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The circuit is built on a small PC board and connects to the parallel port of the computer. Note the resistor array (RN1) adjacent to the IC. PC-controlled 6-channel voltmeter Consisting of just a handful of parts, this simple project plugs into your PC’s parallel port to provide a 6-channel voltmeter. The companion software generates an on-screen display which shows the readings in both analog and digital format. By MARK ROBERTS Two versions of this project are being presented here, the first based on the Motorola MC145041 8-bit analog-to-digital (A/D) converter. This version features three 0-6V input channels and three 0-20V channels and provides 20mV resolution. The second version uses either the 10-bit MAX192 A/D con­verter or the 56  Silicon Chip 12-bit MAX186 chip. It has the same voltage ranges as before but the resolution is improved to 4mV for the 10-bit chip and 1mV for the 12-bit chip. The downside of this version is that the Maxim devices are considerably more expensive than the 8-bit Motorola device. As a guide, the Motorola MC145041 device and the equivalent TLC542CN device from Texas Instruments can be obtained for around $5. By contrast, the MAX192 and MAX186 devices cost around $20 and $55 respectively, so consider carefully whether you really need the extra resolution before opting for the Maxim chips. Note also that the software differs between the two ver­sions. The same software is used for the 10-bit and 12bit Maxim chips, however. As shown in the photos, all the parts are accommodated on a single PC board which also includes the DB25M connector. This connector plugs directly into either LPT1 or (provided that your computer has two parallel ports) into LPT2. The circuit is pow­ ered directly from the parallel port, so no external power supply is required. Fig.1 shows the on-screen display generated by the soft­ware. As can be seen, there are separate “metered” (analog) and digital displays for each input channel. In addition, there is a “button” to toggle the power on or off (just click with the mouse), plus two smaller buttons that let you select the computer port (either LPT1 or LPT2). Finally, there are two digital output buttons and these may be manually toggled on or off using the mouse. When an output is toggled on, it sends its corresponding output on the circuit board high and this can be used to remotely control an external device, either via an optocoupler or some other suitable inter­face circuit. Note that this interface circuit should be suitably buf­fered or isolated to avoid damage to the parallel port. Applications So what are the applications for such a device? A few that spring to mind include: (1) multi-channel analog acquisition; (2) testing or monitoring digital and analog circuits; (3) monitoring security systems; (4) industrial process control; and (5) battery management. In short, you can use this device wherever it is necessary to monitor multiple DC voltages and have them all displayed on a computer monitor. Depending on the readings, you can also elect to remotely control one or two external devices at the click of a mouse button. The 0-6V and 0-20V voltage ranges can be easily altered if necessary, to accommodate higher voltages. This is Fig.1: this is the on-screen display generated by the software. Note that the Channel 0 bezel has changed to red here, indicating an overrange condition. done by changing the voltage divider resistors at the inputs. This does not alter the voltage ranges shown on the “meters” however, so you will have to scale the readings yourself. Circuit details Refer now to Fig.2 – this shows the circuit details of the 8-bit version based on the Motorola MC145041 (or the TLC542) ADC (IC1). IC1 is basically an 8-bit A/D converter with 11 analog input channels, although only six channels (0-5) are used here. The incoming data on each channel is fed to an internal multi­ plexer which selects each channel in turn, depending on the data fed to an internal address latch. The multiplexer output in turn drives the A/D converter section of the chip. The resulting digital data for each channel is then shuf­fled out in serial fashion on the Dout line (pin 15) and fed to pin 13 of the parallel port. It is then displayed on the screen under software control – see Fig.1. Pin 17 of IC1 is the serial data input Fig.2: the 8-bit version is based on the Motorola MC145041 A/D converter (IC1). October 1997  57 Fig.3: the 10/12-bit version is based on the MAX186 and MAX192 chips. The circuit is similar to the 8-bit version. the readings for the 8-bit version will depend on the accuracy and stability of the 5V rail from the computer. By contrast, the Maxim devices feature an internal +4.096V reference so if accuracy and resolution are important, these are the devices to go for. Second, the input impedance is only 156kΩ for the 0-6V channels and 490kΩ for the 0-20V channels. Depending on the circuit being measured, these relatively low input impedances may cause reading inaccuracies due to loading effects. Construction Fig.4: the parts layout for the 8-bit version. Fig.5: the parts layout for the 10/12-bit version. (DIN). This input is driven from pin 2 of the parallel port and