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Making a mailbox with a built-in "scale" to let you know when the mail arrives.

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If your mailbox is outdoors, you will not know if your mail has been delivered. Often, we don't even notice for a while that the mail we've been waiting for has arrived in our mailboxes.

So I wanted to make a device that would notify me when a mail arrived in my mailbox.

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Know the mail by weight

I know there are several ways to know that mail has arrived at your mailbox, but this time I'm going to use a scale to detect the arrival of mail by the change in weight.

I've made a bed exit sensor before that uses a "scale" to detect whether a person is in bed or not.

I'm going to make a similar system using a distortion gauge, HX711, and ESP32.

Materials for scales

材料

I'm going to make the "scale" part first.

I bought the acrylic board from KOMERI because it was cheaper than buying it at the other home improvement store.

A strain gauge (load cell) weighing 500g was purchased from AKIZUKIDENSHI so that it would respond to even the lightest of objects.

The HX711 module, which digitizes the weak signals of strain gauges and allows easy acquisition of the values from a microcontroller, is available on Amazon.

Processing acrylic

Cutting

To process acrylic with a laser cutter, I design the outline of the "scale" in Illustrator. The external dimensions match the internal dimensions of my mailbox. It is quite horizontally long.

A hole for a strain gauge is provided at the right end.

Save the file in SVG format and load it in SmartDIYsCreator. As for the cutting parameters, the cutting material is acrylic board with a thickness of 2mm.

Power: 100%
Speed: 1200mm/min

Cut with a CO2 laser cutter Etcher Laser Pro.

It only takes a few minutes to complete the cut. CO2 laser cutter is very useful. I'm really glad I bought it.

Use countersunk screws for the screw holes of the strain gage. So I used a chamfering drill, which can chamfer the hole into the shape of a countersunk screw.

I learned for the first time that there is a drill bit that can be used to cut out the shape of the head of a countersunk screw. I usually used a slightly thicker drill bit to cut them. This chamfering drill has a 45-degree angle, so you can shape the hole to fit the head of the countersunk screw perfectly.

Assembly

Now that all the parts are in place, it's time to assemble. The holes were drilled with a laser cutter, so they fit perfectly with the position of the holes in the strain gauge.

Screw each of them together to complete the "scale". Hmm? But something is wrong.

The weight of the top panel has caused the acrylic to flex, and the top panel on the opposite side of the strain gauge is in contact with the bottom panel. If the top and bottom boards are touching, the strain gauge will not distort when mail is placed on the top board, so the weight cannot be measured.

Improvement

What the hell. Then why not place a strain gauge in the center? This way, the top and bottom panels do not come in contact with each other.

"Scale" part completed

There is a problem that the weight can be negative, but for now, the "weighing" mechanism is complete. Next, I'd like to connect the HX711 module and use ESP32 to measure the weight.

Added on March 20, 2021 Click here to continue.

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