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gic level convertor is the elegant solution to this nasty issue before producing the circuit you see before you i sketched a rough layout to ensure i could fit everything i wanted in the available space i needed it to fit neatly inside a disguarded ferrero rocher case of course i didn t keep to the plan it kind of evolved as i was making it a tip that may or may not be common knowledge i found that a good way to mount a flat off the shelf integrated circuit ic breakout board like the one i am using pictured chop the legs off a resistor and then run it through the breakout board s hole then solder the leg to that and then onto the strip board look closely and you will see what i mean i have left the odd leg poking out my stripboard circuit was designed in such a way that the i2c lines were available as an expandable bus by this i mean that the two wires clock and serial ran the full width of the board you can make out two resistors in the finished picture these are the pullups for each line and the wires are colour coded as white scl and red sda now in theory i can add further sensors to the bus as and when the pullup resistors are present because the i2c lines are designed to remain in a logic high state these ensure that the line is pulled high the sensors in my circuit are only able to pull things to ground they cannot pull the lines high following the data sheet i cut and soldered each of the 16 pins to their appropriate destinations and tried it out firing it up first off it didn t work this was a result of sloppy workmanship using a multimeter it was apparent that something wasn t quite right further digging revealed the first error i had failed to snip the excess wire off one of my components after soldering this meant that the data line was shorted out to ground having solved that issue i tried again voltages were now as expected i read a few tutorials online and used the wire library to perform some read writes to my accelerometer this failed after many frustrated hours i read the datasheet very carefully basically the wire library was unable to support my hardware the accelerometer uses repeated start this is something unsupported by the wire library so after some searching i discovered another i2c library which quite frankly is far far better than wire it is provided free by peter fleury available on his website here using the data sheet and this excellent library i was able to interface with the accelerometer and the temperature sensor the code and finally the last piece in the puzzle the exciting bit we write some code compile and upload onto the arduino s atmega 328 chip so first off i downloaded my new i2c library and added it to the libraries folder in my arduino directory i think i might have changed the c file extension to a cpp in order to be compatible with the arduino s compiler be sure to include the relevant c library read your datasheet to establish the address of the device you wish to talk to the accel chip is addressed as 0x0d and temp chip addressed as 0x48 this is defined at the top of my sketch a method of letting the compiler know to assign a constant value to an identifier here is a code snippet with most of the variables i defined include i2cmaster h accelerometer define accel_addrs 0x1d 1 this is the mma8450q device address when pin 7 is high define accel_ctrl_reg1 0x38 this is the system control register define standby 0x00 stops measuring used to configure device define resolution 0x01 sets to 2g mode 0 1 define odr 0x03 2 set the output data rate 3 50hz define who_am_i 0x0f define xyz_data_cfg 0x16 configure the data ready register define reg_status 0x04 this gets set when data is available and reset when data is read temperature sensor define temp_addrs 0x48 1 this is the temp device address define temp_reg 0x00 this is the temperature register now avoid blindly copying my code you will be better reading how to use the library and applying that to your application check your data sheet and try to understand what is going on that being said i will proceed with what i got up to in this project the standby and active modes are controlled by the last two bits of the system control 1 register at 0x38 fs1 and fs0 0 0 standby 0 1 active 2g mode 1 0 active 4g 1 1 8g define out_x_lsb 0x05 x register define out_x_msb 0x06 y register define out_y_lsb 0x07 z register define out_y_msb 0x08 define out_z_lsb 0x09 define out_z_msb 0x0a int msb_x 0 int msb_y 0 int msb_z 0 int lsb_x 0 int lsb_y 0 int lsb_z 0 int x_data 0 int y_data 0 int z_data 0 int xx_dat 0 int yy_dat 0 int zz_dat 0 int wire_debug 1 int running2g 0 int data_ready 0 int looopcount 0 int xyz_event 80 bool temp_sensor_read 0 double ambient_temp 0 unsigned long tot_accel 0 unsigned long avg_accel 0 int array_pos 0 const int avg_size 10 unsigned int accel_now avg_size set size of array for storing readings bool movement_detected 0 false means vehicle is stationary unsigned int roll_reset 0 