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PC TALKING VOLTMETER, PT.2 Because most of the work is done by the computer, the PC Talking Voltmeter is a fairly simple device. This month, we give the full construction & adjustment details. By DARREN YATES The PC Talking Voltmeter is built onto a PC board coded SC07110911 and measuring 136 x 58mm. Before starting construction, check the copper side carefully for any breaks or shorts between tracks. If you find any, use a dash of solder or a sharp artwork knife to fix the problem. 60 SILICON CHIP Once you're happy with the board, start construction by installing PC pins at all external wiring points - see Fig.3. When these are in place, install the wire links and the resistors. Note that 1 % resistors are used in the input voltage divider network, as indicated on the wiring diagram. Table 1 shows the resistor colour codes. Alternatively, you can use your multimeter to check the resistor values before installing them on the board. The diodes, ICs and capacitors can now all be installed, followed by the two 3-terminal regulators. Check the regulator type numbers carefully before installing them on the board, so that you don't get them transposed. The 7905 is mounted with its metal tab towards the adjacent lO0µF capacitor, while the metal tab of the 7805 goes towards the edge of the board. You can now complete the PC board assembly by installing transistor Ql. Orient the transistor as shown and push it down onto the board as far as VOLTMETER INPUT it will comfortably go before soldering its leads. Wiring A plastic zippy case measuring 150 x 90 x 50mm is used to house the PC board and its associated hardware. The PC board mounts on the lid of this case and can be used as a template for marking out and drilling the four mounting holes. Once the holes have been drilled, the board is secured in position using machin e screws and nuts, with four nuts used as spacers. The adhesive front panel label can now be secured to the base (ie, the base becomes the front panel) and holes drilled to accommodate the LED, input sockets and rotary switch. You will also have to drill three holes in one end of the case (adjacent to the rotary switch) to accommodate the power switch, a 3.5mm power socket and an optional RCA output socket (if you intend connecting the PC Voice Recorder). It's always best to drill small pilot holes first and then carefully ream each hole to its correct size. The various switches and sockets can then be mounted in position and the wiring completed as shown in Fig.3 and the photographs. Two 4mm banana sockets are used as the front panel input terminals (red for positive and black for negative). Take care when wiring up the rotary switch as it's easy to make a mistake here. The same goes for the LED wiring. You can identify the cathode lead by the adjacent flat area on the LED body. It's also the shorter of the two leads. A 10-way length of ribbon cable about 3-metres long can be used to wire up the DB-25 connector. This job H m = ® Q- I" --mo-. --<at>J-o ~;;; TO PCVOICE - - - RECORDER-!__ GNO - I 1 12VOC - - PLUG-PACK+:---!-,. 03 ' • 07 555 100uF - ~ .... 100uF 01 TLD74 :; 1 ~ ---nm:J-. ';' ...... ..._. 79!~ •I . = 0B25 -<at>D-o ~ ~ ~:<at>: OiaC2 ~~ • ~ ., ---<t::::),4 ~ 100uF : OO EB c3 ..J',.. ~N ,..._._...---....... OA~6soo ' 06 .____. •G CONNECTOR •2 •3 •4 •5 •6 •7 •8 •9 •10 •19 04 0 LED1 Fig.3: here's how to wire up the PC board. Take care to ensure that all polarised parts are correctly oriented & note that the five resistors in the input voltage divider (immediately below IC2) are all 1 % types. is straightforward - just make sure that you connect each wire to the correct pin on the back of the connector (the pin numbers are moulded into the plastic insulation) . The printer port cable emerges through a slot filed in one end of the case (see photograph). This also involves filing a slot in the lip that runs around the inside edge of the lid. If necessary, you can make the cable longer than the 3-metre length specified but don't go beyond about 6 metres. · the voltmeter board to the output marked "pin 14" on the Voice Recorder board (see wiring diagram on page 42 of the August 1991 issue). The output of the voltmeter is thus fed into pin 3 of IC4a on the PC Voice Recorder board via an 82kQ resistor and 0.lµF capacitor. Note that, in this mode, the input section of the PC Voice recorder is not used. Nor do you have to make