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Pt.1: By JOHN CLARKE Voltage Interceptor For Cars With ECUs At one time, the sensors in cars with engine management were regarded as untouchable. But now you can change the signal response of many of the sensors to improve your car’s driveability, throttle response, handling and so on. This voltage interceptor allows you modify and program the response of any voltage sensor in your car, without prejudicing reliability or affecting the ECU in any way. Use it for restoring correct air/fuel ratios after engine modifications, preventing turbo boost cuts or altering sensor signals for improved driveability. Main Features •  Output follows input plus •  •  •  •  •  adjustment value Programmed using a pushbutton controller Easy installation Works with sensors with voltage outputs Adjustable sensitivity Smooth transition between adjacent output points 24  Silicon Chip M ODERN CARS have lots of sens­ ors to closely monitor the engine and other systems and they provide information to the ECU (Engine Control Unit). In turn, the ECU controls the fuel injectors and ignition timing, based on this information. Some of the sensors you can intercept and modify include the airflow meter, oxygen sensor, accelerometers (or G force sensors) used in stability control and active 4-wheel drive sys- tems and the throttle position sensor (TPS). For cars with an electric throttle rather than a throttle cable, modification of the TPS signal can transform the way the car drives. For example, you can alter the signal so that there is less pedal travel required to provide more throttle. This will make the car behave as though it has more power. Alternatively, the signal can be altered so that more pedal travel is siliconchip.com.au required for throttle openings only at smaller throttle settings. This could make the car a lot smoother when moving off and make it safer to drive in wet and slippery conditions. VOLTAGE SIGNAL FIG.1(a) Interceptor concept An interceptor allows the signal from a sensor to be altered before it is monitored by the car’s ECU. Fig.1 shows the concept. Fig.1(a) shows a typical sensor connected to the ECU while Fig.1(b) shows the interceptor connected between the sensor and the ECU. The voltage interceptor does not necessarily modify the sensor signal at all times; when no changes are required, the signal at the output will be the same as at the input. For example, an airflow sensor output may provide 0.9V at idle and 4.1V at full engine load and high RPM. The latter reading will normally result in a very rich mixture. So you might decide to limit the range of the airflow output for signals above 2V (say) and to change the slope. The result would be fuel mixtures that are not quite so rich at large throttle openings and your fuel economy should improve. We have already given the example of modifying the output of the throttle position sensor and there are quite a few other applications. There is one proviso though: each sensor to be modified will require its own interceptor. Want to modify the output of three engine sensors? You will need three of these interceptors. Pusbutton controller The interceptor needs to be programmed and for this you need a Pushbutton Controller which is also described in this article. This controller has nine buttons and a 2-line LCD panel. It connects to the interceptor via a cable and a 25-pin connector which plugs into the PC board. After programming, you disconnect the pushbutton controller and the interceptor is used on its own. The good thing is, regardless of how many interceptors you decide to install, you only need one Pushbutton Controller. We’ll talk about this controller in more detail, later in this article. Note that