How to Make an Ultrasonic Range Finder

Do you need to add a distance sensor to your embedded project ? Build this simple ultrasonic range finder ! This quick & dirty PIC ultrasonic range finder will find a place in numerous projects : presence detector, robotics, car parking, distance measurement...

Instructions

Difficulty: Challenging

Things You’ll Need:

• With a few cheap components and less than 200 bytes of code, this sensor will work from 30 to 200 cm, around 1 cm accuracy, with underflow and overflow indication.

1. Everybody knows the speed of the sound in the dry air is around 340 m/s. Send a short ultrasonic pulse at 40 Khz in the air, and try to listen to the echo. Of course you won't hear anything, but with an ultrasonic sensor the back pulse can be detected. If you know the time of the forth & back travel of the ultrasonic wave, you know the distance, divide the distance by two and you know the range from the ultrasonic sensor to the first obstacle in front of it.
   
   Here we use an ultrasonic piezzo transmitter with its receiver, they are very efficient, easy to find and quite cheap.
   
   First, we have to send the pulse : it is easy to get a 40 Khz pulse from a PIC PWM output. You can drive an ultrasonic transmitter directly from the PIC output, but the sense range will not exceed 50 cm. Using a transistor and a resonator circuit, the ultrasonic transmitter will get around 20 volts and the sense range will be extended up to 200 cm.
2. Second we have to sense the echo : the piezzo receiver can provide a few dozens of millivolt, this will be enough for a PIC ADC with 4 mV resolution without extra hardware.
   
   C1 is a decoupling capacitor
   
   The PWM pulse from the RC2 pin of the PIC drives the T1 transistor base through R1 resistor
   
   A 330 µH inductor is added in parallel to the piezzo ultrasonic transceiver, to form a LC resonnator, the D1 diode protects T1 from reverse voltage.
   
   The ultrasonic receiver is directly connected to the RA1 pin of the PIC (ADC channel number 1), with R3 in parallel as impedance adaptator.
3. Component side
   
