To make the Letta - Lerta I decided to use an Arduino microcontroller. For this project I decided to use a couple of ATTiny 85s. They are basically a computer on a chip. The chip has only 8 pins, so it's very small. At the letterbox end it is hooked up to an HC-12 transmitter which sends a signal to the house when the laser beam in the box is broken.
The circuit board is very simple - apart from the ATTiny and the HC-12 there are only two resistors and a transistor. All the rest is wiring.
The board is contained in a small plastic box which is mounted under the letterbox. There's a small hole in the middle of the underside of the letterbox which the laser pokes through. At the top of the inside of the letterbox is a laser detector.
Every half hour the ATTiny turns on the laser beam for a few milliseconds, and reads the detector. If the detector cannot "see" the laser beam, then there must be mail in the box, so the transmitter is turned on to send "55" to the receiver in the house. If there's no mail it sends "22". (If the batteries go flat it transmits "33"). Then the transmitter switches off and the ATTiny goes into a deep sleep for half an hour and uses extremely low power (0.2 of a microamp).
Meanwhile, in the house the receiver is "listening" all the time, so it's plugged into the mains. When it receives a "55" signal it flashes a multicoloured LED continuously until the reset button is pressed. If there is no mail it flashes red for 30 seconds, and if the transmitter's batteries are flat it alternates red and green flashes.
(If you're interested in the circuit diagrams, go to the next page.)