HCSR04UltrasonicSensor

ElijahJ.Morgan
Nov.162014

ThepurposeofthisfileistoexplainhowtheHCSR04works.Itwillgiveabrief
explanationofhowultrasonicsensorsworkingeneral.Itwillalsoexplainhowtowire
thesensoruptoamicrocontrollerandhowtotake/interpretreadings.Itwillalsodiscuss
somesourcesoferrorsandbadreadings.

1. HowUltrasonicSensorsWork
2. HCSR04Specifications
3. Timingchart,PinexplanationsandTaking
DistanceMeasurements
4. WiringHCSR04withamicrocontroller
5. ErrorsandBadReadings

1.HowUltrasonicSensorsWork
Ultrasonicsensorsusesoundtodeterminethedistancebetweenthesensorandthe
closestobjectinitspath.Howdoultrasonicsensorsdothis?Ultrasonicsensorsare
essentiallysoundsensors,buttheyoperateatafrequencyabovehumanhearing.

Thesensorsendsoutasoundwaveataspecificfrequency.Itthenlistensforthatspecific
soundwavetobounceoffofanobjectandcomeback(Figure1).Thesensorkeepstrack
ofthetimebetweensendingthesoundwaveandthesoundwavereturning.Ifyouknow
howfastsomethingisgoingandhowlongitistravelingyoucanfindthedistance
traveledwithequation1.

Equation1. d = v t

Thespeedofsoundcanbecalculatedbasedontheavarietyofatmospheric
conditions,includingtemperature,humidityandpressure.Actuallycalculatingthe
distancewillbeshownlateroninthisdocument.
Itshouldbenotedthatultrasonicsensorshaveaconeofdetection,theangleof
thisconevarieswithdistance,Figure2showthisrelation.Theabilityofasensorto
detectanobjectalsodependsontheobjectsorientationtothesensor.Ifanobjectdoesnt
presentaflatsurfacetothesensorthenitispossiblethesoundwavewillbounceoffthe
objectinawaythatitdoesnotreturntothesensor.

2.HCSR04Specifications
ThesensorchosenfortheFirefightingDroneProjectwastheHCSR04.This
sectioncontainsthespecificationsandwhytheyareimportanttothesensormodule.The
sensormodulesrequirementsareasfollows.
Cost
Weight
Communityofhobbyistsandsupport
Accuracyofobjectdetection
Probabilityofworkinginasmokyenvironment
Easeofuse
TheHCSR04Specificationsarelistedbelow.Thesespecificationsarefromthe
CytronTechnologiesHCSR04UsersManual(source1).

PowerSupply:+5VDC
QuiescentCurrent:<2mA
Workingcurrent:15mA
EffectualAngle:<15
RangingDistance:2400cm
Resolution:0.3cm
MeasuringAngle:30
TriggerInputPulsewidth:10uS
Dimension:45mmx20mmx15mm
Weight:approx.10g

TheHCSR04sbestsellingpointisitspriceitcanbepurchasedataround$2per
unit.

3.TimingChartandPinExplanations
TheHCSR04hasfourpins,VCC,GND,TRIGandECHOthesepinsallhave
differentfunctions.TheVCCandGNDpinsarethesimplesttheypowertheHCSR04.
Thesepinsneedtobeattachedtoa+5voltsourceandgroundrespectively.Thereisa
singlecontrolpin:theTRIGpin.TheTRIGpinisresponsibleforsendingtheultrasonic
burst.ThispinshouldbesettoHIGHfor10s,atwhichpointtheHCSR04willsend
outaneightcyclesonicburstat40kHZ.AfterasonicbursthasbeensenttheECHOpin
willgoHIGH.TheECHOpinisthedatapinitisusedintakingdistance
measurements.AfteranultrasonicburstissentthepinwillgoHIGH,itwillstayhigh
untilanultrasonicburstisdetectedback,atwhichpointitwillgoLOW.
TakingDistanceMeasurements
TheHCSR04canbetriggeredtosendoutanultrasonicburstbysettingtheTRIG
pintoHIGH.OncetheburstissenttheECHOpinwillautomaticallygoHIGH.Thispin
willremainHIGHuntilthethebursthitsthesensoragain.Youcancalculatethedistance
totheobjectbykeepingtrackofhowlongtheECHOpinstaysHIGH.ThetimeECHO
staysHIGHisthetimetheburstspenttraveling.Usingthismeasurementinequation1
alongwiththespeedofsoundwillyieldthedistancetravelled.Asummaryofthisis
listedbelow,alongwithavisualrepresentationinFigure2.

