Parking Warning System

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This is a device to aid you inparkiar in a garage by providing avisual and audible warning. It iseasily set up by mounting it ontothe wall at the end of the garage.

The device produces a codedinfrared (IR) beam which detectsthe proximity of the vehicle bybouncing IR off it as itapproaches, without beingconfused by other IR sources.When the vehicle is within thepreset range, an audible warningis given and a group of lightemitting diodes (LEDs) are turnedon.

The block diagram in Fig.1shows how the circuit is split upinto separate sections.

INFRARED CODINGA system based on a

continuous IR signal would fail inthis type of application, since thereceiving circuit would be heavily

removed. An HT12D decoderthen decodes the signal to givea steady output.

CODEDTRANSMITTER

Either the HT12A or HT12Btransmitter devices may beused in the coded transmissioncircuit. They work in exactly thesame way except the four dataoutputs of the HT12A areinverted as compared with theHT12B. However, since theseoutputs are not used in thiscircuit, this is of no importance.

Referring to the full circuitdiagram in Fig.2, pins A0 to A7of the transmitter IC2 set thecoded signal for the IRtransmission, which can only beaccepted by a decoder chip(IC3 in Fig.2) with the samesettings. The printed circuitboard is designed so that pinsA0 and A1 are connected to the0V supply line, pins A2 to A7being left unconnected.

Pin 9 of IC2 is connected to0V and pin 18 connected to the

How to avoid having an unwanted rear entrance to your garage!

PARKING WARNING SYSTEMby TOM WEBB

influenced by stray backgroundIR emission from lights etc. Acoded IR signal is better sincethe receiver can be set up toonly accept a specific code.

There are a number ofencoding and decoding ICsavailable, but two from Holtekare used for this circuit. TheHT12B transmitter encodes thesignal and adds a 38kHz carriersignal for greater reliability. Aseparate demodulating sensordetects the coded signal andprovides a clean outputwaveform with the 38kHz carrier

Fig.1. Parking Warning System block diagram.

MONOSTABLEINFRA-REDENCODER &TRANSMITTER

INFRA-REDRECEIVER &DECODER

BUZZERAND/ORL.E.D.s

CAR

Fig.1. Parking Warning System block diagram.

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positive supply, which shouldnot exceed +5V. IC2 pins 11 to14 are not used. The pinslabeled X1 and X2 are theoscillator control pins, andrequire a 455kHz ceramicresonator (component X1) alongwith resistor R6 and twocapacitors, C2 and C3.

The Dout pin provides acoded output superimposed ona carrier signal of 38kHz which,with the aid of Darlingtontransistor amplifier TR2,operates the IR light emittingdiode D1.

Potentiometer VR1 allowsthe transmission power to bevaried. Ballast resistor R8prevents a power supply shortcircuit through D1 and TR2when VR1 is set to minimumresistance.

IC2 pin 10 is connected toground to hold the transmitterperpetually triggered.

INFRAREDRECEIVER

The IR sensor/amplifier/demodulator, IC1, is housed ina package resembling a smallpower transistor. The receiverrejects all IR transmissionsexcept the required 38kHzsignal, and provides a cleanoutput (easily viewed on anoscilloscope). There are threepossible receivers that performthe functions required, but intests the best performer for thiscircuit was the PIC26043S (nota PIC microcontroller!).

When the detector detects asignal having a frequency of38kHz, its output goes high.Transistor TR1 inverts this leveland supplies it to the decoderIC3 at DIN (pin 14).

The code to which IC3responds is set by its pins A0 to

A7. Since pins A0 and A1 ontransmitter IC2 are connected to0V, the same pins on IC3 are alsoconnected to 0V.

Resistor R4 sets theoscillation frequency for IC3 tothe required 150kHz. The value ischosen to suit a power supply ofbetween 45V and 5V. When IC3receives a correctly coded signal,its pin 17 (VT) goes high. Thistriggers the monostable formedby IC5a and IC5b. When the VTpin of IC3 is open circuit (nosignal being received), resistorR7 draws the input pin of themonostable to 0V. Oncetriggered, the monostablesoutput remains high for a periodset by the values of C4 and VR2.

The formula used to calculatethis period is T = 07 x R x C.With VR2 set to 100k, theperiod will be 07 x 01M x 47uF= 329 seconds.

Constructional Project

bc

e

a

a

a

a

a

k

k

k

k

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D3

D4

D5

D1IR

bc

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IC3HT12D

IC5a4001

IC5b4001

IC5c4001

IC5d4001

IC1PIC-26043S

TR1BC184L

TR3BC184L

TR2TIP122 ORTIP121

1

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A0

A1

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VSS

V

VT

OSC1

OSC2

DIN

D11

D10

D9

D8

DD

R247k

R447k

R54k7

R94k7

R310k

R7100k

R610M

R8100

R10330

R1100

VR21M

VR1470

GND

+VE

C1100

C61000

C4472

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6

9 13

5

8 12

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10 11

14

7

N.C. N.C.