any connections between the PC Voice Recorder and the parallel printer port. In addition, the playback section of PC Voice Recorder is now controlled by the software that's used for the PC Talking Voltmeter. Connecting the Voice Recorder As mentioned previously, only a 2wire connection is required between the PC Talking Voltmeter and the PC Voice Recorder. One wire simply connects the two circuit grounds (GND) together, while the other runs from Testing Before plugging the cable into the printer port, connect the plugpack supply and check that the supply TABLE 1: RESISTOR COLOUR CODES 0 0 0 0 0 0 0 0 0 No. Value 4-Band Code (5%) 5-Band Code (1%) 1 1 2 1 3 6 1 1 680kQ 220kQ 180kQ 100kQ 10kQ 4.7kQ 2.2kQ 820Q not applicable not applicable not applicable brown black yellow gold brown black orange gold yellow violet red gold red red red gold grey red brown gold grey black orange brown red red black orange brown brown grey black orange brown brown black black orange brown brown black black red brown yellow violet black brown brown red red black brown brown grey red black black brown NOVEMBER 1991 61 The photo at top left shows the opening menu when you run the software. You can select any one of seven options by pressing the spacebar and then pressing return. The photo at bottom left shows the display when you select the "Basic Voltmeter" mode. In this case, the screen is displaying the voltage from a fresh 9V battery. voltages to the ICs are correct. If you don't get the correct voltages, check the plugpack polarity and for incorrectly oriented ICs and regulators. If the supply voltages are OK, you can now test the board by first connecting a 50kQ pot. (anywhere from 10-300kQ will do) across the ±5V rails. This done, connect the pot. wiper to the positive (+) input, plug the cable into the printer board and boot up the software as described in the· following section (ie, type "GO" and then hit return at the DOS prompt). Select the "Basic Voltmeter" mode from the on-screen menu and check that the front panel LED flashes continuously. If you now vary the pot, the on-screen reading should change. Don't worry at this stage if the reading isn't accurate. This will be fixed when you go through the following setup procedure. Software The software for the PC Talking Voltmeter is available from the author and comes on a 5.25-inch 360Kb floppy disc (see parts list). Included with this disc are the full running instructions and a programming guide as well. y Y 128 I I I I I I I I I I +12.8 ·12.8 · I ·127 ACTUAL CONVERSION : SLOPE WRONG INTERCEPT WRONG I I STEP 1 : CORRECT FOR Y-AXIS INTERCEPT I I STEP 2: ADJUST SCALING Fig.4: this diagram shows how the conversion slope & intercept are adjusted so that we get the correct reading. The X-axis represents the input voltage, while the Y axis represents the 8-bit code from the computer. 62 SILICON CHIP PARTS LIST 1 PC board, code SC07110911, 136 x 59mm 1 360Kb disc with VOLT.BAS software (see panel) 1 plastic zippy box, 150 x 90 x 50mm 1 adhesive front panel label, 151 x87mm 1 DB25 male connector 1 3-metre length of 10-way ribbon cable 1 4-pole 3-position rotary switch 22 PC pins 2 4mm banana sockets 1 set of probes with 4mm banana plugs 1 3.5mm power socket 1 panel-mount RCA socket Semiconductors 1 DAC0800LCN 8-bit DAC (IC1) 1 TL074 quad op amp (IC2) 1 555 timer IC (IC3) 1 7805 +5V regulator (REG1) 1 7905 -5V regulator (REG2) 1 BC548 NPN transistor (01) 1 5mm red LED & bezel (LED 1) 4 1N914 signal diodes (D1 ,D2,D7,D8) 4 1N4004 diodes (D3-D6) The PC board is mounted on the lid of the case, while the input terminals, rotary switch & LED are mounted on the base which thus forms the front panel. Use a 3-metre length of 10-way ribbon cable to wire up the DB-25 connector. When you boot up the software, you can choose any onP. of four diffP.rent operating modes: (1) a basic 8-bit voltmeter with onscreen digital readout; (2) an 8-bit voltmeter that interfaces to the PC Voice Recorder project; (3) a data logger that records volt- age measurements over tim e with adjustable time intervals; and (4) a graphic display mode for data logged measurements. Each of these features, plus the voltage reference setup, can be selected from the main menu. The software is easy to drive. You boot it up by typing "GO" and then hitting return at the DOS prompt. After that, it's just a matter of selecting the required operating mode using the spacebar and return keys, and then