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Dummy Alarm
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This Dummy Alarm project
makes an LED flash briefly once every 5 seconds to
imitate the indicator light of a real alarm. The circuit
is designed to use very little current to prolong
battery life so that it can be left on permanently. An
on/off switch is not included, but could be added if you
wish.
The 7555 timer IC used is
a low power version of the standard 555 timer. A 'super
bright' red LED is used because this provides a bright
flash with a low current. The LED is off for most of the
time so the average total current for the circuit is
less than 0.2mA. With this very low current a set of 3
alkaline AA cells should last for several months, maybe
as long as a year.
The circuit will work
with a standard 555 timer IC (such as the popular NE555)
but this will increase the average current to about 2mA
and the battery life will be much shorter. You can use a
greater supply voltage (15V maximum) for this circuit
but the 1k resistor for the LED should be increased to
keep the LED current low at about 3mA. For example to
use a 9V PP3 battery change the 1k resistor to 3k3. Note
that AA cells will last longer than a 9V PP3 battery. |
Parts Required
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resistors: 1k, 10k,
680k
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capacitor: 10µF
radial
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LED: red super
bright, 5mm diameter
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7555 low power timer
IC
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8-pin DIL socket for
IC
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battery clip
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4.5V battery box for
3 AA cells
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strip board: 8 rows
× 16 holes |
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| Strip board Layout
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Circuit
diagram
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