Door Open Alarm

CONSTRUCTION
ELECTRONICS FOR YOU JULY 2003
Fig. 5: Magnet and reed
switch fitted in the door
PRADEEP G.
C
ommercially available electronic se-
curity alarms with remote control
are generally very expensive. Here
is a circuit, with construction details, for a
low-cost door-opening alarm with remote
control. However, you need not despair as
here we present a low-cost door opening
alarm with remote control. It uses readily
available components and is easy to as-
S
.C
. D
W
IV
E
D
I
CONSTRUCTION
semble.
The circuit
Fig. 1 shows the block diagram of the door-
opening alarm with IR remote control. The
circuit has two main parts, namely, a small
infrared remote
transmitter unit and
a receiver unit with
alarm.
Tr ansmi tter
unit. Fig. 2 shows
the circuit of the re-
mote transmitter
unit. Its working is
very simple. I C
NE555 (IC1) is used
in astable
multivibator mode
to operate a fre-
quency of 1 kHz. A
pnp transistor (T1) drives the IR LED.
Connect a +9V battery to the cir-
cuit. Now on pressing switch S1, the trans-
mitter emits a modulated infrared beam
up to 7 metres without the need of any
lens or reflector.
DOOR-OPENINGALARMWITH
REMOTE CONTROL
Fig. 2: Circuit of IR remote transmitter unit
Fig. 4: Circuit of IR receiver unit with alarm
Fig. 3: Bottom
view of L14F1
IR photo-
transistor
Receiver unit with alarm. The circuit
diagram of the IR receiver unit with alarm
is shown in Fig. 4. IR signals sent by the
transmitter are received by Darlington IR
phototransistor L14F1 (T2) whose bottom
view is shown in Fig. 3. Thus
phototransistor T2 is used here as a sen-
sor.
As IR signals are very weak, these re-
quire amplification. So the signals are am-
plified by the amplifier stage comprising
transistors T3 and T4. Amplified signals
are fed to the triggering circuit comprising
transistors T5, T6, and T7 to trigger the
monostable multivibrator wired around
IC2.
When
IC2 is trig-
gered at pin
2, its out-
put pin 3
goes high
for 5 to 10
seconds.
Time delay
can be set
by a 220k
p r e s e t
Fig. 1: Block diagram of door-opening alarm using IR remote control
CONSTRUCTION
ELECTRONICS FOR YOU JULY 2003
(VR). During this time, transistor T8 con-
ducts to keep reset pin 4 of astable
multivibrator IC3 low. Then the alarm gets
disabled.
Within the preset time period if some-
body opens the door, i.e. the magnet is
moved away from reed switch S2, pin 4 of
IC3 goes low due to the conduction of
transistor T8 and hence the alarm is not
activated.
After completion of the preset time
period if somebody opens the door, reed
switch S2 also gets opened and pin 4 of
IC3 goes high due to non-conduction of
transistor T8 and hence the alarm is acti-
vated.
The actual use of the remote control
is that you can disable the alarm
while you open the door. You can keep
the remote control in your pocket. When
you enter the room or go out from
the room, simply direct remote control
to the sensing phototransistor and
momentarily press switch S1. Thus the
alarm is disabled for 5 to 10 seconds.
So during this time, you can open
the door without activation of the alarm.
After this time duration completes,
if anyone tries to open the door, the
alarm will sound.
Assembling
The door opening alarm uses a
simple magnet-operated two-leads
reed switch as a sensor. Reed switch
S2 is fitted on the door frame using
an adhesive like Feviquick. The
magnet is fitted on the moving part
of the door as shown in Fig. 5.
When the door is fully closed,
the magnet is close to switch S2 and the
internal leads of the switch get shorted.
When the door is opened, the magnet
moves away from the reed switch and
hence the internal leads of the reed switch
get opened.
Assemble the transmitter unit and the
alarm unit on separate PCBs. The PCB of
Fig. 6: Circuit of power supply with battery backup
Fig. 9: Actual-size, single-side PCB for receiver unit
Fig. 10: Component layout of the PCB in Fig. 9
Fig. 7: Actual-size, single-side PCB of
transmitter unit
Fig. 8: Component layout for the PCB in Fig. 7
PARTS LIST
Semiconductors:
IC1-IC3 - NE555 timer
T1, T9 - SK100 pnp transistor
T2 - IR L14F1 photo Darlington
T3-T4 - BC549C npn transistor
T5 - BC558 pnp transistor
T6-T8 - BC548 npn transistor
T10 - 2N3054 pnp power transistor
IR-LED - LD271 infrared LED
LED - 5mm red LED
D1 - 1N4148 switching diode
D2-D5 - 1N4001 rectifier diode
Resistors (all ¼-watt, ±5% carbon,
unless stated otherwise):
R1 - 150-kilo-ohm
R2 - 1.5-kilo-ohm
R3, R4 - 470-ohm
R5 - 4.7-ohm
R6, R15, R17,
R18, R22, R25 - 10-kilo-ohm
R7, R13 - 4.7-kilo-ohm
R8 - 470-kilo-ohm
R9 - 2.7-kilo-ohm
R10, R21, R23 - 1-kilo-ohm
R11 - 100-kilo-ohm
R12 - 120-ohm
R14 - 22-kilo-ohm
R16 - 3.3-kilo-ohm
R19 - 680-ohm
R20 - 47-kilo-ohm
R24 - 2.2-kilo-ohm
R26 - 100-ohm, 1W
VR -220k preset
Capacitors:
C1, C2, C5, C9,
C13, C16 - 0.01µF ceramic disk
C7, C8, C11 - 0.1µF ceramic disk
C3, C4, C14,
C17 - 100µF, 25V electrolytic
C6 - 47pF ceramic disk
C10 - 2.2µF, 25V electrolytic
C12 - 10µF, 25V electrolytic
C15 - 0.047µF ceramic disk
C18 - 1000µF, 25V electrolytic
Miscellaneous:
X - 230V AC primary to 12V-0-12V,
500mA secondary transformer
S1 - Tactile switch
S2 - Reed switch
LS1 - 8-ohm, 1W speaker
LS2 - 5-ohm, 10W speaker
- Magnet
- IC bases
- +12V battery
- +9V battery
the transmitter should
be small. All compo-
nents, excluding timer
IC 555, can be directly
soldered on the PCB.
Use 8-pin IC bases for
timer IC. Try to keep
the length of the wire
between the I R
phototransistor and the
receiver PCB as small
as possible. Don’t over-
heat the sensor while
soldering. Use a 25W
soldering iron for sol-
dering.
The unit requires
back-up during power
supply failure. There-
CONSTRUCTION
ELECTRONICS FOR YOU JULY 2003
Fig. 11: Power amplifier circuit for loud sound
component layout in Fig. 10.
If you want the alarm to sound
loudly during the unauthorised open-
ing of the gate, use the power ampli-
fier circuit shown in Fig. 11 with
another suitable power supply. This
circuit uses another power supply with
a 230V AC primary to 12V-0-12V, 2A
secondary transformer and two di-
odes of 2A rating (D2 and D3). ❑
fore use a 12V DC power supply with
battery for back-up as shown in Fig. 6.
Connect this power supply to the IR re-
ceiver unit with alarm.
The actual-size, single-side PCB for the
transmitter circuit (Fig. 2) is shown in Fig.
7 and its component layout in Fig. 8. The
actual-size, single-side PCB for the receiver
circuit with alarm (Fig. 4) and power sup-
ply (Fig. 6) is shown in Fig. 9, and its
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