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Keep tabs on your car’s battery charge and discharge currents with this dual-display ammeter. It includes a 3-digit display to indicate the current in amperes, as well as a bargraph to show the charge/discharge trend at a glance. By JOHN CLARKE I T’S NOW RARE TO SEE an ammeter installed in a car. Instead, virtually all modern (and not so modern) cars have an “idiot” light to indicate battery charging. Normally, this light is off when the engine is running and only comes on if the alter­nator fails; ie, when no charge is being delivered. Apart from that, it doesn’t provide any other information during normal driving. This means that when the light is out, you have no idea how much current is going into the battery or is being pulled out. And even when an ammeter was fitted, it was hardly what you would call a precision instrument. Most only gave a very rough idea of what happening. However, if you are an enthusiast, you will want to know more about battery charge and discharge rates. This Automotive Ammeter can provide 62  Silicon Chip this information with a high degree of accu­racy. Why is it important? Knowing the charging state of the battery is important since it’s a major component of the cars’ electrical system. If the battery isn’t charging properly, you could be left stranded. When the engine is running, the alternator normally provides all the power for the electrical loads and keeps the battery topped up. However, if there is insufficient charging current, the battery will gradually discharge. This can typically occur if the electrical load is high while the engine is idling, or if the connections to the battery are faulty or the battery itself is on the way out. Measuring the battery current involves measuring the cur­rent flowing in all the leads to one of the battery’s terminals. In addition, it’s necessary to MAIN FEATURES • • • • • • • Compact size Includes 7-LED bargraph display plus 2-digit readout ±0-30A indication on bargraph in 5A steps 1A resolution on 2-digit display Typical 80A maximum reading Dual indication for charge and discharge Automatic display dimming in low light conditions determine the direction of the current, so that we know whether the battery is being charged or discharged. Hall effect sensor The SILICON CHIP Automotive Amwww.siliconchip.com.au Fig.1: the PIC microcontroller (IC1) processes the signal from the Hall effect sensor (Sensor 1) and drives the 7-segment LED displays and the LED bargraph. LDR1, VR1 & IC2b automatically vary the display brightness according to the ambient light conditions. meter measures the battery current using a Hall effect sensor. This monitors the magnetic field produced by current flow in the battery leads. Fig.2 shows the sensor details. A ferrite core is placed around the battery leads, with the Hall sensor positioned in the air-gap. The leads from the battery produce a magnetic flux when ever current flows and this is induced into the ferrite core. This magnetic flux then passes through the sensor, which in turn produces a voltage that’s proportional to the current in the leads. What’s more, the output of the Hall effect device goes positive for one direction of current and negative for the other. So the same sensor can www.siliconchip.com.au determine both the magnitude of the cur­rent and its direction. Main features The SILICON CHIP Automotive Ammeter is housed in a small plastic case and matches the style of our previous PIC-based automotive projects. As before, the readout uses LED displays set behind a Perspex window in the lid. In this unit, there are three 7-segment LED displays and one bargraph display. The 7-segment displays show the current, with the lefthand digit show­ing a minus sign when the battery is being discharged. The vertical LED bargraph on the righthand side of the front panel consists of seven LEDs and operates in dot mode. The centre LED indicates zero amps (0A) while the three LEDs above this progressively light in 10A-steps for currents of 10-19A, 20-29A and 30A and above. The bargraph resolution is increased somewhat by making it possible for more than one LED come on at a time. Thus, the 0A and 10A LEDs both light for currents from 5-9A; the 10A and 20A LEDs both light for currents from 15-19A; and the 20A and 30A LEDs both light for currents from 25-29A. The three LEDs below the 0A LED indicate the discharge cur­ rent and operate in exactly the same manner – but in the opposite