feeds data to the internal multiplexer address latch via an 8-bit data register to select the input channels. For example, Ch 0 is selected by loading $0 into DIN, Ch 1 by loading $1, Ch 2 by loading $2 and so on. The remaining pins connected to the parallel port are VDD (pin 20), SCLK (pin 18), CS-bar (pin 15) and EOC (pin 19). VDD is the supply pin and this is fed from pin 9 of the parallel port which supplies a +5V rail. This +5V rail is also fed to the VREF input at pin 14 to provide a reference voltage. SCLK is the clock input, CS-bar is the chip select input and EOC is the end of conversion output. The incoming voltage signals are fed to the CH0-CH5 inputs via voltage di- vider networks. In the case of the 0-6V channels, the voltage divider networks use 56kΩ and 100kΩ resistors, while the 0-20V channels use 390kΩ and 100kΩ resistors. Finally, the digital outputs are made available at pins 14 and 16 of the parallel port and are fed to the output terminals on the board via 1kΩ isolating resistors. Fig.3 shows the circuit for the 10/12bit version. It is virtually identical to the 8-bit version, the main difference being that the Maxim chips do not provide an EOC output. 58  Silicon Chip Design limitations Before moving on to the construction, we should first point out that this simple design does have a few limitations. First of all, the accuracy of The 8-bit version of the Multi-Channel Voltmeter is built on a PC board coded 07110971, while the 10/12-bit version is built on a board coded 07110972. Figs.4 & 5 shows the wiring details for the two versions. Begin the assembly by fitting PC stakes to the Output 1 and Output 2 terminals and to the adjacent GND terminal. This done, install the wire links, then fit the remaining components. Note that the eight 100kΩ resistors are all contained in a single in-line package which is designated RN1 (for resistor network). Be sure to install this package the right way around; ie, with the common “earth” pin adjacent to pin 10 of IC1. The remaining resistors in the voltage divider networks are installed end-on to minimise board space. Take care to ensure that the IC is correctly oriented. We used an IC socket on the 8-bit version but this Parts List 8-Bit Version 1 PC board, code 07110971, 53 x 42mm 1 DB25M PC-mount connector 3 PC stakes 1 400mm-length 7-way rainbow cable 8 miniature hook connectors 1 MC145041 or TLC542CN 8-bit A/D converter IC 1 22µF 16VW electrolytic capacitor 1 0.1µF MKT capacitor 3 390kΩ resistors 1 8 x 100kΩ resistor network (RN1) 3 56kΩ resistors 2 1kΩ resistors The PC board can be plugged directly into the parallel port or connected to the port via an extender cable fitted with DB25 connectors. Fig.6: the full-size artwork for the 8-bit version. can be considered optional. Complete the board assembly by soldering the DB25M connector into place. The seven input leads (one for each input channel plus ground) can be run using rainbow cable. On the prototype, the ends of these leads were terminated in miniature hook connectors. It’s a good idea to label each lead with the number corresponding to its input channel. Software The software comes on three floppy discs and runs under Windows 3.1x, Windows 95 and Windows NT. It’s easy to install – all you have to do is run the setup.exe file on the first disc (within Windows) and follow the onscreen instructions. In Wind­ows 95, you click Start, Run and then type A:\setup.exe in the space provided Fig.6: the full-zize artwork for the 10/12-bit version. 10/12-Bit Version 1 PC board, code 07110972, 53 x 42mm 1 DB25M PC-mount connector 3 PC stakes 1 400mm-length 7-way rainbow cable 8 miniature hook connectors 1 MAX186 (12-bit) or MAX192 (10-bit) A/D converter IC 1 4.7µF 16VW electrolytic capacitor 1 0.1µF MKT capacitor 1 .01µF MKT capacitor 3 390kΩ resistors 1 8 x 100kΩ resistor network (RN1) 3 56kΩ resistors 2 1kΩ resistors Where To Buy Parts & Software Parts and software for this design are available as fol­lows: (1). MC145041 (TLC542) 8-bit A/D converter ................................................$4 (2). MAX192 10-bit A/D converter ................................................................$20 (3). MAX186 12-bit A/D converter ................................................................Call (4). Software for 8-bit A/D converter (three discs) ........................................$20 (5). Software for 10-bit & 12-bit A/D converters (three discs) ......................$25 (6). Optional LPT2 card for PC .....................................................................$15 Please add $5 for postage. Payment by cheque or money order only to: Mr Softmark, PO Box 1609, Hornsby, NSW 2077. Ph/fax (02) 9482 1565. Note: the software associated with this design is copyright to Mr Softmark. (assuming that the floppy disc is in the A: drive). In Windows 3.1x, you click File, Run and type in A:\setup.exe. Alternatively, you can double-click the setup. exe file from the File Manager or, in Win95, from the Explorer. When you boot the software, you get the screen display shown in Fig.1. Note that the meters and digital readouts will all overrange if the device is unplugged from the parallel port. If any channel overranges, its channel SC number button turns red. October 1997  59