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Keep tabs on your car's battery with this: Digital Voltmeter This digital voltmeter will let you keep tabs on the condition of your car’s battery & charging system. A PIC microcontroller shrinks the circuitry into the smallest available jiffy box and makes it a snack to build. By JOHN CLARKE Flat batteries usually happen at the most inconvenient time, in the most inappropriate place and when the weather is being totally disagreeable. In fact, the battery is probably the most unreliable component in a modern vehicle. To alleviate this problem, some battery manufacturers incorporate a backup unit within the same case, to allow the vehicle to be started if the main unit fails. 24  Silicon Chip A car battery can only deliver peak performance if it is properly maintained. This not only involves keeping an eye on the electrolyte level but also ensuring that the charging voltage operates within strict limits. That means a charging voltage of 13.8-14.4V for a 12V battery, or 27.6-28.8V for a 24V battery. If the battery voltage never reaches 13.8V, then either the charging voltage is too low or the battery is on the way out. This means that the battery will be marginal when it comes to delivering the necessary current during starting, particularly in cold weather. Conversely, if the battery is being overcharged, the electrolyte will gas excessively, leaving the plates dry and reducing the battery’s amp-hour (Ah) capacity. This can not only dramatically shorten the life of the battery but in severe cases (eg, if the voltage regulator has failed) could damage various electronic equipment in the car. So how can you be sure that your car’s battery is being properly charged and that it is in good condition? The answer is to build and fit this Digital Voltmeter. It monitors the voltage across the battery terminals and thus provides an accurate indication of the charging voltage. It also indicates how well the electrical system and the battery cope with extra loads such as lighting, fans and audio systems. In addition, an accurate voltmeter can quickly indicate the overall condition of the battery. For example, if the battery voltage regularly drops below its nominal value of 12V (eg, when the engine is idling or if the engine has been turned off for some time), it indicates that the battery is unable to maintain a charge (assuming that the charging system is OK). Another time to watch the battery voltage is during starting. During this time, the starter motor draws substantial current and the battery voltage will fall below its nominal 12V value. Wouldn’t it be nice to be able to accurately monitor the minimum battery voltage when the vehicle is started? Well, with this Digital Voltmeter you can because we’ve incorporated a minimum hold facility. All you have to do is press the Min/Hold button on the front panel at any time after starting and the lowest measured voltage will be displayed. The display then reverts to normal mode when the button is released. The minimum voltage, which is stored in volatile RAM, is automatically cleared the next time the ignition is turned off. Normally, with a good battery, the voltage should only drop to around 10.5V when starting the engine, although this will depend on the temperature, the cranking current and on the battery itself. In any case, it’s just a matter of using the Min/Hold button to establish a benchmark minimum voltage for your car’s battery and then checking it occasionally to make sure that the battery is in good condition. Be aware, though, that it’s normal for the voltage to go down during cold weather, so keep this in mind before suspecting a faulty battery. In summary, there are good reasons for carefully monitoring the battery voltage and this unit is ideal for the job. It boasts high accuracy, negligible drift with temperature and a 3-digit LED display that reads to the nearest 0.1V in 12V mode. It also features automatic display dimming to suit the ambient light conditions. Only three wires are required to Fig.1 (right): the PIC microcontroller does most of the work. It accepts inputs from the battery (via IC2a) and the Min/Hold switch and drives the 7-segment displays in multiplex fashion. FEBRUARY 2000  25 Parts List 1 processor PC board, code 05102001, 78 x 50mm (150 holes) 1 display PC board, code 05102002, 78 x 50mm (93 holes) 1 front panel label, 80 x 53mm 1 plastic case utility case, 83 x 54 x 30mm 1 4MHz parallel resonant crystal (X1) 1 LDR (Jaycar RD-3480 or equivalent) 3 PC stakes 3 7-way pin head launchers 2 DIP-14 low-cost IC sockets with wiper contacts (cut for 3 x 7-way single in-line sockets) 1 PC board mount click-action push-on switch (S1) 1 9mm tapped brass spacer 3 6mm tapped spacers 2 M3 x 6mm