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Want to control a really big DC motor? This circuit can handle 12V or 24V DC motors at currents up to 40A. 12V-24V High-Current Motor Speed Controller This 12V or 24V high-current DC Motor Speed Controller is rated at up to 40A (continuous) and is suitable for heavy-duty motor applications. All control tasks are monitored by a microcontroller and as a result, the list of features is extensive. T HIS COMPLETELY NEW speed controller is based on a PIC16F88 microcontroller. This micro provides all the fancy features such as battery monitoring, soft-start and speed regulation. It also monitors the speed setting potentiometer and drives a 4-digit display board which includes two pushbuttons. The 4-digit display board is optional but we strongly recommend that you build it, even if you only use it for the initial set-up. It unlocks the full features of the speed controller and allows all settings to be adjusted. The microcontroller will detect whether 30  Silicon Chip the display board is connected and if not, the speed controller will support only the basic functions. In this simple mode, it will function as a simple speed regulated controller with automatic soft-start and with the speed being directly controlled by a pot (VR1). All the other settings will be the initial defaults or as last set (with the display board connected). When connected, the 4-digit display allows you to monitor the speed and the input voltage (useful when running from a battery). It also enables you to navigate through the various menus to adjust the settings. The circuit can run from 12V or 24V batteries and can drive motors (or resistive loads) up to 40A. Furthermore, this is our first DC speed controller (except for out train controllers) incorporating speed regulation under load. In other words, a given motor speed is maintained, regardless of whether the motor is driving a heavy load or not. Monitoring the back-EMF In speed controllers which do not have good speed regulation (ie, the vast majority of designs), the more a motor is loaded, the more it slows down. In order to provide speed regulation, the siliconchip.com.au Pt.1: By MAURO GRASSI circuit must monitor the back-EMF of the motor, since this parameter is directly proportional to its speed. As a result, our new speed controller monitors the back-EMF of the motor. “Back-EMF” is the voltage generated by any motor to oppose the current through the windings. EMF stands for “electromotive force” and is an obsolete term for voltage. Back-EMF is directly proportional to the motor speed and so by monitoring this parameter, we have a means of controlling and maintaining the motor speed. In practice, the main control loop of the microcontroller tries to match the speed of the motor (back-EMF) to the speed set by the pot or recalled from a preset memory. If the measured speed is lower than the set speed, the duty cycle of the pulse width modulation (PWM) signal used to drive the power Mosfets that control the motor is gradually increased. In other words, siliconchip.com.au if the speed tends to drop, more power is fed to the motor and vice versa. The frequency of the pulse width modulation can be set from 488Hz to 7812Hz. This is a useful feature since different motors will have different frequency responses, as well as different resonant frequencies. This is important to reduce the audible buzzing from the pulse width modulation, as these frequencies are well within the range of hearing. By now you’re probably wondering how the microcontroller monitors the back-EMF of the motor, considering that the motor is continuously driven with pulse-width modulated DC. The answer is that the micro periodically turns off the PWM signal to the motor for just enough time for the back-EMF to stabilise. This “window” needs to be wide enough to ensure that we are measuring backEMF and not the spike generated by the last PWM pulse. On the other hand, we don’t want the window so wide that the maximum power to the motor is significantly reduced or that the motor noticeably slows. The compromise value is that the motor is monitored for 200ms every 7.4ms (ie, about 135 times a second), as shown in the scope diagrams in