This is a very simple electric mailbox signal system for rural mailboxes. It uses a high-intensity LED, a 9 volt battery, a 330 ohm resistor and a homemade switch made from a clothespin.
Step 1: Schematic
The parts list is very simple. A 9-volt battery, a 330 ohm resistor, an ultra bright LED, and a custom-made clothespin switch. Update 6/7/2015. If you are an Arduino hobbyist, you can make a nifty flashing light with an ATTiny85-20PU chip (Cost for the chip is about $1.50 on eBay). You can program it to do any kind of blinking you like--just edit the sample Blink sketch. I made one using Morse code that says: Mail is here". I've attached the schematic for that one above.
Step 2: Mailbox Door and Wire
A hole is drilled into the mailbox door about 1 inch from the bottom and into the side of the door. A wire is threaded into the hole and twisted so that it stays there.
Step 3: Clothespin Switch
A wooden clothespin (plastic would be better for weather resistance) is modified by drilling a small hole through each of the jaws on the closed end. Wires are threaded through the holes and glued into place with hot melt glue (or any good glue). The ends protrude inside and make contact with each other. To make the contact better, I soldered a drop of solder on each wire on each side. When the clothespin is shut, the circuit is complete and the LED turns on. To turn the switch off, I put a non-conducting piece of plastic between the two jaws of the clothespin. The plastic piece is connected to the wire leading from the mailbox door and is snatched away from the clothespin when the mailman opens the mailbox door. The clothespin switch then makes contact and turns on the LED.
Step 4: Open Switch
When the plastic piece is pulled away from the clothespin, the clothespin switch closes the circuit and causes the LED to light.
Step 5: Detail of Off and on Positions
The top picture shows the clothespin in the off position. That is, the non-conducting plastic piece or insert is held by the clothespin and prevents the wires on each side of the clothespin from contacting each other. Hence, the circuit is open and the LED is off. When the mailbox door is opened, the plastic piece is pulled away from the clothespin and the circuit is closed making the LED turn on.
Step 6: Showing LED Off and On
You can see in this picture that when the LED is off, there is no light. The on position shows a very bright light. The LED must be high-intensity, otherwise it would not show up in bright sunlight.
Step 7: Privacy
Although it would be a nice service to the neighborhood to let everyone know when the mailman has passed, I felt it was not a good idea to make it too obvious. I made the LED such that only I can see whether it is on or off. The reason being that I don't want any crooks to know when the mailman has left mail in our boxes. So, to this end, I put the LED inside a tube and pointed it at my front door. I'm the only one who can see if the light is on or not.
Step 8: Conclusion
This has been a very handy gadget for me and required no deep knowledge of switching mechanisms like NOR, NAND, OR gates, etc. It's just a very simple electric circuit and fulfills the function of letting me know that the mailbox door has been opened.