countersunk screws or Nylon cheesehead 2 M3 plastic washers 1mm thick or 1 M3 plastic washer 2mm thick 2 M3 x 15mm brass screws 1 2m length of red automotive wire 1 2m length of yellow automotive wire 1 2m length of black or green automotive wire (ground wire) 1 5A 3AG fuse and in-line fuseholder (optional) 1 1kΩ horizontal trimpot (VR1) connect the device to the car’s wiring (+12V, 0V and battery +ve) and the unit is easily calibrated by adjusting a single trimpot. A second trimpot sets the minimum display brightness at night. Circuit details Refer now to Fig.1 for the circuit details. It’s dominated by IC1, a PIC16F84 microcontroller, which forms the basis of the circuit. This device accepts inputs from the battery and switch S1, processes this information and drives the LED displays to give a voltage readout. If you think that the circuit looks similar to the Speed Alarm featured in the November 1999 issue, you’re dead right – it is. The major change, at least 26  Silicon Chip 1 500kΩ horizontal trimpot (VR2) Semiconductors 1 PIC16F84P microprocessor programmed with DVM.HEX program (IC1) 1 LM358 dual op amp (IC2) 1 LM2940-T5.0 5V 1A low dropout 3-terminal regulator (REG1) 3 BC328 PNP transistors (Q1-Q3) 1 BC338 NPN transistors (Q4) 3 HDSP5301, LTS542A common anode 7-segment LED displays (DISP1-DISP3) 1 20V 1W zener diode (ZD1) Capacitors 1 47µF 16VW PC electrolytic 1 22µF 35VW PC electrolytic 1 10µF 35VW PC electrolytic 1 1µF 16VW PC electrolytic 2 0.1µF MKT polyester 2 15pF ceramic Resistors (0.25W, 1%) 3 10kΩ 3 680Ω 1 3.3kΩ 8 150Ω 1 1.8kΩ 1 10Ω 1W Miscellaneous Automotive connectors, heatshrink tubing, cable ties, superglue. Extra parts for the 24V version 1 PC stake 1 22kΩ resistor 5 820Ω 1W resistors as far as the hardware is concerned, is to the input circuitry around IC2a (plus we’ve eliminated some of the switches). And that’s the beauty of using a PIC processor – we can use similar circuitry but get it to perform a completely different function by rewriting the software that controls the internal “smarts” of the device. As a bonus, we can shrink the parts count and that in turns means lower cost. OK, let’s start with the voltage sensing circuit based on IC2a. As shown in Fig.1, the battery voltage is applied to a divider consisting of a 10kΩ resistor and a 1.8kΩ resistor in series with a 1kΩ trimpot (VR1). Assuming a 12V battery, the battery voltage is divided by a factor of 5.1, filtered using 10µF capacitor and applied to pin 2 of comparator stage IC2a. In operation, IC2a compares the voltage on its pin 2 input with a DC voltage on its pin 3 input. This DC voltage is derived by applying a pulse width modulated (PWM) square-wave signal from the RA3 output of IC1 to a 1µF capacitor via a 10kΩ resistor. As a result, pin 1 of IC2a switches low when ever the voltage on its pin 2 is greater than the voltage on pin 3. This signal is then fed via a 3.3kΩ limiting resistor to the RB0 input of IC1. The resistor limits the current flow from IC2a when its output goes high to a nominal 12V, while the internal clamp diodes at RB0 limit the voltage on this pin to 5.5V. A-D converter Most of the complexity of this circuit is hidden inside the microcontroller (IC1) and its internal program. However, among other things, IC1 functions as an analog-to-digital (A-D) converter. In operation, it converts the comparator signal on its RB0 (pin 6) input into a digital value which is then used to drive the 3-digit LED display. The A-D converter used here is a little unusual and only requires two connections to the microcontroller. As mentioned above, the output at RA3 produces a PWM signal and this operates at 1.953kHz with a duty cycle ranging from .075% to 90%. Note that the high output level is at +5V while the low output level is at 0V. The 10kΩ resistor and 1µF capacitor filter the output from RA3 to derive a DC voltage that is the average of the duty cycle waveform. This means that if the duty cycle is 50% (ie, a square wave), the output voltage is 50% of 5V, or 2.5V This voltage is applied to pin 3 of IC2a. Other DC voltages are obtained by using different duty cycles. This DC voltage is connected to pin 3 of IC2a which is used as a comparator. Operation of the A-D converter is as follows: initially, the RA3 output operates with a 50% duty cycle and this sets the voltage at pin 3 of IC2a to 2.5V. At the same time, an 8-bit register inside IC1 has its most significant bit set high so that its value will be 10000000. The 50% duty cycle signal is produced by IC1 for 65.5ms, after which the comparator output (pin 1 of IC2a) is monitored by the RB0 input. Pin 1 of IC2a is low if the divided battery voltage at pin 2 is greater than 2.5V and high if the divided voltage is less than 2.5V. What happens now is that if the divided