SRF05

Arduino library for the SRF05 distance sensor and compatibles.

Description

The library allows to adjust to the speed of sound. Reasons to use a different value are temperature, humidity, type of gas, composition etc.

Default value for the speed of sound is set to 343 m/s. (~20°C)

Several correction formulas for the speed of sound are available on the internet.

// temperature in °C
v = 331.3 + 0.606 * temperature (m/s)
v = 331.4 + 0.606 * temperature + 0.0124 * rel_humidity (m/s)
v = 20.05 * sqrt(273.16 + temperature) (m/s)

In fact humidity has an effect which increases with temperature so the formula is more complex See - https://forum.arduino.cc/t/ultrasonic-sensor-to-determine-water-level/64890/12

temp	10%	20%	30%	40%	50%	60%	70%	80%	90%	notes
-30	313.5	329.5	312.9	329.6	313.0	329.6	313.1	329.7	329.7	extrapolated indication
-25	316.5	332.6	316.0	332.7	316.1	332.7	316.2	332.8	332.9	extrapolated indication
-20	319.5	335.6	319.1	335.7	319.2	335.9	319.3	336.0	336.0	extrapolated indication
-15	322.5	330.5	322.2	330.6	322.3	330.6	322.4	330.7	330.7	extrapolated indication
-10	325.5	333.6	325.3	333.7	325.4	333.7	325.5	333.8	333.9	extrapolated indication
-5	328.5	336.6	328.4	336.7	328.5	336.9	328.6	337.0	337.0	extrapolated indication
0	331.5	331.5	331.5	331.6	331.6	331.6	331.7	331.7	331.7
5	334.5	334.6	334.6	334.7	334.7	334.7	334.8	334.8	334.9
10	337.5	337.6	337.7	337.7	337.8	337.9	337.9	338.0	338.0
15	340.5	340.6	340.7	340.8	340.9	341.0	341.1	341.2	341.2
20	343.5	343.6	343.7	343.9	344.0	344.1	344.2	344.4	344.5
25	346.4	346.6	346.8	347.0	347.1	347.3	347.5	347.6	347.8
30	349.4	349.6	349.9	350.1	350.3	350.5	350.8	351.0	351.2
35	352.4	352.6	353.0	353.2	353.5	353.7	354.1	354.4	354.6	extrapolated indication
40	355.4	355.6	356.1	356.3	356.7	356.9	357.4	357.8	358.0	extrapolated indication
45	358.4	358.6	359.2	359.4	359.9	360.1	360.7	361.2	361.4	extrapolated indication
50	361.4	361.6	362.3	362.5	363.1	363.3	364.0	364.6	364.8	extrapolated indication
55	364.4	364.6	365.4	365.6	366.3	366.5	367.3	368.0	368.2	extrapolated indication
60	367.4	367.6	368.5	368.7	369.5	369.7	370.6	371.4	371.6	extrapolated indication

(table based upon https://www.engineeringtoolbox.com/air-speed-sound-d_603.html)

The library has several ways to improve the quality of the measurements. By taking the average or the median of multiple readings.

The library allows to set a correction factor to compensate for the timing of the pulseIn() function. This has in the end the same effect as changing the speed of sound however it is technically more correct to split the two.

Interface

SRF05(const uint8_t trigger, const uint8_t echo, const uint8_t out = 0) constructor to set the trigger and echo pin. It is not clear yet what the purpose of the OUT pin is.

Configuration

void setSpeedOfSound(float sos) adjust the speed of sound.
float getSpeedOfSound() return set value.
void setCorrectionFactor(float cf = 1) adjust the timing by a few percentage e.g. to adjust clocks. Should not be used to correct the speed of sound :)
float getCorrectionFactor() returns the current correction factor.

normally a single read should be OK.

void setModeSingle() read single time.
void setModeAverage(uint8_t count) read cnt times and take the average.
void setModeMedian(uint8_t count) read cnt times and take the median. cnt = 3..15
void setModeRunningAverage(float alpha) use a running average algorithm with a weight (depends on your application).
uint8_t getOperationalMode() returns the operational mode 0..3.

mode	meaning
0	single
1	average
2	median
3	running average
other	error

Get distance

uint32_t getTime() returns distance in microseconds.
uint32_t getMillimeter() returns distance in millimeter.
float getCentimeter() returns distance in centimeter.
float getMeter() returns distance in meter.
float getInch() returns distance in inches. (1 inch = 2.54 cm).
float getFeet() returns distance in feet. (1 feet = 12 inch).

Experimental - calibration

Experimental, put the sensor at exactly 1.00 meter from a wall, and based upon the timing it will give an estimate for the speed of sound. Current version seems accurate within 3 - 5 %.

float determineSpeedOfSound()

Need to compensate for temperature and humidity.

Experimental - configuration

Performance

Assumes default speed of sound of 340 m/sec. Distance is in meters and equals total distance (forth and back).

Distance	time (us)
1	2.94
2	5.88
5	14.71
10	29.41
20	58.82
50	147.06
100	294.12
200	588.24
300	882.35
400	1176.47

Operational

See examples..

Future

print feet as 3'2" or 3-7/8 feet (is that needed in this lib)
enumerate mode operandi.
investigate purpose of the OUT pin.
example sketch with a DHT22 and the formula for SOS
fix magic conversion numbers.
add float calcSOS(float temp, float humidity = 0);
add float lastValue() ? not all paths update this variable.
add float delta() difference with last value. how to handle different units? or only time?
set default SOS to an SOS from the table iso 340.
add setTriggerLength()

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