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A digital voltmeter for your car Main Fea t Have you ever experienced that sinking feeling when your car won’t start on those cold winter mornings? This digital voltmeter will let you keep tabs on the condition of your car’s battery & the charging system. By JOHN CLARKE Perhaps the most unreliable component in a modern vehicle is its battery. This is not surprising considering the work it has to do, often under quite arduous conditions. On a cold win­ter’s morning, for example, it is expected to deliver enormous cranking currents to the starter motor, this at a time when the battery is at its worst. A car battery will only last well and perform at its best when it is properly maintained. This means keeping an 26  Silicon Chip eye on the electrolyte level and keeping the charging voltage within strict limits. For a 12V battery, the charging voltage should be kept between 13.8V and 14.4V, while for a 24V battery, the charging voltage should be between 27.6V and 28.8V. If the charging voltage is too low, the battery will never fully charge and it will be unable to deliver the necessary current during cold starting. Conversely, if the battery is over­charged, the electrolyte will gas excessively, ures • Compact size • 3-digit LE D readou t • 0.1V reso lution • Suitable for 12V a nd 24V batteries • Leading “ 0” • Display d blanking imming a t night • High acc uracy • Negligible drift with temperature • Can be u sed as a 0-39.9V meter thereby reducing the electrolyte level and shortening the life of the battery. On some vehicles, the charging system is only marginal, particularly in wet weather, with the lights on and in heavy traffic. In these circumstances, the battery is often required to deliver power to all the electrical accessories. This is because the alternator is only Fig.1: block diagram of the Digital Car Voltmet­er. Most of the work is performed in IC1 which is an ICL7107 analog-todigital (A-D) converter. This IC directly drives the 3-digit LED display and produces a reading that corresponds to the voltage at its input. The accuracy of this reading relies on the stability of voltage reference REF1. driven by an idling engine and cannot adequately top up the battery. Similarly, if you make lots of short trips, the battery might not have a chance to adequately charge between starts. The result – a flat battery and you’re left stranded. By fitting this digital voltmeter to your car, you can easily keep tabs on the condition of the battery and the charging circuit. If the battery voltage consistently reads low, for example, then either the battery is on the way out or the charg­ing system is not working correctly. Either way, it’s time to take action. Conversely, if the battery voltage is always high, then the battery is being overcharged, as can easily happen if the regula­tor fails. This can not only damage the battery but, in severe cases, could also damage various electronic systems in the vehi­cle. So there are good reasons for carefully monitoring the battery voltage in a car 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. It also features automatic display dimming when the lights are turned on, to prevent the readout from being excessively bright at night. Fig.2(a) shows the basic method by which IC1 converts the analog input voltage to a digital display value. The two inputs, Vin and Vref, are fed to an integrator via switch S1 which selects between them. To measure the voltage at Vin, S1 is switched to position 1. The integrator initially charges capacitor Cx at a rate set by Vin for a fixed period of time. The higher the voltage at Vin the higher the voltage at Vx at the end of this time period – see Fig.2(b). Note that slope ‘A’ in Fig.2(b) reaches a higher Vx voltage than slope ‘B’ because Vin is higher for ‘A’. At the end of the fixed time period, switch S1 selects the Vref value (position 2) which is opposite in polarity to Vin. Thus, capacitor Cx discharges at a fixed rate as set by Vref. During this “de-integrate” period, a counter is clocked at a fixed rate until the capacitor is fully discharged. The compara­tor then switches and the number in the counter is displayed. This number is directly related to the voltage at Vin. How it works Fig.1 shows