in nature They have a forward bias voltage of about 1 6 volts and they only emit light when the forward bias voltage is at least this big Signal diodes are usually made of silicon with a forward bias voltage of 0 6 Volt An LED will not light when it is reverse biased voltage polarity is reversed In the circuit below only one LED should light at a time the one on the left will light if the op amp output is positive or the one on the right will light if the op amp output is negative LEDs should always have a resistor of 500 in series with them to guarantee that the voltage they see will sag sufficiently at high current to avoid burning out the diode 1 Build the voltage monitor circuit shown above The SPDT switch shown below the 9K resistor controls the capacitor charging The 1k potentiometer is wired to form a voltage divider allowing us to set Vin to anything between 0 and 10V The time constant for charging the capacitor is fixed by the capacitor value and the 9 K resistor and is slow enough that you should be able to notice the longer charging times as Vin is increased Note Vin is what will eventually be the voltage we are trying to measure The capacitor is always charged in this schematic from 0 to some Vin value less than10V But it will take it longer to charge to a higher voltage so the 355 comparator will measure a longer time for V to become greater than V as Vin is increased Make sure the electrolytic capacitors are placed with the correct 2 polarity This means the lead marked positive should go towards the 10V 2 In this configuration the op amp will put out 10V or 10V at pin 6 actually slightly less Measure the real values Since our two LEDs are wired with opposite polarity in either case only one of them will be forward biased and hence emit light Choose one of the LED s red and the other one green and orient them so that the green one lights when the capacitor has a larger voltage than Vin When the switch is flipped to connect the 9 K resistor to the capacitor the charging starts The red LED should initially light then after some delay it should go out and the green one should come on If Vin is increased by adjusting the 1K pot the delay should get longer After the switch is flipped in the other direction disconnecting from the 9K resistor and shorting across the capacitor the red LED should come on immediately since the capacitor is discharged directly to ground without a resistor 3 Making certain the capacitor has the right polarity turn the potentiometer to apply its maximum Vin Flip the switch to start charging You may not see the green light go on This is because the op amp cannot work with input signals very near its power supply voltages Back off a bit on the potentiometer to get Vin a volt or so lower than V and try again C Counting Clock Pulses to Measure Elapsed Time 1 Counting Pulses with the 74LS192 Chip Digital electronic chips of the type known as TTL use 0V for an off signal 0 and 5V for an on signal 1 This is the defining characteristic of digital electronics any input or output can only be in one of two possible states The TTL output of the breadboard s function generator generates a pulse train for use with the TTL family So does an appropriate 555 circuit The TTL chips used in this class are all designated by the letters LS in its JEDEC number Example 74LS00 is a quad NAND gate To count the pulses generated while the capacitor is charging we will use a 74LS192 integrated circuit which is a divide by ten counting chip It is a very capable little guy and costs only about a buck This chip accepts a logic pulse train as input on pin 5 UP COUNT It counts pulses from 0 0000 binary to 9 1001 binary continuously putting out the binary bits corresponding to the number of pulses counted on pins 3 2 6 7 LSB to MSB When it gets past 9 it puts out a carry signal on pin 12 the binary bit outputs roll over to 0000 again and then it repeats It can also count down as well as up but we won t use that here Secure the datasheet for this part by asking your instructor or entering 74LS192 data sheet into Google and picking the appropriate reference The innards of the chip the magic in the sense Arthur C Clarke used the term will be explained in class To familiarize ourselves with this chip we will send signals to it one at a 3 time using a pull up resistor and a debounced switch Read the section Switch Debouncing on pages 506 ff of Horowitz and Hill H H This section explains why we must use a debounced switch to signal the counter so that each switch press sends one and only one pulse into the counter Fortunately the breadboards already provide debounced switches located on the left side 1 Wire the switch and resistor part of the counting circuit above using a normally closed debounced pushbutton switch A 500 pull up resistor should run from 5V to the switch The resistor will then be tied to ground whenever the switch is closed presenting a logic 0 to the COUNT UP input pin 5 When the switch pushbutton is pressed the switch is opened and therefore breaks the resistor connection to ground A 5V input is then presented to the COUNT UP input minus of course the voltage drop across the resistor but remember that inputs to logic chips draw little current so this drop is neglibigle This transition from 0 to 5 V is counted as one pulse 2 Wire the rest of the circuit as shown The 74LS192 is a sophisticated device with a number of inputs we are not using These unused input pins must be tied to either ground or 5V because pins left floating unconnected can drift to random voltages and cause spurious inputs to the chip The COUNT DOWN 4 and LOAD 11 pins must be tied high 5V The CLEAR input pin 14 must be tied to 0V Wire the 74192 binary bit outputs 3 2 6 and 7 to the first four logic indicator LEDs on the right side of the board These logic indicators already have series resistors built in to protect the LED s from overcurrent Make sure the least significant bit LSB pin 3 is wired to the right most LED and the most significant bit MSB pin 7 is wired to the left most LED so that the binary display will read normally 3 Now push and release the switch repeatedly You should see the logic indicators count up one unit for each button press from 0000 to 1001 in binary for 0 to 9 then go back to zero and repeat This should repeat as long as you continue actuating the switch 4 Disconnect pin 5 from the pull up resistor and connect it instead to a TTL signal from