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Have you ever experienced that sinking feeling when your car won’t start on those cold winter’s mornings? This digital voltmeter will show you how your car’s battery is faring. A digital voltmeter for your car By DARREN YATES Imagine this situation. It’s 6:30am, cold, dark and raining outside. It’s also time to go to work so you bolt for the car, fumble through your keys, unlock the door and dive in for all you’re worth! Made it; so far so good. Now to start ‘er up. You put the key in the ignition and crank the engine only to be greeted by an infuriatingly slow “rur, rur, rur” from the starter motor. Blast it! – crook battery. You’re not going anywhere; at least not until the battery has been recharged or replaced. The foregoing is not an unlikely scenario and, with only minor variations, has happened to most motorists. In fact, the battery is the most likely component in your car to fail during the winter months. This project won’t stop the battery from failing but it will tell you when the battery is on the way out. It accurately measures the battery voltage SUPPLY REGULATION V/F CONVERTER IC1 3-DIGIT COUNTER IC3 DISPLAY DRIVER IC4 TIMING CONTROLS IC2 3-DIGIT DISPLAY 24  Silicon Chip Fig.1: block diagram of the Car Digital Voltmeter. The battery voltage is fed to a voltage-to-frequency converter & this drives a 3-digit counter & the LED displays. over an 8-17V range and displays the result on a 3-digit LED display with 0.1V resolution. If the battery voltage consistently reads less than 12V, then either the charging system is not working correctly or the battery has reached the end of its life. Either way, it’s time to take action to avoid getting stranded. The Car Digital Voltmeter can also warn you if the battery is being overcharged, as can happen if the regulator in the alternator fails. By attending to this sort of problem quickly, you can not only avoid battery damage but also avoid damage to your car’s engine management computer. Block Diagram Refer now to the block diagram of Fig.1. This shows the basic circuit sections. Most digital voltmeters now use one of the Intersil 7106/7 series chips but, unfortunately, these are expensive in one-off quantities. As an alternative, our Digital Car Voltmeter uses an older but more economical circuit technique B E C E I GO B Q6 BC557 E C B IC2f .0022 100k .01 +5V 14 10 10 4 3 LE CLK 11 11 CAR DIGITAL VOLTMETER 15 MR 13 8 2 IC3 MC14553 D2 D1 D0 12 12 16 16 +5V IC2e 11 4 IC1b 8 470k .0033 6 10k 470k .0033 E IC2b 2.2M 0.1 CHASSIS 3 IC2a 4049B 2 VR1 2.2k 3.3k 0.1 5 47k 47k 4 100k 100 16VW 7 10k 47k C B Q1 BC547 2 3 .015 IC1a LM358 IC2c 0.1 100 16VW GND OUT 7805 IN +12V VIA IGNITION SWITCH D1 1N4004 IC2d 9 1 100k 0.1 1 10k 100k 10k +5V +5V 10 5 6 8 12 7 +5V C E VIEWED FROM BELOW B 3.3k 1 15 7 9 LE 8 5 C Q4 BC557 E B C Q2 BC557 E 3.3k B 5 6 6 2 1 7 A 3 3.3k 1k 10 10 B IC4 4511 C D f g 14 15 9 e d c b 13 a 4 Fig.2 shows the full circuit details. In addition to the LED displays, it uses three CMOS ICs, an LM358 dual op amp package and a handful of other parts. Let’s see how it all works. Op amps IC1a and IC1b together form the V/F converter sec­tion. IC1a is connected as an inverting integrator while IC1b is configured as a Schmitt trigger inverter. The incoming battery voltage is fed to a voltage divider consisting of a 3.3kΩ resistor and calibration trimpot VR1. From there, the sampled voltage is fed to the inverting (pin 2) input of IC1a via a 100kΩ resistor. It is also further divided by two and fed to the non-inverting input. In operation, IC1a’s output (pin 1) ramps up and down due to the presence of Schmitt trigger IC1b and transistor Q1 in its negative feedback loop. This Schmitt trigger has its upper and E 3 Q3 BC337 C B c d DISP1 HDSP-5303 10 9 2 11 11 6 4 12 7 e f g a b 1k Q5 BC337 3 DISP2 HDSP-5303 5 DP 180  7x 56  16 Circuit diagram Fig.2 (right): IC1a & IC1b form the V/F converter & this clocks IC3, the 3-digit counter. Its multiplexed outputs drive IC4, a 4511 display driver/decoder, & this then drives the displays via 56Ω current limiting resistors. The common cathodes of the displays are driven by the digit driver outputs (D0-D2) of IC3 via PNP/NPN transistor pairs Q2-Q3, Q4-Q5 & Q6-Q7. C 1k +5V Q7 BC337 3 DISP3 HDSP-5303 +5V that uses common parts. It connects directly to the positive and negative termi­nals of the battery and these are the only two connections to the car’s wiring – the circuit is powered directly by the battery it is measuring. As shown in Fig.1, the battery voltage is applied to a voltage regulator circuit and this provides a fixed +5V rail for the counter and display driver circuit. In addition, the battery voltage is applied to a voltage-to-frequency (V/F) converter based on IC1. This