this article. As a result, the fact that we do monitor the back-EMF around 135 times a second means that the power applied to the motor is slightly less than the theoretical maximum, although this effect is negligible. A low-battery alarm is also incorporated to warn when the battery level drops below a preset value. This is especially useful for applications like electric wheelchairs. There are also eight memory speed settings. All settings are persistent, meaning they are retained in nonvolatile memory. Soft start When the motor is switched off, perhaps by an external switch in series with one of its terminals, the voltage at the drain of the Mosfets will be 0V (this is due to the voltage divider used to scale the back-EMF voltage to within the operating range of the microcontroller). The microcontroller converts this analog value to a digital value using an on-board ADC (analogto-digital converter). The firmware detects this 0V con- Main Features • • • • • • • • • • Good speed regulation under load Automatic soft-start and fast switch-off Eight memory settings 4-digit 7-segment display Variable frequency for pulse width modulation (PWM) Battery level meter Low-battery alarm Persistent settings & defaults Rated up to 40A continuous current 12-24V DC input voltage dition and sets the duty cycle of the PWM to 0%. This ensures that when the motor is switched in, its speed will increase gradually from the stationary state to the desired speed setting. Turn-on currents for motors can be very high and it is desirable to reduce these surge currents as much as possible. That is why the automatic softstart feature has been incorporated into the firmware. It will ensure that the motor is brought up to the set speed gradually. Fast switch-off feature Another feature that has been incorporated into the firmware is the so-called “Fast-off” feature. This means that the duty cycle of the PW modulation is set to 0% (turning off the motor) whenever the selected speed setting of the pot goes to 0%. Rather than decreasing the speed gradually, setting the pot to its lowest setting turns the motor off immediately. This design also incorporates our extensive experience with previous speed controllers featured in SILICON CHIP. As a result, it uses four highcurrent Mosfets to do the switching (pulse width modulation), uses very wide tracks on the PC board and heavyduty (40A) terminal blocks to carry the heavy currents. User interface Two pushbuttons on the display board are used to navigate through the menus, while the pot is used both to vary the speed and to vary certain settings. March 2008  31 Parts List 1 PC board, code 09103081, 124mm x 118mm 2 heavy-duty PC-mount terminal blocks (3-way) (Altronics P2053) 1 8-pin DIP IC socket 1 18-pin DIP IC socket 1 SPDT toggle switch (S1) 1 50A 5AG fuse (Jaycar SF1976) 1 60A 5AG fuseholder (Jaycar SZ2065) 1 12-way pin header (Altronics P-5502) 1 PC-mount mini piezo beeper (Jaycar AB3459 or equivalent) 1 220mH inductor (L1) (Jaycar LF1276 or equivalent) 1 10kW 16mm PC-mount linear single-gang pot (VR1) 1 500W horizontal trimpot (VR2) Semiconductors 1 PIC16F88-I/P microcontroller programmed with 0910308A.hex (IC1) 1 MC34063 switchmode DC-DC converter (IC2) 1 BC327 PNP transistor (Q1) 3 BC337 NPN transistors (Q2-Q4) 4 IRF1405 N-channel Mosfets (Q5-Q8) (Jaycar ZT2468) 1 1N4004 diode (D1) 1 1N5819 Schottky diode (D2) 2 MBR20100CT 20A diodes (Jaycar ZR1039) OR 1 40EPF06PBF 40A ultra-fast diode (Farnell 910-1560) (D3) 5 1N4745 16V 1W zener diodes (ZD1-ZD5) 2 1N5364BG 33V 5W zener diodes (ZD6-ZD7) (Farnell 955-8217) 1 3mm red LED (LED1) Capacitors 1 2200mF 50V low-ESR electrolytic (Altronics R-6207) The two pushbuttons are sensitive to two types of presses, short and long. A short press is of the order of half a second or less while a long press is one around one second. To change a setting, a long press is usually needed. This prevents unwanted changes to the settings, which are stored in EEPROM and thus recalled at the next switch on. Because of the capabilities offer­ed by the PIC microcontroller, we have 32  Silicon Chip 1 470mF 16V electrolytic 1 100mF 63V electrolytic 1 100mF 25V electrolytic 1 10mF 25V electrolytic 3 4.7mF 16V electrolytic 1 220nF 100V MKT