7/30/2019 Door Alarm
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Door Alarm
Hangs up on the door-handle
Beeps when someone touches the door-handle from outside
Parts:
R1______________1M 1/4W ResistorR2______________3K3 1 or 2W
Resistor (See Notes)R3_____________10K 1/2W Trimmer Cermet (See
Notes)R4_____________33K 1/4W Resistor
R5____________150K 1/4W ResistorR6______________2K2 1/4W
ResistorR7_____________22K 1/4W ResistorR8______________4K7 1/4W
Resistor
C1,C2__________10nF 63V Ceramic or Polyester
CapacitorsC3_____________10pF 63V Ceramic
CapacitorC4,C6_________100nF 63V Ceramic or Polyester
CapacitorsC5______________22 25V Electrolytic
CapacitorC7____________100F 25V Electrolytic Capacitor
D1,D2,D4_____1N4148 75V 150mA DiodesD3_____________5 or 3mm. Red
LED
Q1,Q2,Q3,Q5___BC547 45V 100mA NPN TransistorsQ4____________BC557
45V 100mA PNP Transistor
L1_________________ (See Notes)L2_____________10mH miniature
Inductor
Hook_______________ (See Notes)
BZ1___________Piezo sounder (incorporating 3KHz oscillator)
7/30/2019 Door Alarm
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SW1,SW2________SPST miniature Slider Switches
B1_______________9V PP3 Battery
Clip for PP3 Battery
Device purpose:
This circuit emits a beep and/or illuminates a LED when someone
touches the door-handle fromoutside. The alarm will sound until the
circuit will be switched-off.The entire circuit is enclosed in a
small plastic or wooden box and should be hanged-up to
thedoor-handle by means of a thick wire hook protruding from the
top of the case.
A wide-range sensitivity control allows the use of the Door
Alarm over a wide varietyofdoortypes, handles and locks. The device
had proven reliable even when part of the lock
comes in contact with the wall (bricks, stones, reinforced
concrete), but doesn't work with all-metal doors.The LED is very
helpful at setup.
Circuit operation:
Q1 forms a free-running oscillator: its output bursts drive Q2
into saturation, so Q3 and the LEDare off. When part of a human
body comes in contact with a metal handle electrically connectedto
the wire hook, the body capacitance damps Q1 oscillations, Q2
biasing falls off andthe transistorbecomes non conducting.
Therefore, current can flow into Q3 base and D3illuminates. If SW1
is closed, a self-latching circuit formed by Q4 & Q5 is
triggered and the beeperBZ1 is activated.When the human body part
leaves the handle, the LED switches-off but the beeper continues
to
sound, due to the self-latching behavior of Q4 & Q5. To stop
the beeper action, the entire circuitmust be switched-off opening
SW2.R3 is the sensitivity control, allowing to cope with a wide
variety of door types, handles and locks.
Notes:
L1 is formed winding 20 to 30 turns of 0.4mm. diameter enameled
copper wire on R2body and soldering the coil ends to the resistor
leads. You should fill R2 body completelywith coil winding: the
final turn's number can vary slightly, depending on different 1 or
2Wresistor types actual length (mean dimensions for these
components are 13-18mm.length and 5-6mm. diameter).
The hook is made from non-insulated wire 1 - 2mm. diameter
(brass is well suited). Itslength can vary from about 5 to 10cm.
(not critical).
If the device is moved frequently to different doors, Trimmer R3
can be substituted by acommon linear potentiometer fitted with
outer knob for easy setup.
To setup the device hang-up the hook to the door-handle (with
the door closed), openSW1 and switch-on the circuit. Adjust R3
until the LED illuminates, then turn slowlybackwards the
screwdriver (or the knob) until the LED is completely off. At this
point,touching the door-handle with your hand the LED should
illuminate, going off when thehand is withdrawn. Finally, close SW1
and the beeper will sound when the door-handlewill be touched
again, but won't stop until SW2 is opened.
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