in turn produces a square-wave signal whose frequency is proportional to the battery voltage. The square-wave signal produced by IC1 clocks a 3-digit counter based on IC3. This counter is stopped and started by a timing circuit based on IC2, so that it essentially functions as a frequency meter. It’s outputs are fed into a 7-segment decod­er/display driver circuit which then drives the three LED dis­plays. June 1993  25 Q2 Q3 3.3k 1k +5V CLK GND 3.3k 1k MR Q6 LE 56  3.3k 1k DISP3 56  IC4 4511 IC3 MC14553 56  56  1 .0022 1 180  0.1 3.3k lower thresholds set to approximately 2/3Vcc and 1/3Vcc respec­tively by its two 100kΩ feedback resistors. When power is first applied, IC1a’s output ramps down line­arly until it reaches the lower threshold of IC1b (about 1.7V). When this point is reached, pin 7 of IC1b goes high and turns on Q1. This pulls pin 2 of IC1a low via a 47kΩ resistor and the voltage on pin 1 now rises as the .015µF capacitor charges in the opposite direction. When it reaches the upper threshold of the Schmitt trigger (about 3.4V), pin 7 of IC1b switches low and Q1 turns off. Pin 1 of IC1a now ramps down again and so the cycle continues indefinitely. As a result, a triangle waveform appears at pin 1 of IC1a, while a squarewave of the same frequency appears at pin 7 of IC1b. The frequency of this square-wave is directly proportional to the input voltage. It not only drives Q1 but also clocks pin 11 of IC3, a CMOS 4553 3-digit counter. IC3 contains three separate decade counters. Its 4-bit outputs appear in multiplexed fashion on pins 5, 6, 7 & 9 (Q0-Q3), while pins 15, 1 & 2 (D0-D2) are the digit driver outputs. The .0022µF capacitor between pins 3 & 4 sets the frequency of an internal oscillator and this in turn sets the speed at which the outputs are multiplexed. The 4-bit outputs are fed into the inputs (A-D) of IC4, a CMOS 4511 IC1 LM358 .015 Q1 10k 10k 10k VR1 1 .01 IC2 4049 1 10k 100uF 100k 470k 100k 100k 100uF 47k D1 26  Silicon Chip DISP2 Q7 7805 +12V VIA IGNITION SWITCH CHASSIS DISP1 Q5 56  56  56  Q4 0.1 0.1 47k 47k 100k Fig.3: install the parts on the two PC boards exactly as shown in this diagram. Make sure that all parts are correctly oriented & note that a small heatsink is fitted to the 7805 regulator to keep it cool. After completion, the two boards are wired together via their +5V, MR, CLK & GND connections. .0033 0.1 2.2M .0033 +5V CLK 470k GND MR LE 7-segment display driver/decoder IC. This converts the 4-bit BCD code into 7-segment outputs which directly drive the three LED displays via 56Ω current limiting resistors. In addi­ tion, pin 5 of DISP2 is permanently connected to the +5V rail via a 180Ω current limiting resistor so that its decimal point is always on. The other two decimal points are unused. Each display is switched on at the correct time via the digit driver outputs (pins 15, 1 & 2). These are active low outputs; ie, for a particular digit to light, its display output must go low. These outputs each drive a PNP/NPN transistor pair and these in turn switch the common cathodes of the display digits to ground. Of course, all this is done at high speed so that, as far as the observer is concerned, the three displays appear to be continuously lit. Timing To get the circuit to count correctly, we need to provide latch enable (LE) and memory reset (MR) timing signals for IC3. This task is performed by IC2a, a CMOS 4049 hex inverter IC. IC2a and IC2b form a basic squarewave oscillator with a frequency of about 2Hz. Its output appears at pin 4 and is cou­pled to pin 7 of IC2c via a .0033µF capacitor. Thus, each time pin 4 switches high, pin 7 also briefly switches high while the capacitor charges. As a result, pin 6 of IC2c generates a train of narrow negative-going pulses and these are fed into the LE input of IC3 (pin 10). Each time a pulse is received, the current count in IC3 is latched into the Q0-Q3 outputs and the display is updated (ie, the display is updated twice every second). Inverters IC2d and IC2e, along with the .0033µF capacitor and the 470kΩ resistor, provide a short time delay to ensure that all data lines are steady before the memory reset takes place. Normally, pin 12 of IC2e is low but when pin 6 goes high (at the end of the LE pulse), pin 12 goes high for a brief period. When pin 12 goes low again, pin 15 briefly goes high and resets IC3 to 000. As soon as the reset signal falls low again, IC3 begins counting the pulses on its clock input from the V/F converter. This continues until a latch enable signal arrives and the dis­play is updated as described above. IC3 is then reset again and so the cycle is continuously repeated every 0.5s, as set by the frequency of the oscillator based on IC2a & IC2b. Power for the circuit is derived directly from the battery