polyester 1 100nF 100V MKT polyester 3 100nF monolithic 1 470pF ceramic Resistors (0.25W, 1%) 2 33kW 1 100W 2 4.7kW 1 56W 1 3.6kW 1 22W 1W 6 1kW 4 15W 2 470W 3 1W Display Board 1 PC board, code 09103082, 73mm x 58mm 1 200mm length 16-way rainbow cable 1 12-way pin header (Altronics P-5502) 2 12-way header plugs (Altronics P-5482) (to terminate cable) 1 SPST PC-mount momentarycontact switch, yellow (Jaycar SP0722; Altronics S-1097) (S2) 1 SPST PC-mount momentarycontact switch, red (Jaycar SP0720; Altronics S-1095) (S3) 1 16-pin DIP IC socket (optional) 1 100nF monolithic capacitor Semiconductors 1 74HC595 shift register (IC3) 4 BC337 NPN transistors (Q9-Q12) 4 7-segment common cathode red LED displays (Jaycar ZD1855; Altronics Z-0190 ) Resistors (0.25W, 1%) 4 470W 8 39W been able to incorporate a large number of features into the firmware, as described in the separate panel later in this article. Circuit description The circuit for the speed controller is shown in Fig.1. As noted previously, it can work with 12V or 24V batteries but has been optimised for operation at 24V. Within the circuit itself, there are two separate voltage rails: +5V for the microcontroller and +16V for driving the gates of the Mosfets. Both are derived from the +24V input supply. The main input supply is filtered by a 2200mF low ESR capacitor, to minimise high-voltage transients which can be produced by the inductance of the battery connecting leads. This capacitor is absolutely vital to the proper operation of the speed controller at high currents. S1 is the power switch and diode D1 protects the low-power part of the circuit (IC1 & IC2) from reverse polarity. A 22W 1W resistor, a 33V 5W zener diode (ZD7) and a 100mF capacitor also protect the MC34063 IC from transients on the supply rail. The filtered supply is then fed to the MC34063 (IC2) which operates in a standard step-down converter configuration to provide the +5V rail. Three 1W resistors between pins 6 & 7 are used to set the maximum switching current. The output voltage is set by the voltage divider associated with trimpot VR2. Only about 200mA is ever drawn from this supply and most of this is used to drive the display. IC1 is the heart of the circuit and is the popular PIC16F88 microcontroller which incorporates a number of peripheral functions. Of these, the timers, hardware PWM (pulse width modulation) and three ADC inputs are used. The three ADC inputs used are at pins 1, 2 & 18. As these need to be within the 0-5V range, voltage dividers consisting of 33kW and 4.7kW resistors are used to scale both the input voltage rail (which could be as high as 29V) and the back-EMF from the motor, to be fed to the ADC inputs at pins 1 & 18. The ADCs convert the monitored voltages to 10-bit values. The +16V rail is used as the gate drive supply for the Mosfets and is derived from the 24V supply via a 1kW resistor and a 16V 1W zener diode (ZD1). Bypassing of this rail is particularly important and is accomplished using 100mF and 100nF capacitors near ZD1 and adjacent to the transistors Q1 & Q2. If the battery supply is to be 12V, the 1kW resistor feeding ZD1 should be reduced to 100W. In this case, the supply will actually be between 12V and 14V (depending on the actual battery voltage); still enough to provide adequate gate drive for the Mosfets and siliconchip.com.au siliconchip.com.au March 2008  33 Fig.1: the circuit uses PIC16F88 microcontroller IC1 to provide PWM drive to power-Mosfets Q5-Q8 which in turn control the motor. The microcontroller also monitors the back-EMF from the motor, to provide speed regulation. IC2 is a DC-DC switchmode converter and this provides a +5V rail to power IC1. a f DISP 3 a a b g e f g e c d b f g e c C B E C Q10 b f C Q11 E E g e d B 8x 39 a c d Q9 DISP 4 b c d B C Q12 16 Vdd 15 Qa 1 Qb SRClr 2 Qc 3 Qd IC3 Sck 4 Qe 74HC595 Rck 5 Qf OE 6 Sin Qg 7 Qh Vss 8 100nF 10 +5V 11 12 13 10 9 8 7 6 5 14 4 B 11 12 2 3 470 E 470 Q9–Q12: BC337 470 470 B E SC  2008 CON2 1 (TO MAIN BOARD) DISP 2 DISP 1 C DC MOTOR SPEED CONTROLLER S2 S3 DISPLAY BOARD Fig.2: the display circuit interfaces to the microcontroller & uses a 74HC595 shift register (IC3) & transistors Q9Q12 to drive four 7-segment LED displays. Switches S2 & S3 are