the block diagram for the Digital Car Voltmet­er. Most of the work is performed in IC1 which is an ICL7107 analog-to-digital (A-D) converter. This IC directly drives the 3-digit LED display and produces a reading that corresponds to the voltage at its input. The accuracy of this reading relies on the stability of voltage reference REF1. Fig.2: how the A-D converter works. To measure the voltage at Vin, S1 is first switched to position 1. The integrator then charges capacitor Cx at a rate set by Vin for a fixed period of time. At the end of this time, S1 is switched to Vref and the capacitor discharges. During this time, a counter is clocked at a fixed rate until the capacitor is fully discharged. April 1997  27 Fig.3: the reference voltage for A-D converter IC1 is derived using an LM336Z-2.5 (REF1). It's output is divided and applied to the REF HI and REF LO inputs. IC2 and its associated parts condition the signal input, while IC3 provides the display dimming feature. This method of A-D conversion is often used in digital voltmeters. It has the advantage that the accuracy is only de­pendent on the accuracy of the reference voltage. Although the technique uses a clock to set the fixed time during the integrate period and the count rate during the de-integrate phase, the stability of the clock is not overly important as far as conver­sion accuracy is concerned. That’s because 28  Silicon Chip the resulting digital value is not dependent on the clock rate. To understand why, let’s consider what happens if the clock is slower than normal. In that case, the Vx value will be higher than expected after the integrate stage and it will take longer to discharge Cx to 0V (ie, the de-integrate stage will take longer). However, that’s compensated for because the counter is clocked at a slower rate over this longer time period. As a result, the same value will be recorded, regardless of clock rate. Of course, if the clock rate is far too slow, the integrator may saturate because its output reaches the limit of the supply voltage. Conversely, if the clock is too fast, Vx will be lower but the counter will be clocked at a faster rate during the discharge period. Thus, any drift in the clock rate over time is cancelled in the conversion process, provided that the clock rate does not drift between conversions. PARTS LIST Fig.4: this is the waveform at the output of the 555 timer (IC3) when the car’s lights are on. Because the waveform is low for only 17% of the time, Q3 is only on for this time and so the displays are dimmed. Returning to Fig.1, the car battery voltage is applied to regulator REG1 and to a signal conditioning circuit based on IC2. The regulator provides a 5V supply rail, while the signal condi­tioning circuit converts the input signal to a voltage range suitable for feeding to IC1 . The display is controlled using dimming and leading “0” blanking circuitry. Leading “0” blanking is a cosmetic feature that blanks the first digit when the reading is below 10V. The leading zero blanking circuit works by detecting when the “f” segment in the most significant display is driven and then switching the whole display digit off. The “f” segment is only driven if 0, 4, 5, 6, 8 and 9 are to be displayed. Since we are only interested in displaying values well below 40.0, blank­ ing the leading digit for values above “3” is of no consequence. The display is dimmed when the dimming input is pulled high. This activates an oscillator which turns the displays on for only 17% of the time, thereby effectively reducing the aver­age display brightness. The switching speed of the oscillator is set high enough so that the display doesn’t flicker. Circuit details Refer now to Fig.3 for the circuit details. At the heart of the design is an Intersil ICL7107CPL 31/2-Digit Single Chip A-D Converter (IC1). It directly drives the three 7-segment LED displays and only requires a few extra components to make it work. The clock components are at pins 38, 39 & 40, while the RC network for the integrator is at pins 27 & 28. To improve accuracy and remove any offsets in the internal op amps, an auto zero capacitor has been included at pin 29. A reference capacitor at pins 33 & 34 is used to store the refer­ ence voltage during the de-integrate stage of the dual-slope