direction. As with our previous instruments, we’ve included automatic dimming and this varies the display brightness according to the ambient light level. That way, the displays are nice and bright for daytime viewing but are June 2002  63 Parts List 1 microcontroller PC board, code, 05106021, 78 x 50mm 1 display PC board, code, 05106022, 78 x 50mm 1 Hall Effect PC board, code 05106023, 20 x 12mm 1 front panel label, 80 x 53mm 1 plastic case utility case, 83 x 54 x 30mm 1 Perspex or Acrylic transparent red sheet, 56 x 20 x 3mm 2 plastic spacers, 1.5mm thick (12 x 7mm) 1 Ferrite core suppressor for 12.5mm cables (DSE D-5375, Jaycar LF-1290 or similar) 1 4MHz parallel resonant crystal (X1) 1 LDR (Jaycar RD-3480 or equivalent) 8 PC stakes 3 7-way pin head launchers 1 5-way 2.54mm DIL jumper launcher 1 jumper shunt (2.54mm spacing) 2 DIP-14 low cost IC sockets with wiper contacts (cut for 3 x 7-way single in-line sockets) 1 9mm long x 3mm ID untapped brass spacer 1 6mm long x 3mm ID untapped brass spacer 2 6mm long Nylon M3 tapped spacers 2 M3 x 6mm countersunk screws 2 Nylon M3 washers (1mm thick) or 1 Nylon M3 nut (2mm thick) 2 M3 x 15mm brass screws 4 150mm cable ties 1 2m length of red automotive wire 1 2m length of black or green automotive wire (ground wire) 1 2m length of 2-core screened cable turned down at night so that they don’t become distracting. The degree of display dimming is adjustable with a trimpot. The accompanying panel shows the other features of the unit. In particular, the maximum reading is 80A and the resolu­tion is 1A. If the current goes above 80A, the unit overloads and displays “OL” on the middle and left 7-segment readouts. Best of all, you don’t need to be a 64  Silicon Chip 1 500kΩ horizontal trimpot (code 504) (VR1) Semiconductors 1 PIC16F84P microcontroller with AMMETER.HEX program (IC1) 1 LM358 dual op amp (IC2) 1 UGN3503 linear Hall Effect sensor (SENSOR1) 1 7805 5V 1A 3-terminal regulator (REG1) 4 BC327 PNP transistors (Q1-Q4) 1 BC337 NPN transistor (Q5) 3 HDSP5301, LTS542R common anode 7-segment LED displays (DISP1-DISP3) 1 10-LED red vertical bargraph (Jaycar Cat. ZD-1704 or equiv.) 1 16V 1W zener diode (ZD1) Capacitors 1 100µF16VW PC electrolytic 1 10µF low leakage (LL) 16VW PC electrolytic or tantalum 1 10µF 16VW PC electrolytic 3 0.1µF MKT polyester 2 15pF ceramic Resistors (0.25W 1%) 3 100kΩ 1 1kΩ 1 47kΩ 4 680Ω 1 10kΩ 7 150Ω 1 3.3kΩ 1 10Ω 1W 1 1.8kΩ Calibration parts (optional) 1 8m length of 0.25mm diameter enamelled copper wire 1 56Ω 5W resistor 1 3.9Ω 5W resistor Miscellaneous Automotive connectors, automotive cable, neutral cure Silicone sealant, heatshrink tubing, cable ties, etc. rocket-scientist to use it, as there are no controls to operate. It’s turned on and off with the ignition and you just read the displays. Simple! Circuit details As already indicated, the circuit is based on a PIC micro­controller which minimises both the cost and the parts count. In fact, the circuit is similar to our previous PIC-based automotive projects. It’s the bits that hang off the microcontroller and the embedded software that make it perform its intended role. Refer now to Fig.1 for the circuit details. IC1 – a PIC16F84 microcontroller – forms the basis of the circuit. It accepts input signals from the sensor (Sensor 1) via comparator IC2a and drives the 7-segment LED displays and the LED bargraph. Most of the circuit complexity is hidden inside the PIC microcontroller and its internal program. That’s the beauty of using a microcontroller – we can easily do complicated (and not so complicated) things with very few parts. A-D converter Among other things, IC1 operates as an A/D (analog-to-digital) converter. In simple terms, this converts the analog voltage produced by the sensor to a digital value which is then used to drive the LED displays. Let’s see how this works. First of all, the DC signal output from the Hall sensor (pin 3) is fed to pin 2 of comparator stage IC2a via a filter consisting of a 47kΩ resistor and 10µF capacitor. This filter circuit removes any ripple voltage from the Hall sensor output. The output from the Hall sensor is nominally at 2.5V when there is no magnetic field applied to it. At the same time, pin 3 of IC2a is biased to 2.5V using two series 100kΩ resistors across the 5V supply. The associated 100kΩ resistor to RA3 of IC1 (pin 2) pulls IC2a’s pin 3 input to 1.67V when RA3 is at ground or to 3.33V when RA3 is at 5V. However, if RA3 is repeatedly switched between +5V and ground