while we have designed this interceptor for use in automotive applications, there are many other siliconchip.com.au ENGINE CONTROL UNIT (ECU) SENSOR +12V MODIFIED VOLTAGE SIGNAL VOLTAGE SIGNAL IN SENSOR VOLTAGE INTERCEPTOR ENGINE CONTROL UNIT (ECU) OUT HAND CONTROLLER FIG.1(b) Fig.1: the Voltage Interceptor is installed immediately after the sensor and modifies the sensor’s signal before it is fed to the ECU. INPUT INPUT PROCESSING (IC1a, IC1b) GAIN OR ATTENUATION WITH VR2 OUTPUT PROCESSING (IC2b, IC3b) GAIN OR ATTENUATION WITH VR7 5V MAXIMUM +12V 0V POWER SUPPLY (REG1, ZD3, ZD4) SENSITIVITY (VR6) ADJUST SIGNAL (IC2a, IC3a) OFFSET WITH VR5 MICROCONTROLLER (IC4) MINIMUM SET (VR3) OUTPUT +11.4V +5.6V +5V –7.5V HAND CONTROLLER VOLTAGE INTERCEPTOR Fig.2: how the unit processes the sensor signal. The signal is first amplified or attenuated and fed to microcontroller IC4. IC4 then digitises the signal and modifies its response to produce a control voltage that’s used to offset the output of IC2b. applications where it is desirable to convert a DC signal into a smaller, larger or non-linear voltage. Interceptor workings The block diagram of Fig.2 shows how the Voltage Interceptor processes the sensor signal. First, the sensor signal needs to be amplified (gain) or reduced (attenuation) to bring it within a range of 0-5V which the microcontroller (IC4) can handle. Yes, inevitably, the Interceptor uses a micro. The Voltage Interceptor’s microcontroller digitises this gain-changed signal from the sensor into a “map” with 256 separate load sites. The output circuitry (IC2b & IC3b) then applies a reverse amount of gain or attenuation to cancel out the gain/ attenuation originally applied in the input processing stage. This ensures that the Interceptor’s output has the same overall voltage range as the sensor itself. The idea is to present a signal to the ECU with exactly the same characteristics as the signal from the sensor. That way, the overall operation of the ECU is not prejudiced in any way and it acts exactly as if the sensor was connected directly to it. You then have the option of changing the Interceptor’s output response at each and every load site. The Pushbutton Controller is used to set this December 2009  25 SC Ó2009 8 A K ZD1–ZD4 20k 10k 2 1k 4 A K D1 1N4004 RB2 RB0 PWM A K 8 6 9 Vdd 3 AN4 7 RB1 14 D1, D2 Vss 5 10nF 22k IC1b IC4 PIC16F88-I/P AN3 MCLR REF– 17 RA0 16 RA7 13 RB7 12 RB6 11 RB5 10 RB4 15 RA6 18 AN1 10k TP4 TP3 1 10 mF +5V 5 6 VR2 100k IC1: LMC6482AIN –7.5V 1 +5.6V 10nF VR3 1k MINIMUM VOLTS 4 IC1a 10k 3 2 470k 5V SET 10k LK1 10k 100W VR5 10k 2.2k 5V MAX TP2 OFFSET LOCK 7 VOLTAGE INTERCEPTOR FOR CARS 10nF VR4 1k THRESHOLD 100 mF 1M RLY1a 1nF A K 2 3 10k V++ 1 A 7 10W 10k 100nF 100nF 43k K 2 3 8 20k 0.5W A K 10nF 9.1k 4 IC2a 470W +11.4V IC2: LMC6482AIN IC2b 10k D3, D4: 1N4148 +11.4V ZD2 15V 1W 5 6 100k 10nF 4 IC3a 8 VR6 50k SENSITIVITY 10k 10nF 2.2k K A LED1 E C B B 100nF +5.6V ZD4 5.6V 1W V+ B Q2 BC547 1 IC3b 7 ZD1 16V 1W E C1 K A B C Q1–Q4 A K Q4 BC327 C E E 100 mF Q3 BC337 C V++ –7.5V +5V IC3: LMC6482AIN 5 6 10nF 10k OFFSET MEASURE TP5 VR7 50k OUTPUT SET D4 ZD3 7.5V 1W 220W OUT ADJ IN LM317T V+ 10 mF 120W K A RLY1 Q1 BC337 TP1 E C –7.5V 5V ADJUST OUT ADJ 100 mF 16V VR1 500W IN B 100 mF A K REG1 LM317T C2 A l LED1 D3 100 mF K K A 1k 1k D2 1N4004 150W RLY1b V+ OUT (CON2b) OUTPUT Fig.3: the Voltage Interceptor is based on a PIC16F88-I/P microcontroller (IC4) which has the ability to adjust the output at 256 points. It accepts the incoming signal from IC1b at its AN4 (pin 3) input and outputs a PWM signal at pin 9. This signal is then filtered and fed to IC2a to produce an offset voltage which is fed to pin 5 of IC2b via VR6. 