voltage is less than 2.5V, the PWM output at RA3 is reduced to a 25% duty cycle to produce 1.25V. The internal register is now set to 01000000. Alternatively, if the divided voltage is greater than 2.5V, corresponding to a low comparator output, RA3’s output is increased to a 75% duty cycle to provide 3.75V. The register is thus set to 11000000, with the most significant bit indicating a 2.5V 50% duty cycle and the next bit indicating the 1.25V 25% duty cycle (adding the two bits gives us the 3.75V). The comparator level is now again checked after 65.5ms, after which the microcontroller adds or subtracts a 12.5% duty cycle (0.625V) and checks against the divided battery voltage again. The register is then set at X1100000 (with the X value a 1 or 0 as determined by the previous operation) if the input voltage is higher than the PWM waveform. If the input voltage is lower than the PWM voltage, the register is set at X0100000. This process continues for eight cycles, the microcontroller either adding or subtracting smaller amounts of voltage (0.3125V, 0.156V, 0.078V, 0.039V and 0.0195V) and the lower bits in the 8-bit register being either set to a 1 or a 0 to obtain an 8-bit A-D conversion. The A-D conversion thus has a resolution of about 19mV (0.0195V) at the least significant bit. In addition, there are 256 possible values for the 8-bit register, ranging from 00000000 (0) to 11111111 (255). In practice, however, we are limited to a range from about 19 to 231. This is because the software must have time for internal processing to take place, to produce the waveform at RA3’s output and to monitor the RB0 input. The two values (ie, 19 & 231) correspond to 1.9V and 23.1V for the 12V measurement mode. This restricted measurement range is not really a problem for a car voltmeter since we only need to measure within a narrow range from about 6-16V for a 12V battery. Following the A-D conversion process, the binary number stored in the Fig.2: the top waveform in this scope shot shows the output from pin 2 of IC1. In this case, the peak-to-peak output is 5.12V and the duty cycle is 50%. The bottom trace shows the resulting filtered waveform on pin 3 of IC2. 8-bit register must be converted to a decimal value before it can be shown on the 3-digit display. Once again, this takes place inside the PIC microcontroller. Note that, in the 24V mode, the 8-bit register value is multiplied by two before being converted to the decimal value. This gives a resolution of 200mV for the measured voltage. The A-D conversion relies on several factors to produce a consistent reading. First, the reference voltage must remain stable and this means that the output from RA3 must swing to the full positive supply rail and all the way to ground. If it doesn’t, then the filtered output from RA3 will vary and give inaccurate results. For the same reason, the duty cycle of the PWM waveform at RA3’s output must remain accurate over each 65.5ms period. In this case, the reference uses the supply from an LM2940T-5 regulator which has excellent long term stability Main Features • • • • • Compact case. 3-digit LED display with automatic dimming. 12V or 24V operation. Optional remote voltage sensing. Minimum hold voltage display. (20mV/1000 hours at 150°C junction temperature and at maximum input of 26V). Its temperature variation is just 20mV over a 100°C range. In addition, the output at RA3 is CMOS and swings to within a few millivolts of the supply rails at no load. As for the duty cycle, this is set by the software and is controlled using a 4MHz crystal oscillator on pins 15 & 16. This means that the resultant voltage reading should be accurate to ±1 digit (±2 digits for 24V operation). The minimum hold switch (S1) is monitored at the RA4 input. Normally, the RA4 input is held high via a 10kΩ resistor to the 5V supply. However, when the switch is closed, it pulls the RA4 input low. This low is then detected by the software which subsequently loads the 7-segment data for the minimum voltage reading into the display register. When S1 is released, RA4 is pulled high again and the current battery voltage is again displayed. LED displays The 7-segment display data from IC1 appears at outputs RB1-RB7. These outputs directly drive the LED displays via 150Ω current limiting resistors while the RA0-RA2 outputs drive the individual displays via switching transistors Q1-Q3. The displays are driven in multiplex fashion, with IC1 switching its RA0, FEBRUARY 2000  27 and off at 1.96kHz, they appear to be continuously lit. Display brightness Fig.3: install the parts on the PC boards as shown here. Note particularly the orientation of switch S1 and be sure to use a BC338 transistor for Q4. The 820Ω resistors (shown in green) are used only in the 24V version. IC2b