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A project log for LoFi

LoFi is a very low cost ($5), small, auto-transmitting module. Preassembled and preprogrammed. Simply attach to your appliances or projects!

david-cookDavid Cook 09/17/2014 at 04:390 Comments

A wireless mailbox notifier lets you know when the postal mail has arrived in your mailbox. This can be particularly helpful if your mailbox is not immediately adjacent to your front door.

The mailbox notifiers I've seen usually rely on detecting when the mailbox lid is opened (hall effect sensor or tilt switch) or when a person approaches (motion sensor). Those are fairly successful methods -- but I didn't want to repeat what other people have already done. Also, those methods don't take into account whether mail is waiting or instead the homeowner simply triggered the lid sensor by anxiously checking their empty mailbox.

Instead, I'd like to check for the presence of something in the box. A hacked postal scale would be highly effective, but would be awkward to fit. So, I'm going to use a reflected sensor: emit infrared light and measure the amount that returns. A white envelope or glossy magazine reflects significantly more light than an empty box. And, because LoFi transmits analog values, a threshold doesn't need to be tweaked between 'detect' and 'non-detect' states.

Here's the schematic: (which is attached to LoFi)

The 22 kilohm resistor forms a voltage divider with the phototransistor. A change in light (either due to sunlight entering due to the box being opened or due to reflection off of mail) changes the conductance of the phototransistor, and thus the division of voltage. That voltage is fed into a LoFi input.

Two infrared emitters (LEDs) are placed in series with a current-limiting resistor to provide a light source. The second emitter is powered for 'free', as it simply uses power that otherwise would have been burned off in the resistor. On average, about 20 mA will flow.


Twenty milliamps will drain the battery pack in less than a week! I thought LoFi was supposed to be low power?

LoFi has a pin that provides GND only when the sensors are being read. Thus, when LoFi is sleeping (98% of the time), the emitters are off. Therefore, the AA pack will last years.

For the case, I found some small plastic jars with colorful screw-on lids (Amazon B00523WBK2). This is attached with yellow Sugru.


As you can see below, the infrared reflective sensor detects mail very well. Over time, it will be interesting to see what different bunches of mail look like. “Either we just received a ton of mail, or one very glossy white envelope."

I'm not sure why the detection increases slightly over time when staring at mail. Either the mail is settling or the phototransistor has both a fast and slow component to its detection properties. Perhaps it follows a curve like a capacitor?

I did encounter one issue, which is that 433 MHz transmitter was completely shielded inside the metal box. I managed to snake the antenna out one of the bottom drain holes, after enlarging the hole slightly with a drill. Fair warning, you may need to seek permission from your spouse and homeowners association. ; )