0V CON1 +12V IN 4 9 10 11 12 13 8 2 3 6 5 CON3 DB25 (CON2a) INPUT TO HAND CONTROLLER 26  Silicon Chip siliconchip.com.au output response during the setting-up procedure. A signal of 0V (min) will be at load site 0 while a 5V signal (max) will be at load site 255. However, most engine sensors do not produce a voltage that goes as low as 0V; the minimum is usually several hundred millivolts above 0V. For example, the minimum might be 320mV DC and the maximum might only be 4V, ie, a range from 0.32V to 4V. After input processing, this signal is amplified to cover a range of 1-5V. For this signal range, load sites from 0 to 51 will not be available and this reduces the overall adjustment points to just 205 compared to the available 256. To improve this, a minimum set adjustment is included to allow the lower adjustment points to be used. Interceptor circuit details Our first Voltage Interceptor was published in 2004 in SILICON CHIP’s Performance Electronics for Cars. Called the Digital Fuel Adjuster (DFA), it proved very popular and is still available as a Jaycar kit (Cat. KC-5385). This Voltage Interceptor is considerably upgraded, with the ability to adjust the output at 256 points instead of the 128 for the DFA. Its circuitry also uses far less components while offering better performance, greater output accuracy and extra adjustments. While the DFA used eight ICs, the Voltage Interceptor uses only four (three dual op amps and the micro) and it fits into a more compact case. Now let’s have a look at the full circuit in Fig.3. The input signal comes in at the top lefthand corner of the circuit and is connected to a relay which initially bypasses the input directly to the output. This is to prevent the car’s ECU from recording a fault code with the Voltage Interceptor, before the engine is started. After the relay, the sensor signal is connected to op amp IC1a. This is set up as an inverting amplifier with a gain of -0.47. Its input impedance is 1MΩ, to provide a minimal load to sensitive sensors (oxygen sensors, for example), that can be affected by loading. IC1b is also an inverting amplifier and it can be set to provide gain or attenuation, using trimpot VR2. The gain range is from -0.2 to -12.2. This is sufficient to boost a 1V signal to 5V or when set to provide attenuation, a 15V signal can be reduced to 5V, suitsiliconchip.com.au Parts List 1 PC board, code 05112091, 105 x 87mm 1 diecast box, 119 x 94 x 34mm, Jaycar HB-5067 or equivalent 1 TO-220 mini heatsink, 19 x 19 x 9.5mm 1 DB25 female PC-mount connect­ or (Altronics P-3250 or equivalent) 4 extension screws, spacers and nuts for DB25 connector 1 2-way pin header with 2.54mm spacing 1 jumper shunt to suit header 1 12V DPDT relay (Jaycar SY4059, Altronics S 4150 or equivalent) 2 2-way screw terminals with 5.04mm spacing 2 3-6.5mm cable glands 3 DIP8 IC sockets 1 DIP18 IC socket 4 M3 x 6mm tapped Nylon spacers 8 M3 x 4mm screws 1 M3 x 6mm screw 1 M3 nut 6 PC stakes Semiconductors 1 PIC16F88-I/P microcontroller programmed with 0511209A.hex (IC4) 3 LMC6482AIN dual CMOS op amps (IC1-IC3) 1 LM317T adjustable 3-terminal regulator (REG1) 2 BC337 NPN transistors (Q1,Q3) 1 BC547 NPN transistor (Q2) 1 BC327 PNP transistor (Q4) 1 16V 1W zener diode (ZD1) able for the 5V maximum required by microcontroller IC4. Op amp IC2b is an inverting amplifier with a gain of -1. A 5V signal from IC1b would produce a -5V level from IC2b’s output. IC2b’s output can be level-shifted by trimpot VR6 but we will give more detail about this later. IC3b is an inverting amplifier with a gain that can be varied from between -0.142 when the wiper of VR7 is set toward the 10kΩ resistor and -3 when VR7 is set towards the 20kΩ resistor. This adjustment has sufficient range to set the maximum output anywhere from 1V up to 12V. 1 15V 1W zener diode (ZD2) 1 7.5V 1W zener diode (ZD3) 1 5.6V 1W zener diode (ZD4) 2 1N4004 diodes (D1,D2) 2 1N4148 diodes (D3,D4) 1 3mm red LED (LED1) Capacitors 5 100µF 16V PC electrolytic 2 10µF 16V PC 3 100nF MKT polyester 7 10nF MKT polyester 1 1nF MKT polyester Resistors (0.25W, 1%) 1 1MΩ 3 1kΩ 1 470kΩ 1 470Ω 0.5W 1 100kΩ 1 220Ω 1 43kΩ 1 150Ω 1 22kΩ 1 120Ω 2 20kΩ 1 100Ω 10 10kΩ 1 10Ω 1W 1 9.1kΩ 7 0Ω links 2 2.2kΩ Trimpots 1 500Ω multi-turn top adjust trimpot (Bourns 3296 type) (code 500) (VR1) 2 1kΩ multi-turn top adjust trimpot (Bourns 3296 type) (code 102) (VR3,VR4) 1 10kΩ multi-turn top adjust trimpot (Bourns 3296 type) (code 103) (VR5) 2 50kΩ multi-turn top adjust trimpot (Bourns 3296 type) (code 503) (VR6,VR7) 1 100kΩ multi-turn top adjust trimpot (Bourns 3296 type) (code 104) (VR2) IC4 is the PIC16F88-I/P microcontroller. The signal from IC1b is fed to its AN4 input (pin 3) and is converted into an 8-bit digital value with 256 possible levels. There are two voltage references for this analog-to-digital conversion: the 5V REF+ and an adjustable voltage, REF-. The 256 conversion levels are measured between these two references. When REF- is set to 0V, the conversion range is 0-5V and the 256 levels are about 19.5mV apart. REF- is adjustable using trimpot VR3 which is connected to pin 1 of IC4 and bypassed using a 10µF capacitor. December 2009  27 Pushbutton Controller Parts List 1 PC board, code 05104073,115 x 65mm 1 front panel label 1 plastic case, 120 x 70 x 30mm with clear lid (Jaycar HB-6082 or equivalent) 1 LCD module (Jaycar QP-5515 or backlit QP-5516) 5 white click-action switches (S1, S2, S5, S7, S9) 4 black click action switches (S3, S4, S6, S8) 1 SPST micro tactile switch with 0.7mm actuator (S10) 1 4017 decade counter (IC1) 1 DIL 14-way pin header 1 DB25 PC-mount right-angle socket 1 1.8m DB25-pin male to DB25pin male RS-232 connecting lead (all pins connected) (Jaycar WC-7502 or equivalent) 4 M3 x 12mm tapped plastic spacers 4 M3 x 6mm CSK screws 2 M3 x 6mm screws 2 M3 x 12mm plastic screws 2 2.5mm thick plastic washers 1 100mm length of 0.7mm tinned copper wire or 2 x 0Ω resistors 1 10µF 16V PC electrolytic capacitor 2 10kΩ 0.25W 1 % resistors 1 7-way, 8-way or 9-way 330Ω terminating resistor array (8-10 leads). Note: six resistors are used in the circuit and one end of each resistor connects to the pin 1 common 1 10kΩ horizontal trimpot (code 103) (VR1) REF- allows us to optimise the analogto-digital conversion of input voltages that do not go as low as 0V. For example, a sensor may have a range from 0.8V to 4V. Gain adjustment for IC1b would be set so the AN4 input receives 1V to 5V from the 0.8V to 4V sensor signal. With REF- set to 1V then, a 1V signal will be converted to a digital value of 0 instead of 51 (as noted above). The 5V would be converted to 255 and so the full 0-255 range of digital sites would be available. Each of those 256 site values can be changed by the Voltage Interceptor. 28  Silicon Chip Specifications Voltage input range: 0-15V maximum Voltage output: typically set for 0-1V or 0-5V but can cover any range up to 12V Minimum input voltage adjustment: from 0-2.5V Output adjustment: ±127 steps Adjustment range: from 0V through to the full output range Adjustment resolution: 39mV for a ±5V adjustment range at maximum sensitivity (finer resolution is available by sacrificing adjustment range) Input adjustment points: 0-255 corresponding to 19.5mV steps for a 0-5V input Output adjustment change response: 512µs response plus 10ms to alter to within 10% of the new value Display update time: 250ms The output changes are manipulated by a control signal from pin 9 of the micro. This is a pulse width modulated (PWM) signal running at 7.843kHz which varies its duty cycle. The PWM signal is