at a fast rate, it follows that pin 3 of IC2a can be set to any voltage between 1.67V and 3.33V, depending on the duty cycle of the switching waveform. In operation, the A/D converter uses IC1 to ensure that the voltage applied to pin 3 of IC2a matches the sensor output vol­ tage applied to pin 2. It does this by producing a 1953Hz pulse width modulated (PWM) signal at its RA3 output, the duty cycle of which is continually adjusted to produce the required voltage on pin 3 of IC2a. For example, if the duty cycle at RA3 is 50%, the average voltage output will be 2.5V. This is filtered by a 0.1µF capaci­tor and applied to pin 3. Other voltages are obtained by using www.siliconchip.com.au different duty cycles, as indicated above. IC2a simply acts as a comparator. Its pin 1 output switches low or high, depending on whether the voltage on pin 2 is higher or lower than the voltage on pin 3. The output from IC2a is then fed to RB0 via a 3.3kΩ limiting resistor. This is included to limit the current flow from IC2a when its output goes high; ie to +12V. The internal clamp diodes at RB0 then limit this voltage to 0.6V above IC1’s 5V supply (ie, to +5.6V). Note the 10kΩ pulldown resistor on RB0. This ensures that the signal on RB0 is detected as a low when pin 1 of IC2a goes low. The A-D conversion process uses a “successive approxima­ tion” technique to zero in on the correct value. This all takes place inside the microcontroller, with the duty cycle for each successive approximation (and thus the valued stored in an inter­nal 8-bit register) controlled by the software. Initially, RA3 operates with a 50% duty cycle and the internal register in IC1 is set to 10000000. IC1 then checks the output of comparator IC2a to see whether it is high or low. It then adjusts the duty cycle at RA3 by a set amount, updates the reg­ister and checks the output of IC2a again. This process continues for eight cycles, each step succes­sively adding or subtracting smaller amounts of voltage at pin 3 of IC2a. During this process, the lower bits in the 8-bit reg­ister are successively set to either a 1 or a 0 to obtain an 8-bit A-D conversion. Following the conversion, the binary number stored in the 8-bit register is processed (we’ll look at this in more detail shortly) and converted to a decimal value so that it can be shown on the 3-digit LED display. Once again, this takes place inside the PIC microcontroller. Note that the possible range of values for the 8-bit register is from 00000000 (0) to 11111111 (255) – ie, 256 possible values. However, in practice we are limited to a range of about 19-231. That’s because the software must have time for internal processing to produce the waveform at the RA3 output and to monitor the RB0 input. Processing the register data OK, let’s now take a closer look at how the PIC microcon­troller processes www.siliconchip.com.au Fig.2: the current sensor consists of a ferrite core placed around the battery leads, with a Hall effect device positioned in the air-gap. A magnetic flux is induced in the ferrite when ever current flows through the leads and this flux passes through the Hall effect device which generates a proportional output voltage. the data in the 8-bit register following con­ version. To do this, it requires several items of information. First, it needs to know the voltage produced by the Hall effect sensor when there is no current flow. This is nominally half the supply voltage (ie, 2.5V) but could be anywhere between 2.25V and 2.75V. This value is determined during the setting up procedure by installing Link 1 which pulls the RB1 line low via a 1.8kΩ resistor. Second, the processor needs to know what the output voltage from the Hall effect sensor is for a known current. This is measured at either 17A, 25A or 30A by installing either Link 2, Link 3 or Link 4 on the RB2, RB3 and RB7 outputs. The Hall effect device’s quiescent output voltage is then subtracted from this measured value to derive a calibration number. For example, let’s say that the Hall effect sensor’s output is 2.5V at 0A and 3.0V at 17A (ie, we are calibrating at 17A). In this case, the calibration factor would be 3 - 2.5 = 0.5 and this is stored by the processor along with the calibration amperage (17A in this case). Once the processor knows this information it can calculate other currents, depending on the output from the Hall sensor. First, it subtracts the sensor’s quiescent voltage from its new output voltage (note: this provides values that can