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Build a talking voltmeter for your PC Fancy a talking voltmeter with an on-screen display? This simple project mates with the PC Voice Recorder featured in the August 1991 issue & plugs directly into the printer port of your PC. venient to have to take your eyes off the circuitry to look at the voltmeter and it's all too easy for the probes to slip (and possibly cause damage) while doing so. Unlike the fancy handheld versions, the SILICON CHIP PC Talking Voltmeter plugs into a computer. This gives it the ability to store and later retrieve By DARREN YATES voltage readings over a given period of time. And by adding the PC Voice Talking voltmeters aren't new and However, they are often much more Recorder board, you can use speech there are now several handheld mod- convenient to use than conventional to relay the information to you. els on the market that can tell you the (mute) multimeters. This particularly What's more, we have replaced the current reading. These all use the lat- applies if you want to take a number American accent of the handheld est in artificial speech technology but, of readings in quick succession or if • models with a good ol ' fashioned unfortunately, this technology is still you have to concentrate on probe "Orstralian" one. You can even use new enough for them to be quite placement (eg, when measuring your own accent if you wish! pricey. voltages on IC pins). Often, it's inconOne very useful feature of the PC 54 SILICON CHIP Talking Voltmeter is that it can be used to monitor a voltage over time; eg, the voltage on an SLA battery under charge. In this mode, it can be set to give an audible alarm if the battery voltage rises above or falls below a preset limit. If you have a graphics card in your computer, you can also print out voltage vs. time graphs. This makes the unit ideal for checking battery performance under load, or for checking the stability of audio amplifiers, to give just two examples. The accompanying panel shows all the relevant specifications of the PC Talking Voltmeter. It has 8-bit accurac;y, better than 0.5% linearity and an input irnpedan.ce of lMQ. It plugs directly into the.parallel printer port of your computer and will measure DC voltages ranging between ±128V over three ranges, with an accuracy of about 2%. If you want the optional speech facility as well, you simply run an extra 2-wire connection to the PC Voice Recorder. Alternatively, you can delete the speech facility and simply operate the unit as a voltmeter with an on-screen digital display. Block diagram Take a look now at the block diagram of Fig.1. A part from the corn puter, the circuit uses a range switching stage, an input buffer stage, a comparator and an 8-bit digital-to-analog converter (DAC). Let's see how this all works. The input voltage is fed through the high impedance (lMQ) range selector and thence to the protected buffer (IC2a). The output of this buffer is then fed to the inverting input of the comparator (IC2b), while the noninverting input is fed from the output of the 8-bit DAC (ICl). This DAC converts the output of the computer which in turn responds to the output of the comparator. In operation, the input voltage from the buffer is compared with a voltage set up by the computer. At the same time, the computer also monitors the output of the comparator. If the comparator output switches from low to high, then the computer-controlled DAC output is too high. The computer then systematically searches for a lower value that is closer to the mark. Even though there· are 256 possible combinations, the car- Fig.1: block diagram of the PC Voltmeter. The input voltage is applied via a voltage divider & buffer stage IC2a to the inverting input ofIC2b where it is compared with the output from an 8-bit DAC (ICl). The DAC converts the output from the computer which in turn responds to the output of the comparator. VOLTAGE RANGE VOLTAGE INPUT □ rect 8-bit value that matches the input voltage can be found in just eight steps. What we really have is an 8-bit successive approximation analog-todigital converter (ADC). It converts our