via a 7805 3-terminal regulator. Diode D1 provides reverse polar­ity protection for the circuit, while the two 100µF capacitors provide supply line decoupling. During operation, the circuit draws approximately 140mA which means that the regulator dissipates about 1.2W. This means that a small heatsink must be fitted to the 7805 to keep it cool. Construction All the components for the Digital Car Voltmeter are installed on two PC boards and these are mounted backto-back on 9mm spacers. The first board (code 04105931) holds the V/F con­verter, power supply and timing circuitry, while the second board (code 04105932) holds the counter circuitry and the LED displays. Before installing any of the parts, carefully check both boards for etching defects by comparing them with the published patterns. When you’re happy that everything is OK, you can start with the display board assembly. Fig.3 shows the parts layout on the two PC boards, with the display board at the top. The first thing to do is to install the 12 wire links. Make sure you get these PARTS LIST Take extra care when installing the transistors on the display board, as it’s easy to confuse NPN & PNP types. The two ICs both face in the same direction, while the displays must be oriented with their decimal points at bottom right. This board carries the regulator & the V/F converter & timing circuitry. It is connected to the display board via a 5-way rainbow cable & the two boards then bolted together on 9mm untapped spacers. The trimpot at lower left allows the unit to be accurately calibrated. in the correct position and don’t forget the small link immediately beneath DISP3. If necessary, you can straighten the link wire by clamping one end in a vyce and then stretching it slightly by pulling on the other end with a pair of pliers. Once the links are in, install the resistors and the .0022µF polyester capacitor. Table 1 shows the resistor colour codes, although it’s also a good idea to check each resis­ tor with a multimeter before installing it on the board (the colours on some brands can be quite difficult to decipher). Note that there are four vacant holes in the display board, to the right of the .0022µF capacitor. These holes are not used in this project. They were originally provided to allow the deci­mal point of DISP1 to be turned on (by installing another link and another 180Ω resistor), a feature that might be handy in future projects based on this board. The six transistors all face in the same direction but be sure to use the correct type at each location. Q2, Q4 & Q6 are all BC557 PNP transistors, while Q3, Q5 & Q7 are all BC337 NPN types. Double check that these are all correctly mounted as it’s easy to get them mixed up. Note that each transistor should be 1 PC board, code 04105931, 102 x 55mm 1 PC board, code 04105932, 102 x 55mm 1 plastic zippy case, 130 x 67 x 42mm 1 red Perspex window, 46 x 20mm 1 front panel label, 125 x 62mm 1 small U-shaped heatsink 1 1-metre length red automotive cable 1 1-metre length black automotive cable 1 40mm-length 5-way rainbow cable 1 2.2kΩ 5mm horizontal trimpot 4 9mm-long untapped spacers 4 9mm-long tapped spacers 4 3mm x 15mm-long machine screws 5 3mm x 6mm-long machine screws Semiconductors 1 LM358 dual op amp (IC1) 1 4049 hex inverter (IC2) 1 MC14553 3-digit BCD counter (IC3) 1 4511 7-segment decoder\ driver (IC4) 1 7805 5V regulator 1 BC547 NPN transistor (Q1) 3 BC557 PNP transistors (Q2,Q4,Q6) 3 BC337 NPN transistors (Q3,Q5,Q7) 1 1N4004 silicon diode (D1) 3 HDSP-5303 common-cathode 7-segment LED displays (DISP1-3) Capacitors 2 100µF 35VW electrolytic 4 0.1µF 63VW MKT polyester 1 .015µF 63VW MKT polyester 1 .01µF 63VW MKT polyester 2 .0033µF 63VW MKT polyester 1 .0022µF 63VW MKT polyester Resistors (1%, 0.25W) 1 2.2MΩ 4 3.3kΩ 2 470kΩ 3 1kΩ 4 100kΩ 1 180Ω 3 47kΩ 7 56Ω 4 10kΩ Miscellaneous Tinned copper wire for links (100mm) June 1993  27 This view shows the two boards stacked together & mounted on the front panel. Make sure that there are no shorts between the two boards when the assembly has been completed. pushed down onto the board as far as it will comfortably go before soldering, so that it doesn’t later foul the front panel. The display board can now be completed by installing the three LED displays. Make sure that these are correctly oriented, with the decimal point of each display to bottom right. V/F converter board The assembly procedure for this board is similar to that for the previous board. Install the three wire links first, followed by the resistors, capacitors and semiconductors. The 7805 regulator is installed with its leads bent at right angles. A small heatsink is then slid under its metal tab and the assem­bly bolted to the PC board using a screw and nut. The two completed PC boards