555 Door Ajar Flasher

A magnet is placed on the door and a magnetic reed switch on the door casing so when the door is closed the circuit is disabled. When the door is open, the LED flashes VERY BRIGHTLY—when closed, the circuit is disabled in such a way that it draws virtually no current for maximum battery life.

Door Ajar LED Light Schematic
555 Door Ajar Flasher Circuit Schematic

Bill of material

555 door ajar BOM

How it works
The flasher is the standard 555 oscillator circuit that drives an ultrabright white LED for daylight visibility. Repetition rate is about 2hZ and the duty cycle is about 10%. Peak LED current is 60mA, but average is 6mA.
Since the door is supposed to remain closed most of the time, the circuit power is controlled by a 2N7000 N Channel MOSFET. When the reed switch is closed (sensing the magnet), it shorts out the gate drive signal to the transistor and turns off the oscillator. At this time the battery drain is only 9uA.
Note that there seems to be no convenient way to turn off the 555 in such a way that it does not draw much current. Grounding pin 4 stops oscillation, but turns on the LED continuously.
C2 provides voltage transient protection for the MOSFET gate.
LED brightness is controlled by R5—YIKES! do not look into the LED like I did…
The reed switch
This circuit uses the commonly available NC (normally closed) reed switch. A NO variety would be easier to use, but such is rare and expensive. If the circuit is packaged inside a small plastic enclosure, the reed switch could be cemented into one edge of the box thus protecting the glass tube from damage. The reed switch I used for the test is packaged in a plastic tube. It is quite sensitive — can sense the magnet at 2 to 3cm.
Door Ajar Flasher Photos

For the future
555 Strobe Circuit

555 datasheet

Comments

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