then filtered using a 100kΩ resistor and a 100nF capacitor to produce an average DC voltage. The resulting output is 0V for 0% duty cycle and +5V for 100% duty cycle. A duty cycle of 50% gives +2.5V and this is the midpoint which results in no change in the sensor signal from input to output. In essence, the resulting control voltage from the micro is used to “offset” the output of op amp IC2b. The offset voltage either adds to or subtracts from the DC voltage that otherwise would have been delivered by IC2b. But first, the DC control voltage from the micro has to be level-shifted by op amps IC2a & IC3a. As stated, at 50% duty cycle, the voltage at pin 3 of IC2a is 2.5V. IC2a amplifies this by 1.91 due to its 10kΩ and 9.1kΩ feedback resistors. With nothing else happening, IC2a’s output would be +4.775, or approximately +4.78V. However, IC3a provides an offset voltage for pin 2 of IC2a so that its output actually sits at 0V. This works as follows. Trimpot VR5 feeds a voltage to pin 3 of IC3a which amplifies it by a factor of 2. In practice, VR5 is set to feed 2.63V to pin 3 of IC3a and so the output at pin 1 is +5.25V. IC2a then amplifies this by -0.91 (9.1kΩ/10kΩ) to give -4.78V and this will exactly cancel the +4.78V which would have been there from the input to pin 3 (of IC2a). Hence, the output from IC2a is set to 0V. Then with the normal duty cycle variation from pin 9 of IC4, IC2a’s output will swing above and below 0V by ±4.78V. However, there is a further adjustment of the signal from IC2a which is fed to Sensitivity trimpot VR6; it sets the overall level of voltage applied to IC2b. When the wiper of VR6 is at 0V, there is no change in output from the Voltage Interceptor with PWM changes. When set so the wiper is towards the 43kΩ resistor, the maximum offset shift for the Voltage Interceptor output is available. Power supply Power for the circuit comes from the car’s 12V supply which can rise to about +14.4V when the battery is being charged. Diode D1 is included to protect the circuit from reverse supply connection. Following this diode we derive the 5.6V supply using zener diode ZD4 and a 470Ω series resistor. A nominal +11.4V supply is provided via a 100Ω resistor and 15V zener diode ZD2. The zener diode is included to suppress transient voltages which are present on 12V car supplies. The 11.4V rail supplies op amp IC3. An LM317T 3-pin adjustable voltage regulator (REG1) is used to derive the main +5V rail. The incoming 12V supply is fed to REG1 via a 10Ω resistor while 16V zener diode ZD1 clamps any voltage transients, with further filtering provided by a 100µF capacitor. Op amp supplies We have specified three LM6482AIN dual op amps for this project. These have a very low input offset voltage of 110µV (typical), an extremely high input impedance of more than 10 Tersiliconchip.com.au DB25 SOCKET +5V 5 14 2 3 15 10k 10k 1 16 Vdd CP0 CP1 IC1 4017B MR 13 12 O5-9 8 Vss O0 O1 O2 O3 O4 O5 O6 O7 O8 O9 3 2 4 7 5 6 10 1 9 11 S1 S3 S2 S5 S4 S7 S6 VR1 10k 10 µF 3 S9 S8 4 S10 6 9 14 13 6 12 8 11 10 10 11 9 12 8 4 6 x 330 Ω* * USES 7 x 330 Ω RESISTOR ARRAY DB7 LCD DISPLAY MODULE DB6 DB5 DB4 DB3 DB2 5 SWITCH FUNCTIONS S7 LEFT 3 S8 STEP LEFT S9 DOWN 6 S4 STEP DOWN S5 VIEW/RUN STEP UP 7 S6 5 8 S1 RIGHT 4 S3 STEP RIGHT 8 PUSHBUTTON CONTROLLER R/W 2 S2 UP SC RS EN DB1 7 DB0 13 2007 CONTRAST 7 S10 RESET Fig.4: the circuit for the Pushbutton Controller is quite simple. It uses 10 switches, an LCD module, a 4017 counter (IC1), a DB25 socket, a 10mF capacitor and a few resistors. Trimpot VR1 sets the display contrast. aohms (>10TΩ), a 4pA input bias current, an output swing to within 10mV of the supply rails and a wide common mode input range which includes the supply rails. In other word, this is a pretty special