input voltage into an 8-bit code that the computer can recognise. Successive approximation Table 1 shows how successive approximation works. What we've done here is to show you how the successive approximation technique works on a step-by-step basis. Say for example that our input voltage is 1.83V. Our DAC has a conversion rate such that each of the 256 steps is worth 0.01 V. OK, let's start with row 1. Our input voltage is set to 1.83V, as we have said. We now set the most significant bit (MSB) of the DAC high, which represents half scale. This sets our DAC to 1.28V. We now check to see if the input voltage is in the upper or lower region of the DAC scale. Because the DAC output is lower than the input voltage (1.83V), the output of the comparator is low. This tells the computer that the input voltage is higher than 1.28V and so we leave the MSB high. We now go to row 2. We know that our voltage is in the upper half of the scale but is it towards the middle or the top end? We now set the next bit high as well, which produces an output voltage from the DAC of 1. 92V (ie, halfway between 1.28V and 2.56V). Since this is higher than our input voltage, the comparator output goes high, telling the computer that this bit must be set low. You can follow the rest of the rows yourself to verify that it works. The method is to start with the MSB and work down, checking at each step to see whether the result is higher or lower than the current input. It may take you quite a bit oftirne to work the Specifications Voltage range ................... ..... Resolution ............................. Input impedance .................... Accuracy ............................... ND converter ........................ Measurement rate ................. ±128V in three decade ranges 0.01 V, 0.1 V & 1V 1Mn approx. 2% 8-bit successive approximation type approx. 10 per second for 10MHz AT (depends on CPU clock speed) Computer requirements • XT/AT/386/486 IBM PC or compatible • CGNEGNVGA video card (for display of logged data only) • 360Kb.floppy disc drive • 256Kb bf memory (minimum) • MS-DOS 3.0 or later with GWBASIC • 1 parallel printer port OCT0BER1991 55 ,~.,~ I Digital/analog scale: LSB Current Bit Set = .01 V; Full scale ~ = 2.56V; Input voltage Comparator Output D/A Output Step 1 MSB 1.28V Step 2 MSB -1 1.28 + 0.64 Step 3 MSB-2 1.28 + 0.32 Step 4 MSB-3 1.28 + 0.32 + 0.16 Step 5 MSB-4 1.28 + 0.32 +0.16 + .08 Step 6 MSB-5 1.28 + 0.32 + 0.16 + .04 Step 7 MSB-6 1.28 + 0.32 + 0.16 + .04 + .02 Step 8 LSB 1.28 + 0.32 + 0.16 + .04 + .02 + .01 = 1.92V High MSB - 1 = 0 = 1.60V Low MSB - 2 =1 Low MSB - 3 =1 = 1.84V High MSB - 4 =0 = 1.8V Low MSB - 5 =1 Low MSB - 6 =1 = 1.76V = 1.82V = 1.83V High/Low LSB = 0/1 = 10110111 = 1.83V comparator to give us a result, this becomes our error. Even so, one step in 256 gives us an accuracy of about 0.4 % but there are other factors which have greater influence on the accuracy, as we shall see shortly. 0. The complete circuit diagram of the PC Voltmeter is shown in Fig.2. It uses just three !Cs, a couple ofregulators and a handful of other components. Circuit diagram The PC Voltmeter is housed in a low-cost plastic utility case & plugs into the parallel printer port of the computer. By connecting it to the PC Voice Recorder, you get a voltmeter that talks. SILICON CHIP =1 MSB result out by hand but the computer can do it much faster! If you look down the decision column in Table 1, you can write out the 8-bit code by starting with the MSB on the left and work across. In our example the 8-bit DAC code is 10110111, with the last bit either 1 or 56 Decision Low Digital word from DAC The last bit to be checked is the least significant bit (LSB). Because the two voltages applied to the comparator have to be different for the = 1.83V The heart of the circuit is !Cl, a DAC0800LCN 8-bit D/ A converter. Its 8-bit digital input comes from the parallel port of your PC (designated port A). By connecting pin 1 of !Cl to ground, we can feed the TTL outputs from the computer directly into !Cl and be assured of logic compatibility. Our reference voltage is set by two signal diodes, Dl and D2. This will give us