making sure that the selected onscreen voltage range matches the setting on the voltmeter. Reference setup A slot must he filed in the inside lip of the lid, together with a matching slot in one end of the case, to provide clearance for the printer port cable . Before you start using the PC Talking Voltmeter, you must first go through the voltage reference setup procedure by selecting it from the main menu. What we've done is to use the computer to store a reference voltage and apply a scaling factor so Capacitors 4 100µF 25VW electrolytics 4 0.1 µF 5mm-pitch metallised polyester Resistors (0.25W) 1 680kQ 1% 1 220kQ 1% 2 180kQ 1% 1 100kQ 5% 1 10kQ 1% 2 10kQ 5% 6 4.7kQ 5% 1 2.2kQ 5% 1 820Q 5% Miscellaneous Hookup wire, tinned copper wire , screws, nuts, washers etc. Where to buy the software The software for the PC Voltmeter, VOLT.BAS, is priced at $35 + $3pp and includes running instructions and documentation . All software come$ on a 5.25-inch 360Kb disc and will run on any PC with at least 256K of RAM and a parallel printer port. Payment should be made by cheque or money order to: Darren Yates, PO Box 134, French 's Forest, NSW.2086. Note: Copyright of the software is retained by the author. NO V EM BER 1991 63 The display shown at top left is presented when you select the "Setup Procedure" from the opening menu. As before, you select the required option by pressing the spacebar & this then gives the display shown at bottom left (note: the voltage shown here is for illustration only). After that, it's simply a matter of following the on-screen instructions. that we get the correct reading. After you select the "Setup Procedure ", you are presented with a second screen that gives step-by-step instructions. You simply select the 12.8V range , connect the input lead to the output of the +5V regulator, press return twice, disconnect the input and press return again. The +5V reference voltage is then stored as an 8-bit code in a small file called VOLTSET.BAS. If the output of your 5V regulator is a bit off, you may like to substitute another into the circuit. Alternatively, you could try an LP2950ACZ 5V regulator IC. This comes in a TO-92 transistor type package and has an initial voltage tolerance of about o. 5 % . However, you should find that most 7805 regulators will do the job just as well. So long as your reference diodes give a voltage of about 1.3V and your 5V regulator is within ±50mV of 5V, the voltmeter will be quite accurate. Fig.4 shows how the scaling process works. Here, the X-axis represents the input voltage and the Y-axis represents the 8-bit code from the computer. Because the scaling factor is linear, the transfer function is simply a straight line. All we need to do now is (1) adjust the intercept (where the line crosses Fig.5: check your PC board carefully against this full-size artwork before mounting any of the parts. 64 SILICON CHTP =UflJlll/JI/J= =IUIIJI'!- PC TALKING VOLTMETER Fig. 7: this fullsize artwork can be us·e d as a template for marking out the front panel. + • + A/D STATUS ±1.28V • • ±12.SV ±128V VOLTAGE RANGE + + + WARNING: Check screen for voltage over-range the Y-axis); and (2) adjust the slope of the transfer function so that we get a 1:1 ratio. Fig.4 first shows the initial conversion from the ADC. The fact that it doesn't intercept zero and has the wrong slope can be caused by a number of factors. This problem is Use PC stakes at all external wiring points on the board, to make the wiring easier. Note the orientation of the two 3-terminal regulators. solved when you initially go through the setup procedure. When you go through the setup procedure and press return with the input disconnected, you are setting the OV reference. We can now add the difference between the two codes to get the correct OV reading. Fig.4 also shows how we get the correct reading for each point on the scale. During the setup procedure, we set the conversion factor of the LSB to 1 OOm V, so that the difference between a OV reading and a 5V reading should be 50. Ifwe get something else, then it is just a matter of scaling by the ratio of the two readings. This may sound complicated but the software takes care of it all for you. You can verify the final result by monitoring the input to the PC Talking Voltmeter with a digital multimeter. When you compare the meter reading with the on-screen voltage, you will find that the two readings are remarkably close. SC NOVEMBER 1991 65