D-A conversion. The reference voltage is derived using an LM336Z-2.5 (REF1). This device is connected between the +5V rail and the REFLO input of IC1. The current through REF1 is set to about 1mA using a 2.2kΩ resistor, while diodes D3 and D4 are used to com­ pensate the reference for temperature variations. Trimpot VR1 is adjusted to set the reference to 2.490V, at which point it has a mini­mum temperature co­efficient. VR2 divides the 2.490V from REF1 to provide a stable 1V refer­ence voltage between REFLO and REFHI. This sets the full scale input for IC1 to 1.999V. However, because we are only using three digits, the display can only show 1 PC board, code 04304971, 117 x 102mm 1 PC board, code 04304972, 88 x 30mm 1 front panel label, 132 x 28mm 1 ABS case, 140 x 110 x 35mm 1 red transparent Perspex sheet, 46 x 22 x 2-3mm 1 small TO220 heatsink, 30 x 25 x 13mm 1 3mm x 6mm long screw plus nut 4 9mm untapped standoffs 4 3mm x 15mm screws 9 PC stakes 1 60mm length of 0.8mm tinned copper wire 3 HDSP-5301 12.7mm high common anode LED displays 2 10kΩ horizontal trimpots (VR1, VR3) 1 50kΩ horizontal trimpot (VR2) Semiconductors 1 ICL7107CPL 31/2 digit A-D converter (IC1) 1 LF351, TL071 single op amp (IC2) 1 555 timer (IC3) 1 7805 5V regulator (REG1) 1 BC548 NPN transistor (Q1) 2 BC328 NPN transistors (Q2,Q3) 1 LM334Z-2.5 reference (REF1) 1 1N4752 33V 1W zener diode (ZD1) 1 1N4732 4.7V 1W zener diode (ZD2) 4 1N914, 1N4148 diodes (D1D4) Capacitors 1 100µF 63VW PC electrolytic 6 10µF 16VW PC electrolytic 1 0.22µF MKT polyester 2 0.1µF MKT polyester 1 0.047µF MKT polyester 1 100pF MKT polyester or ceramic Resistors (0.25W, 1%) 1 470kΩ 2 2.2kΩ 3 100kΩ 3 1kΩ 1 39kΩ 1 390Ω 3 10kΩ 1 47Ω 2 4.7kΩ 1 150Ω 1W 5% Miscellaneous Automotive wire, automotive connectors, solder, etc. April 1997  29 CAPACITOR CODES      Fig.5: the 7-segment displays must be installed with their decimal points at top left, as shown here. Make sure that all polarised parts are correctly oriented. up to 999mV (ignoring the leading zero blanking). The COM pin (pin 32) sits at a nominal 2.8V below the +5V supply rail; ie, at 2.2V. This means that INLO also sits at 2.2V, since it is tied to COM. The 10kΩ resistor between the COM pin and the +5V rail ensures that the Value IEC Code 0.22µF 220n 0.1µF 100n 0.047µF   47n 100pF 100p EIA Code 224 104 473 101 COM pin supply is biased correctly. With no input, INHI also nominally sits at 2.2V. That’s because the 2.2V on COM is applied to pin 3 of op amp IC2 via 1kΩ and 47Ω resistors. This stage operates with a gain of 1.01 due to the 1kΩ and 100kΩ feedback resistors and so its output is biased to 2.2V. IC2 and its associated input stage are also used to process and buffer the battery voltage before it is applied to IC1. The battery voltage is monitored via the ignition switch and is divided by 100 via a 100kΩ input resistor and the 1kΩ resistor connected to COM. This divided voltage is effectively added to the 2.2V bias voltage and then fed to IC2. Let’s say, for example, that 10V is applied to the input. This is divided to 100 and added to the 2.2V bias to give 2.3V on pin 3 of IC2. IC2 then buffers this voltage and applies it to the INHI input of IC2. As a result, the difference between the INHI and INLO inputs is 2.3V - 2.2V = 100mV. This is then displayed as 10.0 (ie, 10.0V) on the LED readouts. Diodes D1 & D2 are included to suppress any voltage spikes which could otherwise go beyond the supply rails and damage IC2. The associated 10µF capacitor also damps any voltage TABLE 1: RESISTOR COLOUR CODES  No.  1    3    1  3    2    2  3  1    1    1 30  Silicon Chip Value 470kΩ 100kΩ 39kΩ 10kΩ 4.7kΩ 2.2kΩ 1kΩ 390Ω 47Ω 150Ω 4-Band Code (1%) yellow violet yellow brown brown black yellow brown orange white orange brown brown black orange brown yellow violet red brown red red red brown brown black red brown orange white brown brown yellow violet black brown brown green black 5-Band Code (1%) yellow violet black orange brown brown black black orange brown orange white black red brown brown black black red brown yellow violet black brown brown red red black brown brown brown black black brown brown orange white black black brown yellow violet black gold brown not applicable The display board is soldered at right angles to the main PC