WD112V

D21N4001

C5100n

C2100p

C3100p

S1

+12V D.C.

+

0V

POWERINPUT

(SEE TEXT)

IC2HT12B

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DOUT

X1

X2

L/MB

D11

D10

D9

D8

DD

X1455kHZ

OUT

IC47805

INOUT

COM

Fig.2. Complete circuit diagram forthe Parking Warning System. The

transmitter is the lower section(IC2/TR2) of the circuit.

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The output from themonostable, at IC5b pin 4, isfed to transistor TR3 via resistorR9. When the output level ishigh, TR3 is turned on anddrives the warning buzzer WD1and turns on LEDs D3 to D5.Diode D2 prevents back EMF

from the buzzer which mightotherwise damage the circuit.R10 is a ballast resistor to limitthe current through the LEDs.

POWER SUPPLYPower to the circuit is

intended to be from a 12V mainsadapter as the circuit will need tobe left switched on for longperiods of time. A supply of 12Vis required in order to power thebuzzer. The power supply isregulated down to 5V by IC4 tosuit the rest of the circuit.

If a buzzer is not being used,then diode D2 can be omitted anda supply of 5V (or 45V) could beused by inserting a wire link in theplace of regulator IC4 (betweenits In and Out pins). However, inthis case, the value of LEDballast resistor R10 should bereduced to about 180 ohms. Thisalso means that batteries couldbe used as the standby current isless than 10mA.

Capacitors C5 and C6decouple the power fed to IC4.Capacitor C1 and resistor R1smooth out the voltage suppliedto the receiver device, IC1.

CONSTRUCTIONApart from the buzzer and

LEDs, all the components arecontained on a single printedcircuit board (PCB). The topsidecomponent layout and full sizeunderside copper foil master areshown in Fig.3. This board isavailable from the EPE OnlineStore (code 7000258) atwww.epemag.com

Begin construction bysoldering in the resistors and thefour wire links. Ensure the correctorientation in the PCB forcomponents C1, C4, C6, TR1 toTR3, D1 and D2. Capacitors C2,C3 and C5 may be connectedeither way round.

Note that on the IR diode,D1, the long leg is likely to bethe cathode (k), but check thiswith the component supplierscatalog.

Infrared receiver IC1 has adome on its sensitive side,which should face outwardsfrom the PCB. Once solderedin, IC1 should be bent back toso that the dome is facingupwards.

Use IC sockets for IC2, IC3and IC5. Do not insert the dual-in-line (DIL) ICs untilconstruction has beencompleted and fully checked.

CASINGTwo plastic cases will be

needed as the LEDs need theirown separate case in order tobe seen through the rearwindscreen of the car.

The circuit board ismounted in its own case onsmall PCB supports which firmlysecure it in place, see Fig.4.Drill holes in the case to suit thepositions of the IR receiver andIR diode, see photographs. Thehole for the IR receiver shouldnot be too small otherwise therange will be reduced. Ifmaximum range is requiredthen the IR receiver should bepositioned right by the hole.

If you prefer to haveplugged connections for thepower supply input and for theoutput to the LEDs, suitableholes should also be drilled fortheir sockets. You also need ahole for the power on/off switchif you decide to use one,although one was not used onthe prototype.

Additionally, two holes arerequired to allow adjustmentaccess to the two presetpotentiometers, using a smallscrewdriver. All holes should bedrilled accurately to correspond


COMPONENTSResistors

R1, R8 100 ohms (2 off)R2, R4 47k (2 off)R3 10kR5, R9 4k7 (2 off)R6 10MR7 100kR10 330 ohms

See also theSHOP TALK Page!

CapacitorsC1 100u radial electrolytic, 25VC2, C3 100p (2 off)C4 47u radial electrolytic, 25VC5 100n ceramicC6 1000u radial electrolytic, 25V

SemiconductorsD1 IR diodeD2 1N4001 rectifier diodeD3 to D5 red LEDs (3mm or 5mm)TR1, TR3 BC184L npn transistors (2 off)TR2 TOP122 (or TIP121) npn Darlington transistorIC1 PIC26043S IR receiverIC2 HT12B (or HT12A) encoderIC3 HT12D decoderIC4 78L05 +5V 100mA regulatorIC5 4001B quad NOR gate

MiscellaneousS1 s.p.s.t. toggle switch (optional)WD1 buzzer, 12VX1 455kHz resonator

Printed circuit board availablefrom the EPE Online Store, code7000258 (www.epemag.com);plastic case to suit (2 off, see text);14-pin DIL socket; 18-pin DILsocket (2 off); connectors forpower and LED cables (see text);PCB pillars (4 off); connectingwire, solder, etc.