used to control the display & for software set-up. ensure minimum heat dissipation (low on-resistance). The PWM output of the PIC16F88 (adjusted by firmware) appears at pin 6 and drives transistor Q3 which then drives complementary transistors Q1 & Q2. Q1, Q2 & Q3 thus provide buffering and voltage level translation for IC1’s PWM output to drive the gates of Mosfets Q5-Q8 via 15W resistors. Note that these resistors need to be relatively low in value (ie, 15W) in order to ensure quick charging and discharging of the gate capacitances. That’s because the gate capacitance of these Mosfets can be quite high, of the order of 5000-10,000pF each. If the gate charging time is too long, the Mosfets will spend too much time between the on and off states and this will lead to much higher heat dissipation. In fact, the gate voltage transitions need to be very fast, of the order of 1ms or less. This has been accomplished, as shown by the oscilloscope screen grab of Fig.4. The specified Mosfets are from International Rectifier, type IRF1405. This is a 55V 169A N-channel Hexfet with an exceptionally low on-resistance (Rds) of 5.3 milliohms (5.3mW) typical. Their pulse current rating is a stupendous 680A. The IRF1405 is specifically intended for automotive use, in applications such as electric power steering, anti34  Silicon Chip lock braking systems (ABS), power windows and so on and is therefore ideal for this speed control application. Why four Mosfets? In fact, since the ratings of this Mosfet are so high, you might think that just one device on its own would be enough to handle the 40A rating of this speed controller project. So why are we using four Mosfets in parallel? As always, real world use brings us down to earth. For a start, we are using these Mosfets without heatsinks, apart from the vestigial heatsink effect of their being bolted to and connected to the copper side of the PC board – not much heatsink benefit there. Their thermal characteristic is 62°C per watt (junction to ambient), assuming that are mounted in free air (which they are not). Assuming an ambient temperature of 25°C and an on-resistance of 10mW (conservative), we can approximate the temperature of the Mosfets at their highest operating current (10A per Mosfet for a total of 40A). At 10A and 10mW on-resistance, the power dissipated is: 102 x .01 = 1W This means that the temperature of the case will be approximately: 25 + 62 x 1 = 87°C This means that at full current, the Mosfets will be very hot to the touch. Careful: they will burn you. Our measurements produced a top temperature of around 77°C after a test period of half an hour. In practice, even with much higher ambient temperatures, the Mosfets should not get quite this hot because in “real world” operation, the speed control is not likely to be providing full power to the motor on a continuous basis. At 24V and 40A, the motor would have 960W applied (ie, more than 1HP) and this equates to relatively high power operation. Protection Zener diodes ZD2-ZD5 are included to protect the Mosfets from excessive gate voltages. In normal circuit operation, these zener diodes do nothing. Additional protection for the drains of the paralleled Mosfets is provided by 33V 5W zener diode ZD6, in parallel with a 100nF capacitor. The zener is there to clip any residual voltage transients which get past the 2200mF low-ESR input filter capacitor. The Mosfets are further protected by fast-recovery diode D3 and its parallel 220nF capacitor. These parts are wired across the motor terminals and are used to suppress the high back-EMF spikes caused by the armature inductance when the motor is switched off by the Mosfets. These components are crucial to siliconchip.com.au Fig.3: the yellow trace is the voltage waveform at the drain of the Mosfets, when a motor is connected. There are narrow spikes up to 31.7V when the Mosfets switch off due to the inductance of the armature. The small windows where the Mosfets are switched off to sense the back-EMF of the motor can also be seen. The two vertical cursors show that the period between such intervals is of the order of 7.6ms. In other words, the speed of the motor is monitored at 131Hz. Fig.4: the yellow trace is the voltage waveform at