is used to control the display brightness. This op amp is wired as a voltage follower and drives a transistor buffer stage (Q4) which is inside the negative feedback loop. Light dependent resistor LDR1 controls the voltage on the pin 5 input of IC2b according to the ambient light level. IC2b drives Q4 which in turn controls the voltage applied to the emitters of the display drivers (Q1-Q3). During daylight hours, the voltage on pin 5 (and thus on pin 7) is close to +5V because the LDR has a low resistance in strong light. This means that Q4’s emitter will also be close to +5V and so the displays are lit at full brilliance Conversely, as the light level falls, the resistance of the LDR increases and the voltage on pin 5 of IC2b decreases. In fact, when it’s completely dark, the voltage on pin 5 is determined by the setting of trimpot VR2 which sets the minimum brightness level. As before, this voltage appears at Q4’s emitter and so the displays are all driven at reduced brightness. Note that, in practice, VR2 is adjusted to give the desired display brightness at night. Clock signals RA1 and RA2 lines low in sequence. For example, when RA0 is brought low, transistor Q1 turns on and applies power to the common anode connection of DISP1. Any low outputs on RB1-RB7 will thus light the corresponding segments of that display. After this display has been on for a short time, the RA0 output is taken high and DISP1 turns off. The 7-segment data on RB1-RB7 is then updated, after which RA1 is brought low to drive Q2 and display DISP2. Finally, RA2 is taken low and new 7-segment data presented to DISP3. This cycle is repeated for as long as power is applied to the unit and because the displays are switched on Clock signals for IC1 are provided by an internal oscillator circuit which operates in conjunction with crystal X1 (4MHz) and two 15pF capacitors. The two capacitors are included to provide the correct loading for the crystal and to ensure reliable starting. The crystal frequency is divided down internally to produce separate clock signals for the microcontroller Resistor Colour Codes          No. 1 3 1 1 3 8 5 1 28  Silicon Chip Value 22kΩ 10kΩ 3.3kΩ 1.8kΩ 680Ω 150Ω 820Ω 10Ω 4-Band Code (1%) red red orange brown brown black orange brown orange orange red brown brown grey red brown blue grey brown brown brown green brown brown grey red brown brown brown black black brown 5-Band Code (1%) red red black red brown brown black black red brown orange orange black brown brown brown grey black brown brown blue grey black black brown brown green black black brown grey red black black brown brown black black gold brown operation and for the display multi­ plexing. Power Power for the circuit is derived from the vehicle’s battery via the ignition switch. A 10Ω 1W resistor and 22µF capacitor decouple this supply rail, while 20V zener diode ZD1 protects the circuit from transient voltage spikes above this value. The decoupled ignition supply rail is then fed to regulator REG1 which provides a +5V rail. This rail is then used to power all the circuitry except for IC2 which is powered directly from the decoupled ignition supply. A 47µF capacitor and a 0.1µF capacitor are used to decouple the regulator’s output. For 24V systems, the supply input is applied via five parallel-connected 820Ω 1W resistors which provide a voltage drop to limit dissipation in the regulator. Note that a low dropout regulator is used to allow the voltmeter to operate down to about 5.5V for 12V systems. A standard regulator would have only allowed measurements down to about 8V before REG1 began to drop out of regulation. OK, so much for the circuitry. Of course, most of the clever stuff takes place inside the PIC microcontroller under software control. For a broad overview of how this software works, take a look at the accompanying panel. Construction Fortunately, you don’t have to understand how the software works to build this project. Instead, you just buy the ready-programmed PIC chip and “plug it in”. All the parts are mounted on two small PC boards: a processor board coded 05102001 and a display board coded 05102002. These are stacked together using pin headers and cut down IC sockets. Fig.3 shows the assembly details. Before installing any of the parts, check the PC boards carefully for etching defects and undrilled holes. Two large holes are required in the display PC board to accommodate a screwdriver to adjust VR1 and VR2. These are just below DISP3 and to the left of S1. Note that two small pilot holes are provided in each location to suit two different trimpot sizes – just drill out the holes to suit the trimpots supplied. The display board (in case at top) plugs into the pin header sockets on the processor board (above). Notice how the bodies of