A Letta - Lerta is a device that alerts you to when there is mail inside your letterbox. We have a long driveway with a bend in it, so we can't see the letterbox. I decided that rather than walking down there several times on mail day to check the box I needed to come up with an electronic solution, and so the Letta - Lerta project was born.
The obvious (to me) answer was a radio link to signal when there was mail in the box. Low power transmitters and receivers for the 433 mhz band are very cheap and easy to come by, so that took care of the first part.
Now I had to figure out how to know if there was any mail in the box. I had several ideas.
I decided against infrared LEDs because ordinary sunlight contains infrared, and if the sun was on a certain angle it could give a false signal. The ultrasonic idea probably would work but it was a bit bulky. With a microswitch there was always the possibility of the mail not hitting it - even if there was a microswitch on both sides of the opening. A hinged flap would probably annoy the post person if s/he had to lift it each time - especially if there was a lot of mail.
Well, you guessed it - I settled for the laser beam idea. It sounds a bit scary, but the laser I used is very low power so there's no chance of the mail catching fire, or the post person being burnt by it.
In the next section we'll look at how it all comes together.