the drain of the Mosfets, while the purple trace is the gate drive. The gate drive goes as high as 15.3V. The rise time of the gates is 526ns while the fall time is 92ns. When switching the Mosfets on and off, it is necessary that the transition be fast, ideally under 1ms, otherwise the Mosfets will dissipate more heat than is necessary. To ensure fast switching of the Mosfets their gate capacitance needs to be charged and discharged very quickly. Fig.5: the yellow trace shows the voltage waveform at the drain of the Mosfets when a motor is connected. The irregular waveform corresponds to the back-EMF monitoring. The Mosfets are then off and the voltage is then directly proportional to the speed of the motor. The window is narrow enough so that the motor’s deceleration is negligible. Turning off the Mosfets to monitor the backEMF is asynchronous to the PWM driving the Mosfets. Fig.6: the yellow trace is the voltage waveform at the drain of the Mosfets and the purple trace is the waveform at the gate of the Mosfets when a motor is connected. Again, the irregular yellow waveform (arrowed) corresponds to the period when the Mosfets are switched off to sense the back-EMF and hence the speed of the motor. You can see from the purple trace that the gate drive during this time is 0V. the operation of the speed controller. Without them, the high voltages generated can and probably would destroy the Mosfets. Other protection measures As already mentioned, diode D1 provides reverse polarity protection for microcontroller IC1 and the switchmode supply (IC2). Zener diode ZD1 is self-protecting in the case of siliconchip.com.au reverse supply connection. However, if the supply is reversed, there will be a heavy conduction path via fast recovery diode D3 and the internal substrate diodes in the four power Mosfets. If you are lucky, the 50A fuse will blow before the Mosfets are damaged but there is no guarantee of this. SO DON’T REVERSE THE BATTERY CONNECTIONS! In a similar vein, if the outputs are shorted while power is applied, high current will flow through the Mosfets. Again, if you are lucky, the 50A fuse will blow before the Mosfets go up in smoke. In reality, the 50A fuse is there to stop a fire! SO DON’T SHORT THE OUTPUTS TO THE MOTOR. If the motor is under heavy load and becomes stalled, high currents will flow in its armature. Depending on the motor’s rating, this may or may not March 2008  35 This view shows the fully assembled main board. The assembly details are in next month’s issue. Fig.7: the yellow trace shows the voltage waveform at the drain of the Mosfets, the purple trace shows the voltage waveform at the gates and the cyan trace shows the voltage waveform at the PWM output of the microcontroller. Note that transistors Q1-Q3 provide voltage translation by stepping up the 5V output from the microcontroller to 12-16V. This higher voltage is needed to ensure that the Mosfets are fully turned on. blow the fuse. If the fuse does not blow during stall conditions of the motor, the Mosfets should survive although they may get very hot. Warning buzzer If the circuit is overloaded, the battery voltage should drop to the point where the warning buzzer will sound. LED1 and its 470W current limiting resistor are switched by a high level on the output of pin 3 of the microcontroller. This is configured as a simple digital output. It also turns on Q4 and the piezo beeper. This output is controlled by the firmware and can be disabled. A 1kW pull-up resistor is used on pin 4 (reset) of the PIC16F88-I/P. This ties the reset pin high which means that the microcontroller is reset only at power-on. Finally, the rest of the outputs of the microcontroller, namely pins 7-17, are used to drive the optional display board. Display board Fig.2 shows the optional display board circuit. It connects to the main board via 12-pin header CON1 and a ribbon cable. The display board consists of two pushbuttons, four 7-segment displays which are multiplexed by the firmware, four transistors and some resistors, as well as a 74HC595 shift register (IC3). Pins 1 & 2 of 12-way connector CON2 supply +5V to the display board. Pin 3 is connected to a digital input The optional display board is connected to the