unsigned short time_stamp void setup serial begin 38400 serial println serial running i2c_init sets up hw pins for i2c delay 100 initialise the sensor put all configs here before we activate i2c_start_wait accel_addrs i2c_write accelerometer device address i2c_write accel_ctrl_reg1 i2c_write standby i2c_stop setup for polling mode i2c_start_wait accel_addrs i2c_write i2c_write xyz_data_cfg i2c_write reg_status i2c_stop delay 500 all configs should be above as we are going online now i2c_start_wait accel_addrs i2c_write accelerometer device address wire_debug i2c_write accel_ctrl_reg1 this addresses the appropriate register if wire_debug 0 success i2c_write odr resolution i2c_stop if resolution 1 running2g 1 else serial println failed to wake the ol girl up error trap delay 500 for int thisreading 0 thisreading avg_size thisreading accel_now thisreading 0 ensures the array is full of 0s when we start end setup i chose to go for a fairly quick serial baud rate anything quicker and my frontend gui was overwhelmed the hw pins 4 5 on the arduino are lost as an i2c interface when we send i2c_init next i address the sensor and tell it to expect a write then i send the register address then i send the value intended to be written to the defined register on the defined slave address various configurations are then set the data rate for example now onto the main programme which runs indefinately so we head into the loop i am still developing bits and pieces this code has a timestamp which is to be passed across the serial for my gui in future i expect that will go and i will keep time on the pc side my data ready flag is initialised as 0 and is set high by the accelerometer everytime the accel reports it has data on the xyz registers a while loop is perfect for polling it will continually keep checking if data is ready until this flag is not 0 the commented section was used during debugging for establishing how many times i could run the while loop before data was ready a significant number of iterations of the while loop was possible so i decided to go and read from my temperature sensor to avoid doing this too often i set a boolean high once temperature has been obtained this flag gets set once outside of the main while loop void loop time_stamp millis while data_ready 0 i2c_start_wait accel_addrs i2c_write accelerometer device address i2c_write reg_status register with data ready flag i2c_rep_start accel_addrs i2c_read data_ready i2c_readnak i2c_stop looopcount looopcount 1 expect a min of 1422 iterations in 2g mode at 1 5 hz n b 1422 even after reading from temp sensor this gives us a great window of opportunity for temp readings if temp_sensor_read 0 ambient_temp get_temperature temp_sensor_read 1 serial println looopcount for debug looopcount 0 data_ready 0 resets flag for next loop iteration temp_sensor_read 0 set this to 0 to tell function to read temp sensor i2c_start_wait accel_addrs i2c_write i2c_write out_x_lsb i2c_rep_start accel_addrs i2c_read binary hex and dec onto some bit shifting computers can t strictly count they can recognise if voltage levels are high or not on or off this is why for a computer all numbers must be made up of only ones and zeroes for example 5 101 in binary basically we are working with a number system to base 2 an unsigned 8 bit number has the possibility of representing 255 integers as well as 0 that is 2 8 so for 5 we have 1 2 2 0 2 1 1 2 0 giving 101 unsigned means the number can t go negative if you want negative you have to use a signed integer an 8 bit signed number ranges between 127 to 127 we have 7 bits available for the value with the final left most most significant bit available for the sign check out two s complement for a more comprehensive description bit shifting is very handy hopefully you understand that numbers are represented as binary at the very basic level on the arduino chip programmers often use hexadecimal hex for short this is purely for simplicity when writing code it has ended up for whatever reason that bits are often packaged as bytes 8 bits are in a byte one byte occupies only 2 characters in hex theoretically we could write 1111 1111 this is what it will look like on the chip or we could say 255 in decimal base 10 is generally used in the real world but ff in hex is the least cumbersome of the three despite all three having the same value in my code i receive 12 bits or 1 5 bytes for each direction 1 byte 1 nibble in order to arrange it meaningfully into one number of the correct magnitude i must do some bit shifting it s like if you shift the decimal number 10 left we add a zero to the end and it becomes 100 the same happens in binary so if we shift b101 left 101 1 it becomes 1010 so we now have decimal 10 that being said the data is read using a multibyte read this reduces the number of physical writes required as the accelerometer chip will auto increment through its registers rather than wasting time and resource using the arduino to request data sequentially