09-05-2012, 01:30 PM
Dummy Alarm
[attachment=21671]
Dummy Alarm Project
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 ‘superbright’ 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 1kW 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 1kW resistor to 3.3kW. Note that AA cells will last longer than a 9V PP3 battery.
[attachment=21671]
Dummy Alarm Project
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 ‘superbright’ 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 1kW 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 1kW resistor to 3.3kW. Note that AA cells will last longer than a 9V PP3 battery.