main board via a 12-way ribbon cable. It displays the motor speed as a percentage of full speed and is used for the software set-up. 36  Silicon Chip of the microcontroller and is pulled high by a 1kW resistor on the main board. Conversely, it is pulled low by the display board. This is used by the microcontroller to detect whether the display board is connected or not. Pins 4-7 of CON1 are used to drive the transistors Q9-Q12 on the display board. These transistors switch the 7-segment display cathodes. Pins 8-10 of CON1 are respectively the CLK, DATA and OUTPUT ENABLE lines and these go to the 74HC595 shift register (IC3). The microcontroller drives these lines to load a new 8-bit value into the shift register. The outputs of the shift register are connected across the four 7-segment displays and drive the anodes. Finally, pins 11 & 12 are connected to pushbuttons switches S2 & S3 on the display board. They are also connected to digital inputs on the microcontroller (which have internal pull-ups enabled) and these inputs are used to monitor the pushbuttons. Next month, we will cover the construction and troubleshooting of the speed controller. In the meantime, take a look at the “Software Features & Set-up” panel on the facing page. siliconchip.com.au DC Motor Speed Controller: Software Features & Set-up T HE STRUCTURE of the firmware for the DC Motor Speed Controller is shown overleaf in Fig.8. The transitions between the various menus are made using the switches on the display board and are indicated with labelled arrows. There are four possible switch presses, either Short or Long and either the Left (L) or Right (R). Thus, for example, “Short R” refers to a short press of the right pushbutton. Main menu The Main menu is as shown in Menu 1. It consists of the letter ‘P’ (for “percentage”) and three digits with a decimal point indicating the range 00.0% to 99.9%. The percentage value indicates the fraction of full speed the motor is currently running at. In this mode, the motor’s speed can be adjusted by varying the pot. The letter ‘P’ will flash while the motor’s speed increases or decreases to the new setting. When the current speed reaches the speed set by the pot, the letter ‘P’ will stop flashing and there will be a short beep (if enabled). Since there is a small periodic window when the pulse width modulation is turned off by the firmware in order to read the backEMF, at full speed the reading will not indicate 99.9% but will achieve its maximum at around 98% or so. Monitoring the input voltage From the Main menu, press “Short R” once and you will be taken to the display shown in Menu 2. It consists of a ‘b’ (for “battery”) followed by three digits with a decimal point indicating a level from 00.0V to 99.9V, to monitor the battery. For good voltage accuracy, it is important that the +5V supply rail be precisely set using trimpot VR2. In practice, with the supply rail to the microcontroller set at 5V, the level will not register any higher than around 40.1V. This is because the voltage divider used to derive the voltage reading consists of 33kW and 4.7W resistors. The relatively high series resistance of 37.7kW was chosen to avoid damaging the input of IC1 if the input voltage goes any higher than around 40V. To go back to the Main menu, either press “Short L” or press “Long R”. If you press “Long L”, you will set the low-battery alarm level to 91.6% of the current voltage input level (and then return to the Main menu). This is a shorthand way to set the low-battery alarm level when you know that the batteries are fully charged. For a typical 12V battery, they are fully charged at around 13.8V (with charger connected) and should not be discharged beyond 11V. Press “Short R” to go to the lowbattery alarm level menu. Setting the low-battery alarm From the Main menu, press “Short R” twice. You will be taken to the low-battery alarm level menu as shown in Menu 3. It consists of an ‘A’ (for “alarm”) followed by three digits which indicate a level between 00.0V and 41.6V. This will show the current setting of the low-battery alarm or rather, the voltage level below which the alarm will sound (if enabled). Whenever the input voltage is below this level, the display will