for each register let s start a multi byte read use the datasheet for the order lsb_x i2c_readack 4 msb_x i2c_readack 8 lsb_y i2c_readack 4 msb_y i2c_readack 8 lsb_z i2c_readack 4 msb_z i2c_readnak 8 i2c_stop x_data get_accel_data msb_x lsb_x xx_dat x_data 10 y_data get_accel_data msb_y lsb_y yy_dat y_data 10 z_data get_accel_data msb_z lsb_z zz_dat z_data 10 i want to keep a rolling average of sum of squares for say 10 ticks at 200 hz tot_accel tot_accel accel_now array_pos allows us to keep rolling accel_now array_pos xx_dat xx_dat yy_dat yy_dat zz_dat zz_dat tot_accel tot_accel accel_now array_pos array_pos if array_pos avg_size array_pos 0 avg_accel tot_accel avg_size roll_reset now we have an average lets check if we think we are stationary if accel_now array_pos avg_accel 2 avg_accel 1 05 accel_now array_pos avg_accel 2 avg_accel 0 95 movement_detected 1 else movement_detected 0 serial print serial print x_data serial print serial print y_data serial print serial print z_data serial print serial print ambient_temp serial print serial print movement_detected serial print serial print time_stamp serial println this pretty much completes it i am developing a quick and easy movement detected algorithm using the sum of squares to identify if acceleration is abnormal compared with the stationary average i push each value out on the serial port it is seperated using a this makes it easy to id where each piece of data resides when using it in the gui the raw data is gathered and arranged using a function you can see it being called in the code above get_accel_data for example this and the temperature data is obtained using the functions written after the void loop as follows double get_temperature double temp_data 0 contains the temperature intended to be displayed as a decimal int temp_data_l 0 low byte of the form xxxx 0000 where 0s are not populated int temp_data_h 0 high byte containing complete data set msb contains sign info 0 ve i2c_start temp_addrs i2c_write accelerometer device address i2c_write temp_reg setup for a read from device s appropriate register i2c_stop i2c_start temp_addrs i2c_read temp_data_h i2c_readack 4 temp_data_l i2c_readnak 4 i2c_stop temp_data temp_data_h temp_data_l temp_data temp_data 0 0625 return temp_data int get_accel_data int msb int lsb int raw_data msb lsb this is a 16 bit number with last 4 bits redundant the arduino is already performing 2 s complement maths on a 2 byte word so once the data is arranged as a word we can shift if back to 12 bits get rid of the first 4 bits which are always zero raw_data raw_data b1000 return raw_data 2024 counts for 1 g there we have it hopefully i will post up how i get on with the gui and how i get on with it in my car very soon questions and comments welcomed have fun if you try something similar derive engine power using obd data posted 20th february 2011 in cars tags calculate carputer cars force how to obd power torque 0 this is a nice project to try out it is quite simple especially for those of you using an obd 2 compliant petrol car if you have a car computer and want to make a power meter here is how to do it step 1 figure out how much fuel you are burning maf reporting gasoline vehicles using an elm tool or whatever you have to read obd data request maf pid 0x10 this is mass airflow g s now divide this maf value by 14 7 this is the magic stoichiometric ratio that fuel needs to burn in air for the sake of an engine power calculation we don t need to worry if the engine is running rich it is only able to physically burn at this ratio so any additional fuel is not turned into useful work at the crank the result of this equation gives fuel mass g s step 1 figure out how much fuel you are burning non maf reporting gasoline vehicles if you don t have a maf sensor reported on your obd you will have to do a speed density calculation this is less error prone if your car is na non turbo note engine size in cc gas constanst r 0 082057 l atm mol 1 k 1 molar mass of air 28 75 g mol estimate volumetric efficiency this is a definate source of error browse the web for expected ve figures for now assume a vtec engine should be getting around ve 0 9 at 6500 rpm and at idle assume ve 0 4 do interpolation to find ve for all other engine speeds or make a lookup table prepare to request the following pids engine speed pid 0x0c rpm raw 4 map manifold air pressure pid 0x0b kpa raw value mat manifold air temp pid 0x0f degc raw 40 work out airflow with the following formula adding 273 to mat converts to degrees kelvin airflow g s map 100 eng_speed 120 engsize 1000 1 mat 273 1 r 28 75 ve fuel flow g s airflow 14 7 step 2 estimate engine power the net calorific value of petrol in the uk is about 43 mj kg depending on what fuel station you go to the rather poor efficiency of our beloved internal engine is ice_eff 0 33 33 is thermal loss to the cooling system and a further 33 goes to bearing and pumping losses power kw 43 fuel flow g s ice_eff power bhp power kw 1 34 look...