can now be placed side-by-side and their +5V, MR, CLK & GND terminals wired together using a short length of rainbow cable. This done, connect the power supply leads to the V/F converter board. Two 1-metre lengths of automotive cable should be used for this job. Use a red cable for the positive lead and a black cable for the negative lead. Once the wiring has been completed, the two boards can be stacked together on four 9mm untapped spacers and held using 12mm-long mounting screws inserted from the V/F converter board side of the assembly. The assembly is then secured by fitting a 9mm tapped spacer to each mounting screw on the display board side – see photo. All that remains now is to install the module inside the specified plastic case. As shown in the photos, the module is mounted on the lid of the case, with the three LED displays visible through a perspex window. The first step is to attach the front-panel label to the lid and use it as CAPACITOR CODES ❏ ❏ ❏ ❏ ❏ ❏ Value 0.1µF .015µF .01µF .0033µF .0022µF IEC Code 100n 15n 10n 3n3 2n2 EIA Code 104 153 103 332 222 RESISTOR COLOUR CODE ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ No. 1 2 4 3 4 4 3 1 7 28  Silicon Chip Value 2.2MΩ 470kΩ 100kΩ 47kΩ 10kΩ 3.3kΩ 1kΩ 180Ω 56Ω 4-Band Code (1%) red red green brown yellow violet yellow brown brown black yellow brown yellow violet orange brown brown black orange brown orange orange red brown brown black red brown brown grey brown brown green blue black brown 5-Band Code (1%) red red black yellow brown yellow violet black orange brown brown black black orange brown yellow violet black red brown brown black black red brown orange orange black brown brown brown black black brown brown brown grey black black brown green blue black gold brown a drilling template for the four mounting holes. This done, drill a series of holes around the inside perimeter of the display cutout area. The centre piece can now be knocked out and the job filed to a smooth finish so that the Perspex® window is a tight fit. You will also have to drill an entry hole for the supply leads, either at one end or at the rear. After that, it’s simply a matter of securing the module to the lid using four 6mm-long machine screws. If necessary, the Perspex® window can be secured by gluing it in position using epoxy resin (don’t use too much). Test & calibration To test the unit, connect the supply leads to a 12V battery and check that the three displays immediately light up. If they do, then the unit is functioning correctly. It can now be cal­ ibrated by first checking the battery’s voltage with your digital multimeter and then adjusting VR1 until you get the same reading on the Digital Car Voltmeter. If it doesn’t work, switch off immediately and check for wiring errors. In particular, make sure that all parts are cor­rectly mounted and that there are no missed solder joints or shorts between tracks due to solder splashes. If these checks reveal nothing, apply power once more and check that the output of the 3-terminal regulator is at +5V. Check that this voltage appears on the supply pins of the ICs and on the emitters of Q2, Q4 & Q6. After that, it’s a matter of trying to isolate the fault to a particular circuit stage. For example, if one display fails to Fig.4: check your PC boards for defects by comparing them against these full-size etching patterns. light, check its two driver transistors. If the display always shows 000, check the V/F converter stage based on IC1 and Q1. If the circuit fails to reset and count correctly, check the values of the resistors and capacitors associated with IC2. Installation Make sure that you install this unit in the car in a pro­fessional manner. + + VOLTS + DIGITAL CAR VOLTMETER + Fig.5: this full-size artwork can be used as a drilling template for the front panel. In particular, all wiring connections should be made using automotive style connectors which should be well-insulated to avoid any possibility of short circuits. The positive supply lead goes to the battery via the igni­ tion switch and a fuse in the fusebox. Finding a suitable point to tap into shouldn’t be too difficult. It’s simply a matter of finding a terminal in the fusebox that goes to +12V when the ignition is turned on (eg, the IGN terminal). Avoid using a terminal that also goes to +12V when the ignition switch is turned to the accessories position, however. Finally, you can reduce the size of the box sitting on your dashboard by mounting the three LED displays on a small satellite board. These can be housed in a small case and wired back to the main unit via an 11-way rainbow cable. Footnote: Dick Smith Electronics has advised us that they will be offering an optional satellite display board with their version SC of this kit. June 1993  29