flash (with increasing frequency as the voltage drops) while if the alarm sound is enabled, there will be a flash from LED1 and a beep. To set the low-battery alarm level . . . continued next page Looking for real performance? NOT A REPRINT – Completely NEW projects – the result of two years research & development • • • • 160 PAGES 23 CHAPTE RS Fr om th e pu bli sh Learn how engine management systems work Build projects to control nitrous, fuel injection and turbo boost systems Switch devices on and off on the basis of signal frequency, temperature and voltage Build test instruments to check fuel injector duty cycle, fuel mixture and brake and coolant temperatures Mail order prices: Aust. $A22.50 (incl. GST & P&P); Overseas $A26.00 via airmail. Order by phoning (02) 9939 3295 & quoting your credit card number; or fax the details to (02) 9939 2648; or mail your order with cheque or credit card details to Silicon Chip Publications, PO Box 139, Collaroy, NSW 2097. siliconchip.com.au er s of Intellig I SBN 9 780 95 $19.80 (inc GST) turbo tient mer 095 852 294 - 4 8 52 29 46 NZ $22.00 (inc GST) TURBO & nitrou BOOST s fuel cont rollers How en g managemine ent work s March 2008  37 DC Motor Speed Controller: Software Features & Set-up . . . continued press “Long L”. The ‘A’ will start flashing and then the low-battery alarm level can be modified by adjusting the pot setting. To turn the alarm off completely, simply set the level to 00.0V. When you have reached the required level, simply press any button and the level will be recorded (and stored in EEPROM). Then there will be a beep (if enabled) and you will be taken to the Main menu. Note that the motor will be turned off automatically when setting the low-battery alarm level. Setting the PWM frequency From the Main menu, press “Short R” three times. You will be taken to the frequency menu as shown in Menu 4. This consists of an ‘F’ (for “frequency”) followed by three digits with a decimal point indicating a level between 0.48kHz and 7.81kHz. This is the current PWM frequency. As the frequency increases, the resolution of the PWM setting decreases. At 0.48kHz (actually 488Hz) the resolution is 10 bits. This decreases to six bits at 7812Hz. Thus, the resolution is at worst six bits or 64 levels and at best 10 bits or 1024 levels. While in this menu, press “Long L” and you will be able to set the frequency. The ‘F’ will start flashing and then the frequency will be modified according to the pot setting. When you have reached the required frequency, simply press any button and the level will be recorded and stored in EEPROM. Then there will be a beep (if enabled) and you will be taken to the Main menu. Note that the motor will be automatically turned off when setting the frequency. Enabling & disabling audible cues From the Main menu, press “Long L”. You will be taken to the settings menu as shown in Menu 8. It consists of ‘A’ (for alarm) followed by either ‘0’ or ‘1’ (0 = disable, 1 = 38  Silicon Chip enable) and a ‘b’ (for beep) followed again by either ‘0’ or ‘1’ (0 = disable, 1 = enable). In this menu, pressing “Short L” will toggle the alarm setting (enable/disable) and pressing “Short R” will toggle the beep setting (enable/disable). When the alarm setting is disabled, there will be no beeping when the input voltage falls below the alarm level. There will still be a warning flashing on the display, however. To disable the latter, simply set the alarm level to 00.0V. When the beep setting is disabled, audible beeps emitted by the firmware at certain points (as when setting certain values or when the desired speed is reached) will be blocked. If you do not want any beeping from the piezo buzzer, simply set ‘A’ to 0 and ‘b’ to 0. In this menu, pressing “Long L” will take you to the Reset Menu, as explained below. Pressing “Long R” will take you back to the Main menu. Reset menu From the Main menu, press “Long L” twice. You will be taken to the Reset Menu as shown in Menu 9. It consists of the letters ‘CL’ (for “clear”) followed by two digits and a decimal point of the form X.X. The X.X represents the current version of the firmware, which for this release stands at 3.0. It is possible that future releases of the firmware will add