# Which optical sensor for detecting moving paper?

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...

You could use an IR LED and a matching phototransistor. The IR LED would be pulsed/modulated, which both saves power and allows to mitigate the effect of other light sources.

• This seems like a good lead, I looked at this adafruit.com/products/164 but saw 30mA -- which I believe is like what you mention but matched together. But pulsing/modulating it seems the way to go! – tar Mar 27 '15 at 11:14
• This part is actually a much more complicated IR range detector. Could be used in principle, but may be overkill. Just take an IR LED ($0,10) and a TSOPxxx ($0,50), and hook them both up to a microcontroller. Should be fairly easy if you have basic µC knowlege (the TSOP does the complicated part of decoding/filtering the IR signals).The range between sender and receiver you want to cover will probably be no more than a couple of centimeters, so that the LED can be driven with very little current, and even when pulsed with a duty cycle of 10% the total (average) consumption would be pretty low. – JimmyB Mar 27 '15 at 12:09
• Oh, to be clear: I'm not talking about actually detecting motion as you asked for, but about detecting if/when an object is blocking the IR beam between emitter and detector. – JimmyB Mar 27 '15 at 12:15
• Hey Hanno, this sounds good (specially the cost part), but, yes, I can't setup anything on the other side, everything has to be in one small device. Mailboxes are usually from 10cm and up to 70cm high. That's why I was looking at the proximity sensors. – tar Mar 27 '15 at 22:42

In my youth I had built something that depending on your mailbox physical dimensions may or may not work.

Essentially it was a thin metal sheet that was fixed inside the box at one end. It was thick enough to hold itself with a bit bending, but thin enough to be bent even more by the lightest envelopes.

You dropped one envelope in there, it bends down and makes contact, an led was lit up.

No current consumption while no mail inside.

• thanks for the suggestion, but trying to avoid anything mechanical or that could be specific to a mailbox :) – tar Mar 27 '15 at 11:23

This assumes you're putting a micro in and making this wireless, because everyone is making everything wireless right now. I would start with an array of reflectivity sensors like this in the base or side(geometry dependent) of the mailbox. (Sparkfun sells a little module.)

Tie their Vccs together, and pair them with a P channel MOSFET so they aren't drawing current all the time. Then you can turn on and poll them ever couple of minutes. If you're worried about false positives, put a debounce on it, where it checks again in another minute before giving you a positive notification. Bonus points if you pair it with a RTC and never poll them outside of main delivery hours, or on days the mail doesn't run.

• So I looked at reflectivity sensors, but isn't their range very limited? As in a few mm? – tar Mar 27 '15 at 22:43