Last winter my father asked me if I could make a mailbox sensor for him. That winter was long and cold with lots of snow and he found it frustrating to make his way to the mailbox only to find it empty and had to make a new trip later in the day.
So this is my Christmas project for my father (hopefully he won’t see this post)
I started out ordering a cheap set of transmitter/receiver http://www.robotshop.ca/on-shine-low-cost-tx-rx-1.html it has a range of 90m but the mailbox is only about 10m from his house so it should be ok. The plan was to only send a high signal when the mailbox lid was opened but it turned out that due to noise I had to send a more reliable signal. So this is what I came up with.
To have a steady pulsetrain to look for I added a 555 timer, two resistors and a capacitor to get a steady square pulse. When the lid is opened the micro switch closes the circuit and it starts transmitting the signal. This way it only draws current when the lid is open so the 9V battery will probably work for years. The circuit board is mounted underneath the mailbox but still exposed to whether so I gave it a coat of black plasti dip to make it more resistant.
The micro switch is mounted at the lid.
Inside the receiver box we have the receiver hooked up to a PICAXE 08M, a piezo buzzer for audio alarm and a LED for visual indication if one misses the alarm. The whole circuit draws less than 3mA when waiting for the signal and a bit less pulsing the LED afterwards.
The PICAXE program is short but functional:
symbol LED = 4
symbol SignalPIN = 1
symbol Periode = 10
symbol Threshold = 8
symbol Frekvens = w0
tune 0, 4,($06,$06,$08)
if Frekvens > Threshold then goto Post
tune 0, 4,($05,$03,$05,$03,$05,$03,$05,$03,$05,$03)
Well, that’s my Christmas present for my dad at least. I admit it’s not a great project and I see that you could order something premade that is probably better for $49,- but what is the fun in that?