I am building a mailbox notifier and looking for sensor and thresholding circuit suggestions to detect when mail is dropped in, here are my requirements:
- very low power (when no motion at least) sub 1mA at 3.3/5V
- output LOW when no motion
- output HIGH during motion
I have started testing with an off the shelf PIR sensor board (https://www.adafruit.com/products/189), but there are a few issues, mostly the PIR sensor does't always trigger when an envelope is dropped it, even with the sensitivity at maximum. I believe this is due to the PIR sensor not working well with paper temperature.
I am considering the following options:
- photo diode/transistor -- because luminosity changes when the lid/door is opened
- photoresistor -- maybe too slow to respond?
- integrated active proximity/IR sensor (http://www.adafruit.com/datasheets/vcnl4000.pdf), but will it detect a thin piece of paper?
- What other options should I consider?
The problem with sensing luminosity is mailboxes have thin openings and if it's on a door, and the door is opened or lights are switched on/off outside, it may get confused.
For the threshold circuit, I need to basically only trigger when mail falls in/move, and optionally the door is opened/closed (luminosity goes up/down at once), and go back to LOW as soon as things are still.
The ideal would be detecting when paper is moving, but I am looking for ideas to do this on a low power circuit...