the electrolytic capacitors on the processor board are bent over, so that they lie parallel to the board surface. You can now start the assembly by installing the parts on the processor board. Begin by installing all the wire links, then solder in all the resistors using the accompanying resistor colour code table as a guide. It’s also a good idea to use a digital multimeter to measure each one, just to make sure. Note that the seven 150Ω resistors Capacitor Codes    Value IEC Code EIA Code 0.1µF 100n 104 15pF   15p   15 are mounted end on. Note also the different values for the resistor immediately below VR1. The two horizontal trimpots (VR1 & VR2) can go in next, followed by PC stakes at the four external wiring points. This done, solder in a socket for IC1 (but don’t install the IC yet), then install IC2 by soldering it directly to the PC board. Make sure that both the socket and IC2 are correctly oriented. This done, install zener diode ZD1 and transistors Q1-Q4. Be careful here – Q4 is a BC338 NPN type while Q1-Q3 are BC328 PNP types, so don’t get them mixed up. Zener diode ZD1 can now be FEBRUARY 2000  29 How The Software Works We have already described the operation of the A-D converter in the main article and this forms a major part of the software operation. Other sections of the software come under two headings: (1) MAIN and (2) INTRUPT. The accompanying flowchart shows the MAIN and INTRUPT programs. The MAIN program operates when the processor is reset after first powering up. It sets up the RB0 and RA4 ports as inputs and the RB1-RB7 and RA0-RA3 ports as outputs. It then reads the value stored in memory for 12/24V mode and places it in a flag register. After this, it looks for a pressed switch which is used to change the 12/24V option. If the switch is pressed, it toggles from the current option to the other (ie, if the unit was in 12V mode, it toggles to 24V mode and vice versa). The new option is then written to memory for storage. Interrupts are now allowed which starts the program skipping to the INTRUPT section when ever the internal timer triggers an interrupt. We interrupt via an internal timer which can be preloaded so that the period between interrupts can be adjusted. This feature is used to generate the pulse width modulation output at RA3. If we want the RA3 output to be low for a short time, we load the timer with a value close to 255. Then, when the counter increases and overflows from 255 to 0, we have another interrupt. The converse happens for a high output from RA3. In this case, the timer is preloaded with a value of 255 minus the value used for the RA3 low output time. When the next interrupt occurs (ie, when the count rolls over from 255 to 0 again), RA3 goes low and the cycle start all over again. The value that is loaded into the counter is called LOW_TIME and is the same value as used in the 8-bit register for the A-D conversion. This A-D conversion is detailed in the circuit description and its operational block is shown in the MAIN and INTRUPT program flowchart. The display is updated in the multiplex routine when the total 255 counter period has expired. This occurs on each second timer overflow interrupt. The multiplexing lights the next display and switches off the previous one. The left digit is blanked if the value for the display is below 10.0V. After the A-D conversion, in the Main program, the software tests the minimum hold switch. If it is pressed, the LOW_1 value (ie, the lowest value) is displayed. If the switch is open, the REAL_V value, which is the value arrived at during the A-D conversion, is compared with the current LOW_1 value. If the REAL_V value is the lower of the two, it replaces the current LOW_1 value (ie, the LOW_1 value is updated). A check as to whether the 12V or 24V flag is set determines whether or not the value for display is multiplied by two, as required for the 24V setting. Finally, the values are converted to decimal for the display. The process then continues with another A-D conversion to measure the voltage again. Full software for the Digital Voltmeter can be obtained from our website and is called DVM.ASM. This may be used by readers who are interested in the programming details. 