be either positive or negative, depending on the current direction). The result is then multiplied by the calibration amperage and divided by the calibration factor to get the final result. This is best illustrated by another example. Let’s assume that the calibration factor is 0.5 and that the calibration amperage is 17A. Further, let’s assume that the sensor output is at 3.4V. In this case, the current would be (3.4 - 2.5) x 17/0.5 or 30.6A. This result (to the nearest amp) is shown on the LED displays and on the bar­graph. Driving the displays The 7-segment display data from IC1 appears at outputs RB1-RB7. These directly drive the display segments via 150Ω current-limiting resistors, while the RA0, RA1, RA2 & RA4 outputs drive the individual displays in multiplex fashion via switching tran­ sistors Q1-Q4 (more on this shortly). As shown, the corresponding display segments are all tied together, while the common anode terminals are driven by the switching transistors. Similarly, the cathodes of the LEDs in the bargraph display (LED­BAR1) are also connected to the display segments. What happens is that IC1 switches its RA0, RA1, RA2 & RA4 lines low in sequence to control the switching transistors. For example, when RA0 goes low, transistor Q4 turns on and applies power to the common anode connection of DISP3. Any low outputs on RB1-RB7 will thus light the corresponding segments of that dis­play. After this display has been lit for a short time, RA0 is switched high and June 2002  65 DISP3 turns off. The 7-segment display data on RB1-RB7 is then updated, after which RA1 is switched low to drive Q3 and display DISP2. RA2 is then switched low to drive DISP1 and finally, RA4 is switched low to give the LED bargraph its turn. Note that IC1’s RA4 output has a 1kΩ pullup resistor con­nected to the emitter supply rail for transistors Q1Q4. This is necessary to ensure that Q1 switches off fully, since RA4 has an open-drain output. Between driving DISP1 and the LED bargraph, the RB1-RB7 outputs are set as inputs. These have internal pullup resistors that hold them high unless pulled low via one of the links (ie, Links 1-4) and the associated 1.8kΩ resistor. By monitoring the state of these RB inputs, we can determine whether one of the links has been installed for calibration. Link 1 tells the processor that the voltage from the Hall effect sensor is at the quiescent level (ie, when there is no current flow through the battery lead). The other three links set the current level used for calibration (you only have to choose one). For example, if Link 2 is installed, the processor knows that the voltage output from the Hall sensor corresponds to a 17A current flow. Links 3 and 4 are respectively used for the alter­native 25A and 30A current calibration levels. This view shows the fully assembled display board. Note that the three 7-way pin headers are all mounted on the copper side of the board, with their leads just protruding through from the top. Display dimming Trimpot VR1, light dependent resistor LDR1 and op amp IC2b are used to control the display brightness. As shown, IC2b is wired as a voltage follower and drives buffer transistor Q5 to control the voltage applied to the The pin headers on the display board plug into matching in-line sockets on the microcontroller board. Note that the three electrolytic capacitors are mounted so that they lie horizontally across other components. Table 1: Resistor Colour Codes  No.   3   1   1   1   1   1   4   7   1 66  Silicon Chip Value 100kΩ 47kΩ 10kΩ 3.3kΩ 1.8kΩ 1kΩ 680Ω 150Ω 10Ω 4-Band Code (1%) brown black yellow brown yellow violet orange brown brown black orange brown orange orange red brown brown grey red brown brown black red brown blue grey brown brown brown green brown brown brown black black gold (5%) 5-Band Code (1%) brown black black orange brown yellow violet black red brown brown black black red brown orange orange black brown brown brown grey black brown brown brown black black brown brown blue grey black black brown brown green black black brown not applicable www.siliconchip.com.au Fig.3 (left): install the parts on the micro­ controller PC board as shown here. Table 2: Capacitor Codes    Value IEC Code EIA Code 0.1µF   100n   104 15pF   15p   15 emitters if the display driver transistors (Q1-Q4). When the ambient light is high, LDR1 has low resistance and so the voltage on pin 5 of IC2b is close to the +5V supply rail delivered by REG1. This means that the voltage on Q5’s emitter