The following code shows a method of reading a class of sonar rangers. These rangers requires a trigger pulse. Shortly after receiving a trigger they transmit a noise pulse and set the echo line high. When the echo is received the echo line is set low.

### SETUP

The ranger used is a SRF05 (check the pinouts, there are many variants).
The fritzing diagram shows the back of the ranger, i.e. pin 1 is the rightmost.
Pin 1 is 5V.
Pin 2 is the trigger line.
Pin 3 is the echo line.
Pin 4 is out (unused).
Pin 5 is ground.

### CODE

`#include <stdio.h>
#include <pigpio.h>
/*
P1 Name gpio used for
2 5V --- 5V
6 GND --- Ground
24 CE0 8 Sonar echo
26 CE1 7 Sonar trigger
*/
#define SONAR_TRIGGER 7
#define SONAR_ECHO 8
/* forward prototypes */
void sonarTrigger(void);
void sonarEcho(int gpio, int level, uint32_t tick);
int main(int argc, char *argv[])
{
if (gpioInitialise()<0) return 1;
gpioSetMode(SONAR_TRIGGER, PI_OUTPUT);
gpioWrite (SONAR_TRIGGER, PI_OFF);
gpioSetMode(SONAR_ECHO, PI_INPUT);
/* update sonar 20 times a second, timer #0 */
gpioSetTimerFunc(0, 50, sonarTrigger); /* every 50ms */
/* monitor sonar echos */
gpioSetAlertFunc(SONAR_ECHO, sonarEcho);
while (1) sleep(1);
gpioTerminate();
return 0;
}
void sonarTrigger(void)
{
/* trigger a sonar reading */
gpioWrite(SONAR_TRIGGER, PI_ON);
gpioDelay(10); /* 10us trigger pulse */
gpioWrite(SONAR_TRIGGER, PI_OFF);
}
void sonarEcho(int gpio, int level, uint32_t tick)
{
static uint32_t startTick, firstTick=0;
int diffTick;
if (!firstTick) firstTick = tick;
if (level == PI_ON)
{
startTick = tick;
}
else if (level == PI_OFF)
{
diffTick = tick - startTick;
printf("%u %u\ ", tick-firstTick, diffTick);
}
}
`

### BUILD

`cc -o sonar sonar.c -lpigpio -lrt -lpthread
`

### RUN

`sudo ./sonar >sonar.dat &`
While the program is running you can capture the waveform using the notification feature built in to pigpio. Issue the following commands on the Pi.
`pigs no
pig2vcd </dev/pigpio0 >sonar.vcd &
pigs nb 0 0x180 # set bits for gpios 7 and 8
`

Move an object in front of the sonar ranger for a few seconds.

`pigs nc 0`

The file sonar.vcd will contain the captured waveform, which can be viewed using GTKWave.

Overview

Reading circa every 10ms

One reading, circa 400us

another

The file sonar.dat will contain pairs of timestamps and echo length (in us). The following script will convert the timestamps into seconds.

`awk '{print $1/1000000, $2}' sonar.dat >sonar-secs.dat`

Gnuplot is a useful tool to graph data.

plot 'sonar-secs.dat' title 'Sonar'

plot [10:25] 'sonar-secs.dat' title 'Sonar'