the function generator running at a low frequency Observe that the binary outputs now count up 4 without your button pushing Next tie the CLEAR pin 14 to a pull up resistor and a DIP switch see note below and note its function Lastly tie the CARRY CO pin 12 to a fifth logic indicator LED and note its behavior Keep this circuit together Note on DIP switch pull up setup Some people would call this a pull down resistor since it connects to ground rather than 5V as in the debounced switch setup above To set this up use the row of DIP switches on the lower left of the prototyping board The slide switch on the far right internally connects one side of all the DIPs to either 0 or 5 V Make sure this switch is set to 5V To avoid shorting the 5 V supply directly to ground a 500 resistor to ground is needed From the high side 5V of this resistor run your input to the CLEAR pin or where ever else in the circuit a manually operated logic input is needed 2 Switching the input to 74LS192 On and Off with a NAND Gate The voltmeter will work by counting the pulses from the 555 between the time the capacitor charging starts and the time the 355 comparator detects that the capacitor voltage has surpassed V in and therefore changes its output state To accomplish this we will use a simple NAND gate in the circuit shown above The NAND gate was discussed in class A NAND is a NOT AND boolean circuit element The truth table is given in the diagram above The pull up resistor will provide a logic 1 to input A of the NAND when the switch is closed this imitates the situation we will have while the capacitor is still charging and 0 if the switch is open Input B of the NAND is connected to the pulse train output of the 555 The output of the NAND will therefore follow the 555 pulse train inverted if the switch is closed and the NAND output will just sit at logic 0 otherwise The signal from the pull up resistor and switch is said to gate the counting of the 555 pulse train 1 First explore the operation of the NAND Find a 74LS00 IC and also its datasheet The 7400 actually has four independent NAND gates in one package Just pick one of them for this exercise To drive the NAND assemble the 555 timer circuit from page 287 of H H You could use the TTL generator but the 555 will be needed later anyway Select resistors and 5 capacitors to give a pulse repetition rate of a few Hz so that you can see the output change visually Then connect the 555 output pulse train to one input of the NAND and the second input to the DIP switch with a pull up resistor 2 Verify with a scope that the output of the NAND only oscillates when the DIP switch is closed and the input is therefore pulled up to 5 volts logic 1 3 Now wire the NAND output to the COUNT UP pin of the 74LS192 counter from the preceding section Verify that the counter only counts when the DIP switch is pulled up If the 555 pulse repetition rate is too high you won t be able to see the four indicator LEDs switching as the count progresses They will look as if they are all on at the same time with a reduction in brightness When the DIP switch is off though the LED s should freeze in a steady pattern 4 Use two probes and a scope to verify that the 555 signal is being inverted by the NAND You will need to display both scope channels at once D Improving the Analog Input Transduction Section 1 Constant Current Charging Why is it unsatisfactory to charge the capacitor through a fixed resistor using a fixed voltage Hint V V0 1 exp t RC We now need a constant current capacitor charging circuit that can be turned on and off and reset to zero volts by the start and stop charging signals This is a complex task and thus part D will have several sub sections As explained at the very start of this write up the capacitor must be charged linearly with time so there is a fixed voltage rise per unit of time How do we charge a capacitor linearly Recall that V q C and that 1 Amp 1 Coulomb Sec So only if we charge the capacitor with a constant current do we increment the charge by the same amount in each second hence also incrementing the voltage by a constant amount in each second This gives a linear V t For the constant current drive we will use the LM334 a three terminal current source that just requires one external resistor to set the current An ideal current source would provide a fixed current no matter what load it was connected to But as with the voltage divider any real current source will sag under certain conditions Notice VIN and V on the diagram below The VIN terminal is the power supply voltage input to the chip The V terminal is the output pin from which the current ISET will be supplied to our load the capacitor As the signs indicate V must be less than VIN If the loading is such that V gets too close to the supply voltage the output current will sag or cease altogether For correct operation we must satisfy the condition VIN V 0 5V Naturally this means we cannot charge our capacitor above VIN 5 V As with most IC s there are many possible applications and circuits for the 334 The figure below shows the simplest configuration Vin can be anywhere between 1 and 40V RSET determines the current by the following formula ISET 227 V K Tambient RSET For this project assume Tambient 300K In a real instrument we would have to build in temperature compensation to make the charging current independent of the ambient temperature 6 1 Using the above formula calculate RSET to produce a current of 0 5mA 2 Secure a copy of the datasheet for the LM334 so you can identify the leads Assemble the little fragment shown above powered by VIN 5 V and measure the output current to verify your calculations The 334 is temperature dependent so warm it a little with your finger and see how the current changes It should be a small change 3 Put the above constant current source into the capacitor charging circuit of section B1 in place of the 9K resistor to the positive supply as shown in the full circuit diagram on the last page of this writeup Verify the function of the circuit by placing an ammeter in series with the current source and capacitor and watching the voltage on the capacitor with the oscilloscope You should observe on the scope that the capacitor never charges above VIN 5 V 4 5V as mentioned previously The ammeter should show a 5mA reading until the capacitor voltage levels out near 4 5 V then it should read zero Replace the 5V supply voltage with 10V Notice th