op amp package. Furthermore, the three op amp packages have different supply voltage requirements. For IC1a, the output is expected to swing from 0V and negative by no more than -7.05V. IC1b is required to swing from 0V and positive up to 5.00V. Hence, IC1 uses the +5.6V and -7.5V supply rails. For IC2a, the output will swing over a maximum of ±4.78V, as noted above. IC2b will typically swing from 0V to -5V. Hence, IC2 uses the main +5V rail and the same -7.5V rail as used by IC1. IC3a’s output is usually fixed at close to 5V, as noted above, while IC3b’s output is designed to swing from a typical 12V maximum down to 0V. Supply for this is 0V for the negative supply and a nominal 11.4V for the positive supply. This 11.4V supply will vary with the car battery voltage and must be over 13V (as it normally siliconchip.com.au This is the view inside the completed Voltage Interceptor unit. It’s build on a single PC board and is housed in a sturdy diecast metal case. The construction details will be in Pt.2 next month. December 2009  29 Using The Pushbutton Controller As already noted, the controller has a 2-line 16-character Liquid Crystal Display (LCD) and nine pushbutton switches to do the programming. In fact, it is the same controller that was used in the Programmable Ignition System from the March, April & May 2007 issues of SILICON CHIP and it is available as a Jaycar kit, Cat KC-5386. On the top line, the display shows OUTPUT then the output value and either (dV) or LOCK. The OUTPUT refers to the up or down adjustment made and is 0 when there is no change in the output compared to the input. Values can be altered in number by up to ±127. The (dV) is the delta Voltage and is an abbreviation for the change in voltage made to the output. If the word LOCK is displayed instead, it means that a jumper link has been installed preventing any adjustment to the output settings using the push button switches. On the lower line of the display, it shows INPUT and then a number (from 1-255) and then either /RUN/ or (VIEW). The INPUT number refers to the way the input value has been divided up into 256 sections from minimum through to maximum and shows the particular input value load site and its corresponding output value (on the top line). The /RUN/ display shows input load sites in real time as they follow any input voltage variation. A (VIEW) display does not show the current input value that is connected to the Voltage Interceptor but the input value selected by the pushbutton switches. This display allows the whole input/output map to be viewed by scrolling through each value. The display is changed between /RUN/ and (VIEW) using the Run/View switch. Pressing this switch toggles between the two alternatives. Up and Down switches are used to change the output value for each input value. A fast up and fast down switch is also included to increment the values in steps of 4 instead of steps of 1. Scroll left and scroll right buttons provide for changing the input value when set for the view display. These switches do not operate for the run display. A reset button is included and must be accessed using a small probe that inserts into a hole in the front panel. Pressing this switch for 4s resets all output adjustments back to 0. The display shows RESET on the top line when this reset is successful. will be) for the output to reach 12V. So how do we generate the -7.5V rail? This task is performed by the micro, as well as doing its main function in providing the main Interceptor offset function. It delivers a 975Hz square wave output from its RB2 output at pin 8. This drives a charge pump circuit comprising transistors Q2, Q3 & Q4, diodes D3 & D4 and capacitors C1 & C2. Transistor Q2 and the 2.2kΩ resistor act as a