WiringtheRoadRippersMonsterTruckforexternallytriggered
motorpower
TheRoadRippersMonsterTruckwas$6.99frommylocalWalgreens.Versionssell
onebayfor$14.99.IthastwoAAbatteriessupplying3V,twoLEDheadlights,a3Ω
DCmotor,andan8Ωspeaker.Thewiresareallstranded,andsmallerthan22AWG
–maybe24or28–Istartedstrippingwireswithasharpedgeandthenusedmy
fingernailsafterotherwisepullingtheentireendofthewireoffmultipletimes.
Aftercuttingouttheprintedcircuit
boardinside,thewiringinthemonster
truckIhadwas:
• Red/Red+BlackStripe
=3VDC/Ground
• White/White+BlackStripe
=Motorin/Ground
• White/White+YellowStripe
=Speakerin/Ground
• Red/Black/White
=LEDhigh/LEDlow/LEDlow
Shoppinglist
People(atleastintheU.S.)withtimetoorderpartsinadvancewillsavealotof
moneybuyinginbulkfromDigiKey/Mouser/JamecoinsteadofRadioShack.
×2
Itisprobablybettertobuyacircuitboardtosolderto,andoneofthestandstohold
theparts,soyoudon’tburnyourfingerslikeIdid:
Wiringdetails
Useapairoftransistorstoswitchthemotoron/offusingthemicro::bitboard.The
RadioShack2N4401SwitchingTransistorhasabouta200×currentgainfromthe
basetotheemitter(theRadioShackspecssay100to300times),andamaximum
basecurrentof600mA.ThephotoshowstwotransistorsinaDarlingtonPair–so
thattheemitterofthefirstonefeedsintothebaseofthesecondone–makingthe
currentgainsomethinglike40,000(uptothemaximum600mA).
Thetransistorhasthreeports:aBase(wherethemaincurrentflowsin),aCollector
(wherethesignalcurrentflowsin),andanEmitter(wherethemain+signalcurrent
flowout).Thereissomethinginsidephysicallyblockingcurrentflow(aregionof
extrapositivecharge)thatopenswhenyouflowelectronsinthroughtheCollector,
allowingthemaincurrenttogothrough.
Inputfrommicro::bit=3Volts
2×collector→emitterdiodes(inside)
=2×0.7=1.4Volts
Currentthroughresistor=
(3V-1.4V)/220kΩ=.0073mA
Transistorgain=200(well100-300)
200×200=40,000
40,000×0.0073mA=290mA
Thisisthewholecircuitdiagram.ThedrawingwasmadeinDigiKey’sfreewebbasedSchemeIttool:http://www.digikey.com/schemeit/.
Pagelinkingto2N4401transistordatasheetfromFairchildSemiconductor
AudioAmplifier
ThedatasheetfortheICchipinthissectionlistssupplyvoltagesfrom4-18V.This
probablymeansthe3VfromthetwoAAbatteriesintheRoadRipperMonsterTruck
won’tbeenough.ButIdidn’ttry…
Thereareintegratedcircuit(IC)chipsthatcanamplifyaninputsoundsignal.Thisis
mucheasierthando-it-yourself(althoughifyoucaretotryitthisistheeasiestoneI
foundonline).Don’tsolderdirectlytoanICoryou’llcookit.Insteadusearetention
contactandjustpushthechipinaftereverythingiscool.Youcanmelttheplasticon
theretentioncontacttooifyoutaketoolongsobequickandcareful.Thisislikea
middle-schoolorabovetaskforsure.
There’sasmalldivotinthetopoftheICchipandasimilarsmalldivotinoneendof
theretentioncontacttogettheorientationcorrect.Thechipfitsbothwayssoallyou
havetogobyisthelittledivot.
RadioShackshoppinglist
Wiringdetails
Usetherecommended“minimumwiring”optionfromthechipmanufacturer,Texas
Instruments(actuallyNationalistheonethatmakestheRadioShackLM386chip
butit’sthesamespecifications)
Vsstandsforthesuppyvoltage–whichcanbefrom4Vto18V.Theextrapinsare
forbettercontrolovertheamplificationifyouwanttousethembutthey’refinethe
waytheyare.
YouneedtouseapowerresistorattheoutputWresistor,whichwillreceive
P=V2/R=25Volts2/10Ohms=2.5Watts
Linktothewholedatasheet–wiringexamplesareattheend–andotherdetailsand
ideascomebeforethat…