board, as shown here (see text). Note the U-shaped heatsink fitted to REG1. This should be securely fastened to the board so that it can’t short against other parts. spikes. Trimpot VR3 is used to adjust the offset of IC2’s output so that the display reads 0.0 when the input is connected to ground. The LED displays are common anode types and are all con­ trolled by Q3. In addition, the leading digit (DISP1) is con­trolled by Q1 and Q2. Normally, the “f’ segment output from IC1 is high and so Q1 & Q2 are on and DISP1 is turned on via Q3. However, if the “f” segment output for the DISP1 digit goes low (eg, if a zero is to be displayed), Q1 turns off. This then turns off Q2 and so DISP1 also turns off to provide the leading zero blanking feature. Display dimming When the car’s lights are off, pin 4 of 555 timer IC3 is pulled low and so its pin 3 output is also low. This means that Q3 is on and so the displays run with a 100% duty cycle for full brilliance. When the lights are turned on, pin 4 of IC3 is pulled to 4.7V (as set by ZD2) and so IC3 begins to oscillate. Its operat­ing frequency is set to about 244Hz while the duty cycle is about 83%, as set by the RC timing components on pins 2, 6 & 7. This means that pin 3 is low for only about 17% of the time. And since Q3 is only on when pin 3 is low, it follows that the displays only operate with a 17% duty cycle. This reduces the display brightness, so that they don’t become intrusive at night. Power supply Power for the circuit is derived from the car’s battery via the ignition switch. The 15Ω resistor and zener diode ZD1 provide transient suppression, while the 100µF capacitor provides filter­ing. The filtered voltage is then fed to a 3-terminal regulator which produces a 5V supply for IC1, IC2 and IC3. Normally, the supply voltage to the SPECIFICATIONS • • • • • • • Voltage range 8-33V (0-39.9V when separately powered) Resolution 0.1V (100mV) Accuracy within 0.1V Temperature drift less than 0.5% from 0-60°C Quiescent current 130mA <at>15V, 150mA <at> 30V (full brightness) Input impedance 100kΩ Input current -27µA <at> 0V, 0µA at 2.2V, 122µA <at> 15V April 1997  31 Alternatively, if a separate power supply is used to drive REG1, the circuit can accurately measure input voltages down to 0V. As a result, the +12V supply and input terminals are not connected on the PC board so that the unit can be used in appli­cations where low voltage measurements are required. Construction Another view of the completed module, showing how the two boards are soldered together. Note how the 10µF electrolytic capacitors are bent over so that they clear the base of the case. The completed module is mounted upside down in the case, so that the display decimal points are at bottom right. The board is secured on 9mm spacers using 12mm-long screws which go into integral standoffs on the base of the case. circuit is connected to the input so that the battery voltage can be measured. However, if the input voltage to the regulator drops below about 8V, the circuit will give misleading results because of low voltage to the ICs. This is of no concern for a car battery voltmeter. DIGITAL CAR VOLTMETER 32  Silicon Chip Building this unit is easy since most of the parts are mounted on a main PC board coded 04304971. The only parts not on this board are the three 7-segment displays. These go on a sepa­rate display PC board coded 04304972 and this is then soldered to the main PC board at right angles. Before mounting any of the parts, carefully check the PC boards for any shorts between tracks or broken sections. If necessary, cut out the rectangular section at the front of the main board, where it meets the display board. Fig.5 shows the assembly details. Start by installing PC stakes at the four external wiring points and at test points TP1-TP5. This done, install the wire links and the resistors. Table 1 shows the resistor colour codes but it is also a good idea to check each value using a digital multimeter, just to make sure. Next, install the ICs, followed by the capacitors, diodes, zener diodes and the transistors. Make sure that all these parts are correctly oriented and that the correct type number is used at each location. In particular, don’t confuse transistors Q1 and Q2. The regulator (REG1) is mounted