1. SetTRIGtoHIGH
2. SetatimerwhenECHOgoestoHIGH
3. KeepthetimerrunninguntilECHOgoestoLOW
4. Savethattime
5. Useequation1todeterminethedistancetravelled

Source2
 Tointerpretthetimereadingintoadistanceyouneedtochangeequation1.The
clockonthedeviceyouareusingwillprobablycountinmicrosecondsorsmaller.Touse
equation1thespeedofsoundneedstodetermined,whichis343meterspersecondat
standardtemperatureandpressure.Toconvertthisintomoreusefulformuseequation2
tochangefrommeterspersecondtomicrosecondspercentimeter.Thenequation3can
beusedtoeasilycomputethedistanceincentimeters.

Speed
Equation2. Distance = 170.15m Meters 1e6S
100cm 170.15m
58.772S
cm

Equation3. Distance = time s
58 = s/cm = cm

4.WiringtheHCSR04toaMicrocontroller
Thissectiononlycoversthehardwareside.Forinformationonhowtointegrate
thesoftwareside,lookatoneofthelinksbeloworlookintothespecificmicrocontroller
youareusing.

TheHCSR04has4pins:VCC,GND,TRIGandECHO.
1. VCCisa5vpowersupply.Thisshouldcomefromthemicrocontroller
2. GNDisagroundpin.Attachtogroundonthemicrocontroller.
3. TRIGshouldbeattachedtoaGPIOpinthatcanbesettoHIGH
4. ECHOisalittlemoredifficult.TheHCSR04outputs5v,whichcoulddestroy
manymicrocontrollerGPIOpins(themaximumallowedvoltagevaries).Inorder
tostepdownthevoltageuseasingleresistororavoltagedividercircuit.Once
againthisdependsonthespecificmicrocontrolleryouareusing,youwillneedto
findoutitsGPIOmaximumvoltageandmakesureyouarebelowthat.

5.ErrorsandBadReadings
Ultrasonicsensorsaregreatsensorstheyworkwellformanyapplications
whereothertypesofsensorsfallshort.Unfortunately,theydohaveweaknesses.These
weaknessescanbemitigatedandworkedaround,butfirsttheymustbeunderstood.The
firstweaknessisthattheyusesound.Thereisalimittohowfastultrasonicsensorscan
getdistancemeasurements.Thelongerthedistance,theslowertheyareatreportingthe
distance.Thesecondweaknesscomesfromthewaysoundbouncesoffofobjects.In
enclosedspacesitispossible,ifnotprobablethattherewillbeunintendedechos.The
echoscanveryeasilycausefalseshortreadings.InFigure2apulsewassentout.It
bouncedoffofobject1andreturnedtothesensor.Thedistancewasrecordedandthena
newpulsewassent.Therewasanotherobjectfartheraway,sothatwhenthenewpulse
reachesobject1,thefirstsignalwillreachthesensor.Thiswillcausethesensortothink
thatthereisanobjectcloserthanisactuallytrue.Theoldpulseissmallerthanthenew
pulsebecauseithasgrownweaker.Thelongerthepulseexiststheweakeritgrowsuntil
itisnegligible.Ifmultiplesensorsarebeingused,thenumberofechoswillincrease
alongwiththenumberoferrors.Therearetwomainwaystoreducethenumberoferrors.
Thefirstistoprovideshieldingaroundthesensor.Thispreventsechoscominginfrom
angleoutsidewhatthesensorshouldactuallypickup.Thesecondistoreducethe
frequencyatwhichpulsesaresentout.Thisgivesmoretimefortheechostodissipate.

 WorksCited

Source1.
HCSR04User's_Manual.docs.google.CytronTechnologies,May2013Web.5Dec.
2009.
<https://docs.google.com/document/d/1YyZnNhMYy7rwhAgyL_pfa39RsBx2qR4vP8s
aG73rE/edit>

Source2.
Attiny2313Ultrasonicdistance(HRSR04)example.CircuitDB.n.a.7Sept.2014
Web.5Dec.2014.<http://www.circuitdb.com/?p=1162>

Links

Thesearenotformattedyouwillneedtocopyandpastethemintoyourwebbrowser.

WanttolearnaboutUltrasonicSensorsingeneral?
http://www.sensorsmag.com/sensors/acousticultrasound/choosingultrasonicsensorprox
imityordistancemeasurement825

AllabouttheHCSR04

http://www.circuitdb.com/?p=1162
http://www.micropik.com/PDF/HCSR04.pdf
http://randomnerdtutorials.com/completeguideforultrasonicsensorhcsr04/
http://www.ezdenki.com/ultrasonic.php
(^fantastictutorial,explainsalotofstuff)
http://www.elecrow.com/hcsr04ultrasonicrangingsensorp316.html
(^thisonehassomecoolcharts)