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I want a simple circuit do an alarm light that tell me someone visited me. When someone visits me and in the case no one answer the door, I want to know that.

So I want a press button switch outside the door that will turn on a LED inside the house and will not turn off until I reset it inside the house. How simple that can be done?

Best to make use of my existing rechargeable USB battery that is used for phones.

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  • \$\begingroup\$ Is that your question - "How simple that can be done?" \$\endgroup\$ – HandyHowie Nov 7 '18 at 8:01
  • \$\begingroup\$ if the button also operates the door bell we need to understand your doorbell before we can make a detailed suggestion. \$\endgroup\$ – Jasen Nov 7 '18 at 8:31
  • \$\begingroup\$ no need for a door bell. Just keep light up the LED so I know someone visited me before. \$\endgroup\$ – DriveCarefully Nov 7 '18 at 8:40
  • \$\begingroup\$ want the simplest way. Is this circuit hard to do so? \$\endgroup\$ – DriveCarefully Nov 7 '18 at 8:40
  • \$\begingroup\$ What Jasen is asking, is how do you plan to activate the circuit. Are you connecting it to your door bell? Or will you have a second separate button next to the door bell people will press \$\endgroup\$ – Doodle Nov 7 '18 at 9:53
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What you are looking for is called a latch.

Before you get there through, break down your project into smaller parts.

Start by learning how to turn on an LED using a button to activate a mosfet.

Then add the latch. And finally a battery (don't recommend using rechargeable one for safety).

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    \$\begingroup\$ If you make a latch circuit using something like a 74HC00 you can drive the LED directly as long as you limit the current, no need for a MOSFET/BJT \$\endgroup\$ – Doodle Nov 7 '18 at 8:15
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    \$\begingroup\$ Good point by @Doodle. Also remember to use a current limiting resistor for your LED. \$\endgroup\$ – A.S. Nov 7 '18 at 8:20
  • \$\begingroup\$ Why not rechargeable battery, it now can be used to light up a USB LED after inserting into it. Its output is typically 3.7V \$\endgroup\$ – DriveCarefully Nov 7 '18 at 8:34
  • \$\begingroup\$ So it is just a circuit including the followings:- battery source, IC, limiting resistor, a push button and and a on/off switch(inside house to reset) and a LED. And that is "done"??? \$\endgroup\$ – DriveCarefully Nov 7 '18 at 8:45
  • \$\begingroup\$ @DriveCarefully how are you going to recharge the battery once it's depleted though? Li-ion batteries found inside phones are not a trivial task to charge safely... \$\endgroup\$ – Doodle Nov 7 '18 at 8:55

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