will also be close to +5V and so the displays operate at full brightness. As the ambient light falls, the LDR’s resistance increases and so the voltage at pin 5 of IC2b falls. As a result, Q5’s emitter voltage also falls and so the displays operate with reduced brightness. At low light levels, the LDR’s resistance is very high and the voltage on pin 5 of IC2b is set by VR1. This trimpot sets the minimum brightness level and is simply adjusted to give a com­ fortable display brightness at night. Fig.4: the parts layout on the sensor board is shown above, while at left is the display board. Clock signals Clock signals for IC1 are provided by an internal oscilla­tor which operates in conjunction with 4MHz crystal X1 and two 15pF capacitors. The two capacitors are there to provide the correct loading for the crystal, to ensure that the oscillator starts reliably. The crystal frequency is divided down internally to produce separate clock signals for the microcontroller and for display multiplexing. Power supply Power for the circuit is derived from the vehicle’s battery via a fuse and the ignition switch. This is fed in via a 10Ω resistor and decoupled using 0.1µF and 100µF capacitors. Zener diode ZD1 provides transient protection by limiting any spike voltages to 16V. It also provides reverse polarity protection – if the leads are reversed, ZD1 conducts heavily and blows the 10Ω resistor. The decoupled supply is fed to 3-terminal regulator REG1 to derive a +5V rail. This rail is then further filtered using 0.1µF and 10µF capacitors www.siliconchip.com.au and applied to IC1, Sensor 1 and the collector of Q5. Op amp IC2 derives its power from the decoupled +12V rail. Software We don’t have space to describe how the software works here but if you really must know, you’ll find the source code posted on our website. Of course, you really don’t have to know how the software works to build this project. Instead, you just buy the prepro­grammed PIC chip and plug it in, just like any other IC. So let’s see how to put it all together. Construction Fig.3 shows the assembly details. Most of the work involves assembling three PC boards: a microcontroller board coded 05106021, a display board coded 05106022 and a sensor board coded 05106023. The latter carries just three parts: the Hall effect sensor (Sensor 1), a 0.1µF capacitor and three PC stakes and can be built in next to no time at all. The assembled display and microcontroller boards are stacked together piggyback fashion using pin headers and cut down IC sockets to make all the interconnections. This completely eliminates the need to run wiring between the two boards. Begin by inspecting the PC boards for shorts between tracks and for possible breaks and undrilled holes. Note that a “through-hole” is required on the display board to accommodate a screwdriver to adjust VR1 which mounts on the microcontroller board. This hole is just below the decimal point for DISP3 (see photo). Note also that the two main boards need to have their corn­ers removed, so that they clear the mounting pillars inside the case. The sensor board can be assembled first. Install the ca­pacitor and the three PC stakes first, then complete the assembly by mounting the Hall effect sensor. Mount the sensor with its leads at full length and be sure to mount it with the correct orientation. June 2002  67 micro­controller board to do this – just connect a 12V supply to the board and check that there is +5V on pins 4 & 14 of the socket. If this is correct, disconnect power and install IC1 in its socket, making sure that it is oriented correctly. Display board assembly Fig.5: this diagram shows how the two PC boards are stacked together and secured to the bottom of the case using screws, nuts and spacers. Be sure to use nylon spacers and washers where specified. This is the completed board assembly, ready for mounting in the case. The top of the LDR should be about 3mm above the displays. The microcontroller board is next. Being by installing the nine wire links, then install the resistors. Table 1 lists the resistor colour codes but we recommend that you check each value using a digital multimeter, just to be sure. Note that the seven 150Ω resistors at top right are mounted end-on. Trimpot VR1 can go in next, followed by a socket to accept IC1 – make sure this is installed the right way around but don’t install IC1 just yet. IC2 is soldered directly to the board – install this now, followed by zener diode ZD1 and transistors