a voltage which is close to about 1.3V. By now, some of you may be cringing about the accuracy of this reference but, before you give the idea away, we should point out that any error that occurs due to this source is corrected by the software. We built the prototype using components that came straight from the shelf and our PC voltmeter compared favourably with our Fluke multimeter. Obviously, it doesn't have the same resolution but the difference between the readings was only about 50mV on the 12.8V range! That said, this project is not designed to replace the multimeter - far from it! In fact, a digital multimeter will be very useful for checking out the final accuracy. !Cl is powered by ±5V supply rails, which we'll talk more about later. Its output takes the form of a differential current flow from pins 2 and 4. These outputs are fed to op amp IC2c, which is connected as a current to voltage converter as well as a subtracter. The resulting output from pin 7 of IC2c is a voltage which can take any ,_._ _ _ _ _ _ _ _ _ _ _....__ _ _ _ _ _ _ _ _....__ _ +5V VOLTMETER INPUT 680k 1% -_g. 4.7k 220k 1% 100k !.12.BV S1 180k 1% 1128V -5V .,. 02 IN914 +5V 01 1N914 PARALLEL PRINTER PORT 0825 CONNECTOR (2) 00 (3) 01 (4) 02 4.7k 4.7k 14 .,. .~ 13 12 4 11 I 10 GNO 2 IC1 OACOBOOLCN (5) 03 4.7k (6) 04 GNO I (19) GNO~ 0.1+ 15 4.7k .,. PCVOICE T O.,. ~ -5V .,. ~f 3 A VIEWED FROM BELOW OUT t=-"-'--1--- ♦SV + 100 25VWI (9) 07 (10) ACK ELJc ~K OUT REG1 IN 16 (8) 06 B IN 03 1N4004 ,.. .,. (7) 05 .. ffi 5 8 D5 .,. 1N4004 IC3 555 12VDC PLUG-PACK -i .,. 0.1 . 01I .,. PC TALKING VOLTMETER Fig.2: all the elements depicted in the block diagram can be directly related to the main circuit shown here. ICl, an 8-bit DIA converter, is the heart of the circuit. It accepts an 8-bit digital input from the computer & generates a differential output which is fed to IC2c, IC2c in turn drives the non-inverting input of comparator IC2b. value (in 10mV steps) between plus and minus the reference voltage; ie, between ±1.3V. This voltage is fed into the non-inverting input (pin 10) of IC2b, which acts as our decision comparator. The inverting input is derived from the input isolating buffer (IC2a), which is another TL074 op amp. Input divider The input voltage is fed into the buffer via a voltage divider with three positions: divide-by-100, divide-by10 and divide-by-1. To make sure that the input voltage range of the buffer is not grossly exceeded, a lO0kQ resistor is connected in series with the input. In addition, diodes D7 and DB ensure that the input voltage to IC2a can not rise more than 0.6V above the supply rails. IC2a is wired as a buffer stage and provides an extremely high input impedance (>10 12 ohms). This means that the input impedance is determined· by the voltage divider resistance (ie, lMQ). The output from IC2a appears at pin 1 and is fed directly into the inverting input (pin 9) of decision comparator IC2b. The decision comparator works like this. When the voltage applied to its non-inverting input from the DAC (and thus from the computer) is greater than the voltage on its inverting input, the output at pin 8 goes high. This lights LED 1 and turns on transistor Ql which then pulls the -ACK line of the printer port to ground. This -ACK line is checked by the computer during each cycle of the conversion process. It tells the computer whether the curreni. digital count is higher or lower than the input volt· age, as described previously. Power for the circuit is derived from a 12VDC plugpack supply, the same as for the PC Voice Recorder. This drives a 7805 3-terminal regulator via polarity protection diode D3 to derive a +5V supply rail. In addition, a -5V rail is derived using IC3 and a -5V regulator. IC3 is a 555 timer which is connected for astable operation. It produces a 750Hz squarewave at its pin 3 output and this drives a diode pump (D4 & D5) which is wired to produce a negative voltage (-9V approx.) at the anode of D5. This voltage is then fed to a 7905 regulator to derive the -5V supply rail. That's all we have space for this month. Next month, we will conclude with the constructional details. SC OCTOBER 1991 57