new features or refinements to critical sections of the code. While in this menu, press “Short L”, “Short R” or “Long R” to go back to the Main menu. Note, however, that pressing “Long L” will reset all settings to the default values and the speed controller will lock until power is turned off. When a power-on reset next occurs, the default values for the frequency, low-battery level alarm and audible beeps will be restored. This feature is useful for initialising the firmware variables and for making sure that you begin from a known state. Most of the time, it will not be used. Memory speed mode From the Main menu, press “Short L” to enter memory mode. The display will be as shown in Menu 6. It consists of the letter ‘C’ (for “constant”) followed by a digit from 1-8 (indicating one of the eight available memories), in turn followed by two dashes. Now adjusting the pot will select one of the eight memories. When the pot becomes stable for a short period, the speed of the motor will be set according to the current value of that memory. The display will change as shown in Menu 7. This display still consists of the letter ‘C’ followed by the number of the memory but it will then have a decimal point followed by two digits representing the speed percentage from 00% to 99% (the first two letters will flash until the set speed is reached). Adjusting the pot will now change the selected memory and the speed setting will be recalled from one of the eight stored memory speed settings (after a short beep, if enabled). To go back to normal mode, where the motor speed is controlled directly by the pot, simply press any key, long or short. Setting the memory To set one of the eight memory speed values you press “Long R” from the Main menu. The display will change as shown in Menu 5. It consists of the letter ‘C’ (for “constant”) followed by a digit from 1-8 (indicating one of eight memory settings) and two dashes. Now adjusting the pot will select one of the eight memory settings to store the current value of the speed of the motor. When the pot becomes stable for a short period, the speed of the motor will be stored at that particular memory. This can be recalled later by entering memory mode, as explained in the previous section. There will be a short beep (if enabled), indicating that the value has been stored and you will be taken SC back to the Main menu. siliconchip.com.au siliconchip.com.au HALT STATE. TURN POWER OFF AND BACK ON TO RECALL DEFAULT SETTINGS. LONG L) MENU 9: LAST TWO DIGITS SHOW THE FIRMWARE VERSION. PRESS LONG L TO RESET ALL SETTINGS TO DEFAULT VALUES. (PRESS (PRESS SHORT L) (INACTIVE OR ACTIVE POT) (PRESS ANY KEY OR INACTIVE POT) (SET FREQUENCY WITH POT AND PRESS ANY KEY TO (PRESS RETURN LONG L) TO MAIN MENU) (SET ALARM LEVEL WITH POT AND PRESS ANY KEY TO RETURN (PRESS TO MAIN LONG L) MENU) MENU 5: SET MEMORY MENU. CHANGE MEMORY NUMBER BY VARYING POT. ONE OF EIGHT MEMORY PLACES CAN BE CHOSEN. CURRENT SPEED IS STORED IN THE CHOSEN MEMORY. (PRESS LONG R) SCREEN SHOWING THE LOW BATTERY WARNING. IT SPELLS “Lo” FOR LOW BATTERY. MENU 7: SHOWS THE MEMORY NUMBER CURRENTLY RECALLED AND THE CURRENT SPEED AS A 2-DIGIT PERCENTAGE. MENU 6: RECALL MEMORY MENU. CHANGE MEMORY NUMBER BY VARYING POT. THE STORED SPEED WILL BE RECALLED. (PRESS ANY KEY) (PRESS SHORT R) (PRESS SHORT R) (PRESS SHORT R) MENU 4: CURRENT FREQUENCY IS SHOWN IN KILOHERTZ. MENU 3: CURRENT ALARM LEVEL IS SHOWN IN VOLTS. (PRESS ANY KEY EXCEPT LONG L TO RETURN TO MAIN MENU) (PRESS LONG R OR SHORT L) MENU 2: INPUT VOLTAGE IS SHOWN. USEFUL FOR MONITORING BATTERY (PRESS LONG L TO LEVEL. SET ALARM LEVEL* ) (PRESS LONG R OR SHORT L) Fig.8: this diagram shows the structure of the firmware for the DC Motor Speed Controller. The transitions between the various menus are made using the switches on the display board and are indicated with labelled arrows. (PRESS ANY KEY EXCEPT LONG L TO RETURN TO MAIN MENU) (PRESS LONG L) MENU 8: DISABLE OR ENABLE AUDIBLE CUES. 0=DISABLED, (PRESS 1=ENABLED. A=ALARM, LONG L) B=GENERAL BEEP. PRESS SHORT L OR R TO TOGGLE SETTINGS. (PRESS LONG R) MENU 1: MAIN MENU. SPEED SHOWN AS THREE DIGIT PERCENTAGE. VARY SPEED WITH POT. * ALARM LEVEL IS SET TO 91.6% OF CURRENT INPUT VOLTAGE. March 2008  39