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555 Door Ajar Flasher Circuit Schematic

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

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

9 Comments

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  • Michael

    Judging from the number of mistakes by this author, I would NOT try to build this project!

    “About the REED switch…

    As a matter of fact it’s the other way around: NO(normally-open) type is the jelly bean kind, and the NC(normally-closed) is difficult to find.”

    “Repetition rate is about 1.41Hz and the duty cycle is about 1%, NOT 10% !

    C3 (not C2) provides voltage transient protection for the MOSFET gate.

    Peak LED current is 60mA, but average is 0.6mA(not 6mA) because duty cycle is 1%.

    Be aware!”

    Mihai: Thanks for catching this clown’s errors!

  • Joe Vogel

    very nice site. Found what i think i can use.(door ajar light flasher) i’ll be using it on an overhead garage door. This will require the LED or the door sensor to be remote mounted about 15 – 20 feet. I hope this will not cause any problems.
    I haven’t played with these discreet circuits in 30 years. It’s nice to see that there are still people still interested in it and the parts are available.
    Thanks for the kick start,
    Javu

  • Mihai

    Repetition rate is about 1.41Hz and the duty cycle is about 1%, NOT 10% !

    C3 (not C2) provides voltage transient protection for the MOSFET gate.

    Peak LED current is 60mA, but average is 0.6mA(not 6mA) because duty cycle is 1%.

    Be aware!

  • tgor152

    The picture with the parts laid out shows a different schematic, which one do I use?

  • prabhakar kumar

    led driver trasformer winding which core used

  • prabhakar kumar

    how do u know that in dc circuit…used capacitor 450uf 50v and 20 omhs resistance used any calculation formula..please give me solution…i want to made230ac input and output 4odcv ,9vdc,12vdc,11vdc,

  • Mihai

    About the REED switch…

    As a matter of fact it’s the other way around: NO(normally-open) type is the jelly bean kind, and the NC(normally-closed) is difficult to find.

    • Jim Keith

      Woops, you are correct on that one.

      I got the switch description incorrect, but operation correct “When the reed switch is closed (sensing the magnet)…”

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