30  Silicon Chip Specifications Range: about 5.5-23.1V when powered from a 12V battery; 1846.2V when powered from a 24V battery. Display Resolution: 100mV in 12V mode, 200mV in 24V mode. Update time: 0.52s The pin headers are installed on the copper side of the display board using a fine-tipped soldering iron. These headers plug into matching sockets on the processor board. Crystal X1 also mounts horizontally on the PC board. It is secured by soldering a short length of tinned copper wire between one end of its metal case and an adjacent PC pad to the right of transistor Q2. Finally, the three 7-way in-line sockets can be fitted. These are made by cutting two 14-pin IC sockets into single in-line strips using a sharp knife or a fine-toothed hacksaw. Clean up the rough edges with a file before installing them on the PC board. Display board This view shows the completed module, with the two PC boards stacked together in “piggyback” fashion. Make sure that none of the parts on the processor board contact the back of the display board. installed, followed by REG1. The latter is installed with its metal tab flat against the PC board and with its leads bent at rightangles to pass through their respective mounting holes. Be sure to accurately align the hole in the regulator’s metal tab with its hole in the PC board. The capacitors can go in next, mak- ing sure that the electrolytic types are all correctly oriented. Note that the electrolytics must all be mounted so that they lie parallel with the PC board. In particular, the 22µF & 47µF capacitors at bottom right lie across the regulator leads, while the two 10µF capacitors lie across the adjacent 1.8kΩ and 10kΩ resistors. Now for the display board: install the seven wire links and the resistors first, then install the three 7-segment LED displays with their decimal points at bottom right. Note that the links all go under the displays, which is why they’re shown dotted on Fig.2. The 820Ω 1W resistors (shown in blue) are required for the 24V version only. The LDR is mounted so that its top face is about 3mm above the displays. Install it now (it can go in either way), then install S1 with its flat side oriented as shown. Finally, complete the display board assembly by installing the pin headers. These are installed from the copper side of the board, with their pins protruding about 1mm above the top surface. You will need a fine-tipped iron to solder these pin headers. You will also have to slide the plastic spacers along the pins to give sufficient room for soldering. Preparing the case Fig.4: the two PC boards are secured together using spacers, a 2mmthick washer and several machine screws. Work can now begin on the plastic case. First, use a sharp chisel to remove the integral side pillars, then slide the processor PC board into place and use it as a template to drill two mounting holes in the base – one through the hole in REG1’s metal tab and the other immediately below the 0.1µF capacitor on the far lefthand FEBRUARY 2000  31 small dabs of super glue along the inside edges. Finally, a hole is also required in the rear (base) of the case for the power leads. Testing Fig.5: this full-size front panel artwork can be used as a drilling template. It’s a good idea to check the power supply before plugging the microcontroller IC into its socket. To check the supply, first unplug the display board and put it to one side. Now connect automotive hookup wire to the +12V and GND (chassis) inputs on the processor board. This done, apply power and use a multimeter to check that there is +5V on pins 4 & 14 of IC1’s socket (you can use the metal tab of REG1 for the negative connection). If this is correct, disconnect the power and install IC1 in its socket, ensuring that it is oriented correctly. This done, plug the display board back into the pin headers on the processor board and reapply power. The LED displays should light and show “L0”, indicating that the input voltage is below 1.9V (ie, not connected). You can test the dimming feature by holding your finger over the LDR. Adjust VR2 until the display dims. Calibration Fig.6: check your boards carefully against these full size PC artworks before installing any of the parts. side. This done, use an oversize drill to countersink these holes at the rear of the case, to suit the specified M3 x 6mm CSK screws. Next, plug the display board into the processor board and secure them together as shown in Fig.4. Check that the leads from the parts on the display PC board do not interfere with any parts on the processor PC board. If necessary, trim the leads of the parts on the display board to avoid this. The front panel artwork can now be affixed to the panel and used as 32  Silicon Chip a template for drilling the LDR and switch holes and for making the display cutout. It’s best to drill a small pilot hole for the switch first and then carefully enlarge it to the correct size using a tapered reamer. The display cutout is made by first drilling a series of small holes around the inside perimeter, then knocking out the centre piece and filing to a smooth finish. Make the cutout so that the red Perspex or Acrylic window is a tight fit. This window can then be further secured by applying several The calibration procedure for both versions is straightforward. Basically, the procedure involves