Q2-Q5. Watch out here – Q5 is an NPN BC337 type, while Q2-Q4 are all PNP BC327s. Don’t mix then up. REG1 is mounted with its metal tab flat against the PC board and its leads bent at right angles to pass through their respective holes. Make sure that its tab lines up with the mount­ing hole in the PC board. The capacitors can go in next but make sure that the elec­trolytics are mounted with the correct polarity. Note that the 10µF capacitor below VR1 must be a low-leakage (LL) 68  Silicon Chip type. It is installed so that its body lies horizontally across the adjacent 680Ω resistors. It’s a good idea to bend its leads at rightangles using needle-nosed pliers before mounting the capacitor on the board. Similarly, the two electrolytic capacitors below REG1 must be in­stalled so that their bodies lie over the regulator’s leads (see photo). Crystal X1 mounts horizontally on the PC board and can go in either way around. It is secured by soldering a short length of tinned copper wire to one end of its case and to a PC pad immediately to the right of Q3. Finally, you can complete the assembly of this board by fitting PC stakes to the external wiring points and installing the three 7-way in-line sockets. The latter are made by cutting down two 14-pin IC sockets into in-line strips. Use a sharp knife or a fine-toothed hacksaw for this job and clean up any rough edges with a file before installing them. Before plugging in IC1, it’s a good idea to check the supply rails on its socket. You don’t need to have any other circuitry connected to the Now for the display board. Install the eight wire links first (note: six of these mount under the displays), then install the three 7-segment LED displays. Make sure that these are prop­erly seated and that their decimal points are at bottom right before soldering them The LED bargraph can go in next – this mounts with the corner chamfer at bottom right (ie, labelled side towards the edge of the PC board). This done, install LDR1 so that its top face is about 3mm above the displays. The remaining parts, including the 5-way DIL pin header, can now be installed. The shorting jumper can be installed in the “OFF” position (at right) for safe keeping, at this stage. The three 7-way pin headers are installed on the copper side of the PC board, with their leads just protruding above the top surface. You will need a fine-tipped soldering iron to solder them in. Note that you will have to slide the plastic spacer along the pins to allow room for soldering, after which the spacer is pushed back down again. Final assembly Work can now begin on the plastic case. First, remove the integral side pillars with a sharp chisel, then slide the micro­ controller board in place. That done, mark out two mounting holes – one aligned with REG1’s metal tab and the other diagonally opposite (to the bottom left of IC2). Now remove the board and drill the two holes to 3mm. They should be slightly countersunk on the outside of the case to suit the mounting screws. In addition, you will have to drill two holes in the bottom of the case to accept the power leads and the shielded cable for the Hall effect sensor. These two holes should be located so that they line up with the relevant PC stakes. The display board can now be plugged into the microcon­troller board and the assembly fastened together and installed in the case as shown www.siliconchip.com.au Another view of the completed PC board assembly, prior to mounting in the case. Make sure that the displays are oriented correctly (decimal point to bottom right). in Fig.4. Be sure to use a 2mm nylon washer (or spacer) in the location shown. Once it’s all together, check that none of the leads on the display board short against any of the parts on the microcon­ troller board. Some of the pigtails on the display board may have to be trimmed to avoid this. The front panel artwork can now be used as a template for marking out and drilling the front panel. You will need to drill a hole for the LDR plus a series of small holes around the inside perimeter of the display cutout. Once the holes have been drilled, knock out the centre piece and clean up the rough edges using a small file. Make the cutout so that the red Perspex window is a tight fit. A few spots of superglue along the inside edges can be used to ensure that the window stays put. That done, you can affix the front panel label and cut out the holes with a utility knife. The power supply and sensor leads are soldered directly to their