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Smoke, alcohol or LPG alarm
This circuit raises an alarm if it
detects smoke or LPG cooking gas
leakage, or even alcohol vapours.
This is achieved by using the same
circuit with one of several sensors
designed to detect smoke, LPG or
alcohol.
So different alarms can be made
by simply changing the sensor. For a
smoke alarm, use the MQ2 sensor; to
detect alcohol, use the MQ3; or use
the MQ6 sensor for LPG.
The MQx sensors each have six
pins. The heater filament, between
pins H-H, is powered from the 5V
rail. Two pairs of A-B pins connect
across the sensing element; it doesn’t
matter which pair you use.
Half of the LM358 dual op amp
the lug using a screw in a hole already
drilled for the purpose. The wiring
is hidden under foam acoustic pads
inside the ear cups.
The cable connecting to the phone
has two wires, pink and white, plus
a screen braid, with a three-way jack
plug on one end. Do not mix up the
microphone’s white return wire with
the white wire in the cable back to the
phone. If necessary, put a label on the
microphone's white wire, change its
colour with a marking pen, or solder
it to the common lug first.
Circuit
Ideas
Wanted
98
Silicon Chip
(IC1) is wired as a comparator. A reference voltage set using potentiometer VR1 is applied to the inverting
input (pin 2) while the sensor voltage goes to the non-inverting input
(pin 3). The sensor produces a current that is converted to a voltage by
the 10kW resistor between the B pin
and ground.
Output pin 1 of the op amp swings
high whenever the sensor voltage
goes above the reference voltage. A
small amount of hysteresis is provided via positive feedback using
a 10MW resistor so that the output
doesn’t vacillate when the sensor
voltage hovers around the trigger
threshold.
When pin 1 of IC1 goes high,
The centre (ring) contact on the jack
plug connects to one side of the string
of two induction coils and the 150W
resistor. The opposite end of that string
goes to the common solder lug. The
third and longest contact (sleeve) at
the base of the jack plug is returned via
the screening braid, which you should
also connect to the common lug.
When finished, check for around
157W between the jack plug ring
and sleeve contacts. Depending on
the probe polarity, there should be
around 1.2-2.5kW between the tip
transistor Q1 releases the reset signal on 555 timer IC2, which is configured as an astable multivibrator,
so it starts oscillating. The frequency
depends on the value of the capacitor connected to pin 6 and the two
resistors connected to pin 7. The
resulting square wave is AC-coupled
to a small 8W speaker to produce the
alarm tone.
After switching on the 5V supply,
wait about ten seconds for the filament to heat up. Then adjust VR1
until the alarm just stops sounding.
Enclose the PCB in a suitable box
with vents so that fumes can circulate near the sensor.
Raj K. Gorkhali,
Hetadu, Nepal. ($75)
and sleeve. If all indications are OK,
plug your induction headset into a
suitable phone and press talk to hear
a dial tone.
Using my Silicon Chip Hearing
Loop Tester/Level Meter (November
& December 2010; siliconchip.au/
Series/15), I found that the dial tone
was smack bang on 0dB or 0.1A/m.
The phone also has a volume control
that can reduce the volume below
that level.
Anthony Leo,
Cecil Park, NSW. ($90)
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