$29Approx. CostGuidance Only

PotentiometersVR1 470 ohm miniature horizontal skeleton presetVR2 1M miniature horizontal skeleton preset

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with their respectivecomponents.

The LEDs mounted in theirseparate case can be connectedto the circuit using singlescreened wire, as shown in


Fig.5.

TESTINGThe first check

is to make sure the voltageregulator IC4 is the correct wayaround. Connect the circuit tothe 12V power supply and thencheck that 5V is present on theoutput pin of IC4. If it is, thendisconnect the power and insert

the remaining chips, correctlyorientated.

Testing of the IR modulespresents a problem as if onedoesnt work then the other willseem not to be working as well.If in doubt use a voltmeter oroscilloscope as follows:

Test the voltage on the VTpin (pin 17) on IC3 of thereceiver module. It should

ak

R9

R3 R

2

R1

R7

R8

R6

C1

D1

C4

C6

TR3

TR2

TR1 IC4

IC1bce

+

+

+

R10

R4

R5

C5

C2

C3

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kX1

D5(k)

D3(a)

WD1

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V++

0V0VVR1

VR2

e

c

b

b

c

e

OUTCOM

IN

+VEGND OUT

258

D2

VIA S1

Fig.3. Printed circuit board topside compo-nent layout and (top right) approximately

full-size underside copper foil master.

D4 D5D3a aa k kk

L.E.D.S CONNECTEDIN SERIES

OPTIONALJACK PLUG

SCREENEDCABLE

Fig.4 (below). Wiring from the circuitboard to the optional LED jacksocket and power connector.

HOLE TO ADJUSTTIME BUZZER ORL.E.D.S ARE ON

INSIDE VIEW

+

+

+

POWER

BUZZER

OPTIONAL JACKSOCKET TO CONNECTTO L.E.D. BOX

OPTIONALPOWERCONNECTOR

HOLE TOADJUSTRANGEOF IR

Fig.5 (above). The LEDs mounted ina separate case and connected viascreened cable and jack plug to theww

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normally be at 0V but change toabout 5V when a signal isreceived. Now check thevoltage on the pins of IC1. Pin 3should be at 5V and pin 2 at 0V.When a signal is not beingreceived, pin 1 (the output pin)should be at just under 4V.When a signal is received thisvoltage should fall by about 1V.

Note that as the signal is

oscillating, a voltmeter providesa rather approximate guide tovoltage. If an oscilloscope isavailable it should be possibleto view the encoded signal, inwhich case the trace will riseand fall between 4V and 0V. Ifthis test fails then try sending asignal from a TV remote controlunit. The signal will not bedecoded, but you will at least

know if the receiver IC isworking, and hence determine ifthe fault lies in the transmitter orreceiver or both.

If the output from IC1 isworking, test the signal at pin 14(Din) of IC3 on the receivermodule. It should be at about0V when no signal is received,rising to about 13V (as seen ona voltmeter) when a signal isreceived. Again, an oscilloscopewill show that the signal actuallypulses to about 5V.

If the VT pin on the receiveris working then simple voltmetertests should establish theposition of any other faults.

If the circuit is triggeredstraight away then IC1 may bereceiving IR straight from the IRdiode D1, through strayreflection inside the case. If thishappens the transmitter shouldbe surrounded by a rolled pieceof black card.

SETTING UPThe presets VR1 and VR2

can be adjusted to suit theusers own particular needs. Thefollowing is a summary of theirfunctions:

VR1: Adjusts the range ofthe IR beam by decreasing orincreasing the power goingthrough IR diode D1. Reducingthe resistance extends therange.

VR2: Sets the time thebuzzer and LEDs stay on bycontrolling how muchrecharging current is input to themonostable. Reducing theresistance reduces the time.

COMMONPROBLEMS

Typical mistakes include dryjoints and bridged pads, i.e.adjacent pads accidentally


Completed circuit board mounted inside its box.

The transmitter/receiver case and the smaller LEDbox with their lids removed. Note the tube of blackcard to stop stray reflections from reaching the IR

receiver chip.

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joined together with solder.Other problems include failureto insert wire links. Also checkthat the components arecorrectly placed, and the correctway round. Note again thatsome IR LEDs are unusual inthat the longer lead denotescathode (k).

IN USE

This Parking WarningSystem should be set up withthe IR sensors lining up with theextremity of the car, e.g.bumper. The LED box shouldbe positioned so as to be seenthrough the rear windscreen.The time the LEDs and buzzerare on, and the range of the IRcan easily be changed using ascrewdriver to adjust the presetsVR1 and VR2.


Completed remote LEDwarning box.

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Parking Warning System

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