respective terminals on the back of the micro­ controller board. be +5V on pin 1, 0V on pin 2 and nominally 2.5V on pin 3 (this could be between 2.25V and 2.75V, depending on the particular sensor). You can test the dimming feature by holding your finger over the LDR. Adjust VR1 until the display dims to the correct level. This trimpot is best adjusted when it’s dark, to obtain the correct display brightness. Calibration The first calibration setting to be made is for the quies­cent Hall effect output level. This is done by placing the jumper shorting plug across the “0” DIL launcher located on the display PC board. Just make sure the sensor is not located near any magnets when this is done. The display should indicate “CAL” and the 0A LED should be lit on the bargraph display. Now remove the shorting plug after about one second and place it in the off position. The display will now return to normal operation and show a “0”. Note that the off position is just a position to store the shorting plug and it does not form any connection to the circuit. The unit must now be calibrated using a known current flow. The first step is to position the Hall effect sensor in the air gap of the ferrite core as shown in Fig.7. In this case, the ferrite core is sim- Testing Before testing the unit, you have to connect the Hall sensor leads to the microcontroller board. These connections, along with the power supply connections are made on the copper sides (see photo). Now apply power – the display should show two dashes (- -). After about 5 seconds, the display should then show a value on the 7-segment LED displays and one or more LEDs should light in the bargraph. If this doesn’t happen, check the voltages on the Hall effect sensor. There should www.siliconchip.com.au The PC board assembly fits neatly into a small plastic utility case and matches the style of our previous PIC-based automotive projects. June 2002  69 Table 4: Total Load With Lights On (Typical) Parking Lights + licence plate....................................................25W (2.1A) Reversing Lights.........................................................................42W (3.5A) Main brake Lights.......................................................................42W (3.5A) Main brake light + high level brake light.....................................60.4W (5A) Headlights (high beam, no low beam) + all brake lights + parking + licence plate...........................................................205.4W (17A) Headlights (high beam with low beam) + all brake lights + parking + licence plate...........................................................315.4W (26A) ply a voltage spike protector which is designed to clip over power leads to limit noise spikes. This unit uses a split core encased in a plastic housing that can be opened to accept the lead and then clamped shut again. Fig.7 and the accompanying photos show how the Hall effect sensor is installed sandwich fashion between the two ferrite cores. The sensor board can be encapsulated in heatshrink tubing and attached to the side of the plastic case using a cable tie. By the way, it’s good idea to glue a couple of 1.5mm-thick plastic spacers either side of the Hall effect sensor, to prevent stressing the ferrite core when the case is closed. Once the current sensor has been made up, clamp it to the battery lead(s). You can now calibrate the ammeter using either of two methods: (1) the “rough ‘n ready” way using the current drawn by the car’s headlights; or (2) the precise way by winding turns through the core to simulate a higher current. We’ll look at the rough ‘n ready way first. Tables 3 & 4 show typical lamp ratings in cars and the currents drawn with various combinations of lights switched on. If you want better accuracy, check the ratings for the various lights in your vehi­ cle, You should be able to get this information from the owner’s handbook or from a service manual. As stated previously, you need to Fig.6: this is the full-size artwork for the front panel. Discharge Current (A) (cutout for LED displays) Charge Parking lights (front)............................................................................... 5W Tail lights................................................................................................ 5W Licence plate.......................................................................................... 5W Dashboard parking indicator............................................................... 