applying a suitable input voltage and adjusting trim­pot VR1 until the reading on the display matches the reading obtained on a digital multimeter. Let’s look at the 12V version first. The step-by-step procedure is as follows: (1). Connect the “To Battery +ve” terminal to the “+12V Via Ignition Switch” terminal using a short length of wire. (2). Connect a 12V (approx.) supply to the “+12V Via Ignition Switch” terminal and ground. (3). Compare the reading against a digital multimeter and adjust VR1 for the same reading. Note that the Digital Voltmeter only updates about every 0.5 seconds, so adjust VR1 slowly during this procedure. If you don’t have a digital multimeter, connect the “To Battery +ve” terminal to the output of REG1 and adjust VR1 for a reading of 5.0V. This should give a reasonably acc­ urate calibration, to within ±150mV. Truscott’s • RESELLER FOR MAJOR KIT RETAILERS • PROTOTYPING EQUIPMENT • COMPLETE CB RADIO SUPPLY HOUSE • TV ANTENNA ON SPECIAL (DIGITAL READY) • LARGE RANGE OF ELECTRONIC COMPONENTS Professional Mail Order Service Truscott’s The Perspex window should be a tight fit in the front panel cutout and can be further secured by applying spots of super glue along the inside edges. The calibration procedure for the 24V version is only slightly more complicated. In this case, you have to “switch” the unit to 24V mode first before calibration can take place (the 12V mode is the default). The step-bystep procedure is: (1). Connect the “To Battery +ve” terminal to the “+24V Via Ignition Switch” terminal. (2). Press (and hold down) the Min/ Hold switch and apply 18-30V to the Digital Voltmeter. The display will show an “H” to indicate that the 24V mode has been set. This setting will now remain even if the supply is subsequently switched off and on again. (3). Compare the Digital Voltmeter reading against the reading obtained on a digital multimeter and adjust VR1 for the same reading. Be sure to adjust VR1 very slowly – as before, the Digital Voltmeter updates only about twice every second. Note also that the reading will only show an even number after the decimal point (ie, it indicates in 200mV steps). This means that a 24.1V supply may show 24.0 or 24.2V but not 24.1V. (4) If you don’t have a digital multimeter, connect the “To Battery +ve” terminal to the output pin of REG1 and adjust VR1 for a reading of 5.0V. Once again, this should give a reasonably accurate calibration. By the way, if you want to revert to the 12V mode, all you have to do is again press the Min/Hold switch as power is applied. The display will now show an “L”, indicating that the 12V setting mode has now been selected. ELECTRONIC WORLD Pty Ltd ACN 069 935 397 Ph (03) 9723 3860 Installation Be sure to use automotive cable and connectors when installing the unit into a vehicle. The +12V supply is derived via the ignition switch and a suitable connection can usually be made at the fusebox. Be sure to choose the fused side of the supply rail, so that the existing fuse is in series. The ground connection can be made by connecting a lead to the chassis via a solder eyelet and a self-tapping screw. The “To Battery +ve” input can also go to the fused side of the ignition switch. Alternatively, this connection can be run directly to the positive terminal of the battery via an in-line automotive fuseholder (mount this fuseholder close to the battery terminal). This reduces the voltage drops across the wiring of the ignition supply and gives a more accurate reading of the battery voltage, particularly when starting. The only drawback with the direct connection method is that there will be a constant 1mA drain from the battery. However, this current is so low that it really shouldn’t cause any problems, even if the battery is left for extended periods without recharging. Note: When using the voltmeter with 24V vehicles, the five 820Ω resistors will become quite hot. To alleviate this, we recommend replacing them with 10 1.8kΩ 1W resistors. The five added resistors can be installed on the SC underside of the PCB. Amidon Stockist Fax (03) 9725 9443 27 The Mall, South Croydon, Vic 3136 (Melway Map 50 G7) email: truscott<at>acepia.net.au www.electronicworld.aus.as P.C.B. Makers ! • • • • • • • • • If you need: P.C.B. High Speed Drill P.C.B. Guillotine P.C.B. Material – Negative or Positive acting Light Box – Single or Double Sided – Large or Small Etch Tank – Bubble or Circulating – Large or Small U.V. Sensitive film for Negatives Electronic Components and Equipment for TAFEs, Colleges and Schools FREE ADVICE ON ANY OF OUR PRODUCTS FROM DEDICATED PEOPLE WITH HANDS-ON EXPERIENCE Prompt and Economical Delivery KALEX 40 Wallis Ave E. Ivanhoe 3079 Ph (03) 9497 3422 FAX (03) 9499 2381 • ALL MAJOR CREDIT CARDS ACCEPTED FEBRUARY 2000  33