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Dummy Alarm with Flashing LED

2014-12-11 05:32  
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Figure 1 

ALEDflashing briefly once every 5 seconds to imitate the indicator of a real alarm is the function of this dummy alarm project with the use of a 7555 timer IC.

To prolong battery life, the circuit was designed to use very little current in order to be left ON permanently. Optionally, an ON/OFF switch may be included if desired. The circuit uses a low power version of the standard 555 timer, the 7555 timer IC. A bright flash with a low current is provided by a super bright redLED. The average total current for the circuit is less than 0.2mA since theLEDisOFFfor most of the time. A set of 3 alkaline AA cells should last for several months or a year with this very low current.

A standard 555 timer IC can also be used in the circuit which will increase the average current to about 2mA while the battery life will be much shorter. If a greater power supply voltage at 15V maximum is used in this alarm, the 1K resistor for theLEDshould be increased to keep theLEDcurrent low around 3mA.

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