1.4W Reversing lights.................................................................................... 21W Main brake lights.................................................................................. 21W High level brake light......................................................................... 18.4W Dashboard brake indicator.................................................................. 1.4W Headlights (high beam/low beam)................................................ 60W/55W Dashboard high beam indicator.......................................................... 1.4W 30 20 10 0 10 20 30 Table 3: Typical Lamp Ratings In Cars calibrate at either 17A, 25A or 30A. From Table 3, you can see that if you switch on the headlights at high beam along with the brake lights and the parking lights, you will get a total current drain of about 26A (assuming a 12V battery). This value should be satisfactory for calibrating the unit at 25A – just place the shorting jumper into the 25A position. The display will show “CAL” and the 25A discharge LEDs will light on the bargraph. That done, remove the jumper plug and replace it in the OFF position. And that’s it – the calibration is completed! Note: some cars switch the lowbeam lights off when the headlights are at high-beam and so the total current will only be around 17A. In this case, you calibrate the unit by placing the shorting plug in the 17A position. Precise calibration A more accurate calibration can be Fig.5: here are the fullsize etching patterns for the PC boards. 70  Silicon Chip www.siliconchip.com.au This view shows how the Hall effect sensor and the adjacent plastic spacer (or washers) are attached to the ferrite core. at 214mA and the 80 turns simulates 17A through the core. In this case, calibrate the unit using the 17A shorting position, then remove the jumper shorting plug after about one second. Fig.7: you can accurately calibrate the unit at low current using the set-up shown here (see text). Use silicone sealant to seal the assembly after clamping it to the battery leads and to protect the sensor board. made at much lower cur­ rent using either the car’s battery or an adjustable or fixed 12V power supply. In this case, we simulate a higher current flow by winding many turns of wire through the ferrite core (see Fig.7). For example, if you want to simulate 30A, wind 30 turns on the ferrite core and set the current through these turns to 1A. If you have an adjustable power supply, install a 3.9Ω 5W resistor in series with the power supply and the winding and set the output voltage to 3.9V. If you’re really fussy, add a multi­meter in series with the wiring and set the current to exactly 1A by adjusting the supply voltage. When the current is at 1A, install the jumper in the 30A position. The display will show “CAL” and the 30A discharge LED will light. Remove the jumper short after about one second and the unit is accurately calibrated. If you are using a fixed 12V supply, you can connect a 56Ω 5W resistor in series with 80 turns around the ferrite core. The 56Ω resistor sets the current The current sensor clamps onto the battery lead(s) as shown here. Make sure that all the leads to one battery terminal are included. www.siliconchip.com.au Installation The Ammeter can be installed into a vehicle using automo­tive style terminators to make the connections to the ignition supply and ground. Note that the ignition supply connection must be made on the fused side. The ground connection can be made to the chassis with an eyelet and self tapping screw. Use twin core shielded cable for the 3-wire connection to the Hall sensor. The Hall effect sensor should be attached to the ferrite core as shown in Fig.7, with the spacers installed and the assem­ bly clipped together place. You can attach the core to either the positive or negative battery lead but all wires connecting to one battery terminal must pass through the core. Check that the ammeter display shows the “-” sign when the battery is discharg­ing. You can check this by switching on the headlights when the engine is off. If the minus sign is off, simply open the ferrite core, flip the assembly 180° and replace it over the wire or wires. Finally, the Hall effect sensor assembly should be tied together with cable ties and covered with a layer of sili­cone sealant to keep dirt and moisture out. The PC board and wiring should also be covered with the Silicone and the lead secured with cable SC ties. June 2002  71