level shifter, converting the 5V square wave from RB2 to a 12V square wave. Complementary transistors Q3 & Q4 buffer this 12V square wave and drive the charge pump. When the square wave is high at about 12V, Q3 switches on and charges C1 to almost 12V via diode D3. When the 12V square wave is at 0V, transistor Q4 switches on and the positive side of C1 is pulled down to 0V. The negative side of C1 is therefore close to -12V and this charges capacitor C2 (negatively) via the now conducting diode D4. The process repeats with C1 charging and delivering its charge to C2. This provides the nega30  Silicon Chip tive voltage supply that is regulated to 7.5V using zener diode ZD3 and the 220Ω series resistor. Relay operation As already noted, relay RLY1 bypasses the Interceptor circuitry before the car’s engine is started, to avoid the ECU recording a fault condition. The relay is switched on when IC4’s RB1 output goes high and drives transistor Q1 via a 1kΩ resistor. Q1 then drives the relay coil and indicator LED1. Diode D2 clamps the reverse voltage from the coil when it is switched off to prevent damage to Q1. The relay is switched on at a preset supply voltage. This can be either as soon as a 12V supply is connected to the Voltage Interceptor or at a higher voltage. The higher voltage threshold prevents the relay from switching until the car engine has started, after which the alternator increases the supply rail, ie, as the battery is charged. The relay switching voltage is set using trimpot VR4. Its wiper is monitored by the AN3 input (pin 2) of IC4 and the setting can be measured at test point TP4. The voltage is scaled so that 1.3V at TP4 gives a 13V threshold. Similarly, for a 12V threshold, TP4 would be set to 1.2V with VR4. The battery supply voltage is measured at the AN1 input (pin 18) of IC4. The 12V input is divided down by a factor of three using the 20kΩ and 10kΩ resistors and so the AN1 input will be at 5V for a 15V supply. Pushbutton controller The 25-pin socket shown on the lefthand side of the circuit (Fig.3) is for connection to the Pushbutton Controller. A jumper link (Lock: LK1) at the RB0 input to IC4 prevents the pushbutton controller from making any changes to the output. In this LOCK state, the input and output values can be viewed but not altered. The circuit for the Pushbutton Controller is shown in Fig.4. It comprises an LCD module, a 4017 decade counter (lC1), a DB25 socket and several pushbutton switches. This unit connects to the 25-pin connector in the main Voltage Interceptor circuit via a standard DB25 RS232 cable. Signals from the microcontroller in the Voltage Interceptor drive both the LCD module and counter IC1. IC1 has 10 outputs and each output independently goes high in sequence as it is clocked at its clock input (pin 14). A high at the reset (MR, pin 15) sets the “0” output at pin 3 high. Each output connects to a switch. When a switch is closed, it pulls pin 9 of the DB25 socket high whenever its corresponding output on IC1 is high. This allows the microcontroller in the Voltage Interceptor to recognise which switch is closed. The LCD is driven using data lines DB7-DB4. The display readings are entered via the data lines and are controlled via the EN and RS (Enable and Register Select) inputs. Note that the data lines and the EN and RS lines are all connected to ground via 330Ω resistors. These resistors terminate the signals correctly to prevent false data at the LCD from the long DB25 cable interconnection. The resistors also tie all inputs low when the DB25 cable is not connected Finally, trimpot VRl is used to adjust the display contrast. That’s all for this month. Next month, we’ll give the construction details and describe how it’s used. SC siliconchip.com.au