   Solder side
   
   Take care to align as best as possibe the transmitter with the receiver

Tips & Warnings

• Here is the mikroC source code : /
• file : sonar.c
• project : Simple UltraSonic Range Finder
• author : Bruno Gavand
• compiler : mikroC V6.2
• date : september 30, 2006
• description :
• This is a basic ultrasonic range finder, from 30 to 200 centimeters
• target device :
• PIC16F877A with 8 Mhz crystal
• or any PIC with at least one ADC and PWM channel
• configuration bits :
• HS clock
• no watchdog
• no power up timer
• no brown out
• LVP disabled
• data EE protect disabled
• ICD disabled
• see more details and schematic on http://www.micro-examples.com/
• / /
• MACRO DEFINITIONS
• / /
• ultra sonic pulse length in microseconds
• / #define PULSELEN 300 /
• circular buffer size for samples averaging
• / #define BUFSIZE 10 /
• LCD PORT
• EasyPic2, EasyPic3 : PORTB
• EaspyPic4 : PORTD
• / #define LCDPORT PORTD #define LCDTRIS TRISD /
• GLOBAL VARIABLES
• / unsigned char outOfRange ; // out of range flag : set when no echo is detected unsigned int buf[BUFSIZE] ; // samples buffer unsigned char idx = 0 ; // index of current sample in buffer /
• INTERRUPT SERVICE ROUTINE
• This ISR handles TIMER1 overflow only
• / void interrupt(void) { if(PIR1.TMR1IF) // timer1 overflow ? { outOfRange = 1 ; // set out of range flag PIR1.TMR1IF = 0 ; // clear interrupt flag } } /
• MAIN LOOP
• / void main() { ADCON1 = 0 ; // enables ADC TRISA = 0xff ; // PORTA as inputs PORTA = 0 ; TRISC = 0 ; // PORTC as outputs PORTC = 0 ; // TIMER1 settings T1CON = 0b00001100 ; // prescaler 1:1, osc. enabled, not sync, internal clk, stopped #ifdef LCDPORT // init LCD Lcd_Init(&LCDPORT) ; // use EP2/3/4 settings Lcd_Cmd(Lcd_CLEAR) ; // clear display Lcd_Cmd(Lcd_CURSOR_OFF) ; // cursor off Lcd_Out(1, 1, "UltraSonicRanger") ; Lcd_Out(2, 5, "cm") ; #endif // init PWM Channel : 40 Khz, 50% duty cycle PWM1_Init(40000) ; PWM1_Change_Duty(128) ; INTCON.GIE = 1 ; // enable global interrupts INTCON.PEIE = 1 ; // enable peripheral interrupts PIE1.TMR1IE = 0 ; // disable timer 1 interrupt PIR1.TMR1IF = 0 ; // clear timer 1 interrupt flag // forever for(;;) { unsigned char i ; // general purpose byte unsigned long cm ; // distance in centimeters unsigned char str[4] ; // string for range display // prepare timer T1CON.TMR1ON = 0 ; // stop timer outOfRange = 0 ; // reset out of range flag TMR1H = 0 ; // clear timer1 TMR1L = 0 ; T1CON.TMR1ON = 1 ; // start timer 1 PIE1.TMR1IE = 1 ; // enable timer 1 interrupts on overflow // send pulse PWM1_Start() ; // enable PWM output : transducer is pulsed at ultrasonic frequency Delay_us(PULSELEN) ; // during PULSELEN microseconds PWM1_Stop() ; // stop PWM Delay_us(PULSELEN
• 2) ; // do nothing for twice the pulse length duration to prevent false start while(Adc_Read(1) < 1) // while no pulse detected (no signal on ADC channel 1) { if(outOfRange) break ; // to late, out of range } T1CON.TMR1ON = 0 ; // stop timer 1 PIE1.TMR1IE = 0 ; // disable timer 1 interrupts on overflow #ifdef LCDPORT if(outOfRange) // is overrange condtion detected ? { Lcd_Out(2, 8, "OverRange") ; // display overrange message } else if(TMR1H < ((PULSELEN
• 6
• Clock_kHz()) / (1000
• 4
• 256))) // is underrange condition detected ? { Lcd_Out(2, 8, "UnderRnge") ; // display underrange message } else // good reading { buf[idx] = TMR1H ; // build a 16 bit value from timer1 buf[idx] <<= 8 ; // MSB buf[idx] += TMR1L ; // LSB // circular buffer idx++ ; // next location if(idx == BUFSIZE) // the end is reached ? { idx = 0 ; // back to start } cm = 0 ; // prepare centimeter averaging for(i = 0 ; i < BUFSIZE ; i++) // for all samples in buffer { cm += buf[i] ; // add to sum } cm /= BUFSIZE ; // average samples /
• cm contains now the number of clock cycles
• from the start of the ultrasonic transmission
• to the first echo detection
• the duration in second is s = cm / (Clock_Khz()
• 1000 / 4)
• if we admit that sound speed in the air is 340 m/s
• the distance in centimeters (forth and back) is d = s
• 340
• 100 / 2
• or d = 340
• 100 / 2
• cm / Clock_khz() / 1000
• 4
• d = 34
• 2 / Clock_Khz()
• / cm
• = 34
• 2 ; // now converts to centimeters cm /= Clock_Khz() ; ByteToStr(cm, str) ; // convert to string Lcd_Out(2, 1, str) ; // print string Lcd_Out(2, 8, " ") ; // clear error message } #endif Delay_ms(10) ; // 10 milliseconds delay before next sample } }
• Of course, this ranger is very basic and have a few drawbacks : A little shock to the piezzo receiver cell could lead to a wrong measurement, Since ultrasonic pulse is not coded, any other ultrasonic source will put the mess : => Unwanted underflow or overflow conditions may happen This is the price of the very simple design of the ranger.