horizontally on the PC board with its leads bent at rightangles. It is then secured to both the board and a U-shaped heatsink using a screw, nut and lockwasher. A second heatsink should also be fitted to the copper side of the board if the unit is to be used with a 24V battery. Make sure that this second heatsink doesn’t short out any of the tracks. The display board can now be Fig.6: this full-size front panel artwork can be used as a template for cutting out the display window. functioning correctly and you can proceed with the calibration. Calibration Fig.7: check your etched PC boards against these full-size artworks before installing any of the parts. quickly assembled by installing the three LED displays. These must all be oriented with their decimal points at top left, as shown on Fig.5. Final assembly The unit is housed in a small ABS case measuring 140 x 110 x 35mm. This is fitted with a self-adhesive front panel label, while a red Perspex window covers the display area. The main job in the final assembly is to solder the two PC boards together at right angles. To do this, first mount the main PC board upside down on the base of the case and secure it on 9mm spacers using 3mm x 12mmlong screws. This done, the display board is butted against the main board and the two large end pads soldered. Make sure that the two boards are at rightangles and that the bottom edge of the display board rests against the case before making these connections. The PC board assembly should now be removed from the case and the remaining edge pads soldered together. Apply a generous fillet of solder to the two large end pad connections to ensure sufficient mechanical strength. Now for the smoke test but first go back over your work and carefully check for any errors. In particular, check that all parts are correctly oriented, that the correct part has been used at each location and that there are no missed solder joints. If everything is correct, apply power and check that the display lights up (note: only the last two digits should light). If it doesn’t, check transistor Q3. Now check for +5V at the output of the regulator (REG1), at pin 1 of IC1, at pin 7 of IC2 and at pin 8 of IC3. Next, check that the display dims when +12V is applied to the LIGHTS input. If it does, the unit is probably The calibration procedure is quite straightforward – just follow this stepby-step guide: (1) Connect a multimeter between TP1 and TP2 and adjust VR1 for a reading of 2.490V (this will give the minimum temperature drift for REF1). (2) Connect a multimeter between TP1 and TP3 and adjust VR2 for a 1V reading. This calibrates the full scale reading for the A-D converter. (3) Connect the INPUT terminal on the PC board to GND and adjust VR3 for a 0.0V reading. This sets the offset output of IC2. (4) Connect the INPUT and +12V terminals together and connect the multimeter between these terminals and GND. Check that the dis­ play shows the same reading as the multi­ meter. If not, adjust VR2 slightly until the readings are the same. That completes the calibration. Connect suitable flying leads to the four external wiring terminals and drill a small hole in the rear panel to provide an exit for these leads. The board assembly can now be finally secured to the base of the case. Finally, complete the construction by fitting the front panel. One approach is to substitute a piece of red Perspex for the whole of the front panel, with the area outside the display panel suitably masked (eg, with a stick-on label). Alternatively, you can cut a display window out of the existing panel and fit this with a red Perspex window for the displays. Installation The Digital Car Voltmeter can be installed on the dashboard of the vehicle. It is wired to the ignition, lights and ground connections on the fused side of the fusebox. Use automotive connectors for all wiring. The ground connection can be made to the chassis using an eyelet crimp-lug which is secured to the metal using a self-tapping screw. The separate INPUT connection to the voltmeter can be made at the fusebox, at a point which is switched via the ignition switch but which has a low current drain. This will ensure that the voltmeter is not measuring a low voltage due to drops across the vehicle wirSC ing. April 1997  33