My last visit to the dentist convinced me that I should really brush my teeth at least two minutes. I decided to build a special toothbrush timer: it would detect when a brush is taken out, measure two minutes and notify when the time is up.

I chose to use Atmega328p, a weight sensor, brass cups, a tray, old alarm clock's chime and some plywood. I would like to tell how this device was made: what electrical components were used and how I assembled the enclosure.

Here is a video with how it all works togather:

An algorithm is quite basic:

Wake up
Read current weight with the precision of 3 grams
If current weight is less then the previous one play following sequence:
1. Turn on all lamps
2. Turn of a lamp every 20 seconds
3. Chime every 30 seconds
Sleep for 8 seconds

Step 1: Circuit Design

The circuit is designed around a wight sensor. I got one from a used digital scales. It is a pretty basic device: there are four wires: one is input current, one is ground and voltage difference between the other two wires is determined by a weight of an object on the scales. The difference, however, is really small, so one needs to use an amplifier to be able to tell that the weight has changed.

Here are components that I chose:

ATMega328p with 16Mhz crystal – I chose this micro-controller because it is extremely easy to use. There is also a good community support, as ATMega328p is used in one of Arduinos.

Ina12P amplifier – this device is created specifically for amplification of sensors' signals. It is not hard to use it. The only thing that I found tricky is figuring out the right resistance value for amplification gain.

TPS2020 switch – this is a great switch with a very low standby current ( <10 μA). I use it to control power of the servo that chimes.

MAX1555 charger – this device controls charging of device's lithium-ion battery.

2N3904 transistors – used to control lamps' power.

Lamps – I chose incandescent lamps out of esthetic reasons. Since I wanted to give the project a "vintage" look LED lamps did not quite fit the design. Although these lamps consume more power the feeling they evoke is quite worth it.

This device is able to work 3 weeks on a single charge.

Full schematic could be found on Github.

Step 2: Printing the Circuit

I used photo-resistive method to etch the circuit.

Step 3: Programming

There program is pretty strait-forward. Source code could be found on GitHub.

041.TIF
Download

Step 4: Enclosure Design

The design and assembly of project's enclosure was almost as exciting as building and programming its circuit. I started with a new document in Inkscape and a new sheet of plywood I got from a hardware store. I have rented a laser cutter in Techshop to cut the plywood according to my design. The ease with which the cutter created my parts is quite fascinating:

I used glue to put everything together. All of the edges were reenforced with triangles cut from the same plywood.

I have applied two layers of Minwax wood finish and a layer of their Polyurethane to protect the surface.

I have applied some hot glue to all of the edges: this sealed the enclosure and made it somewhat water-proof. Importance of this step can not be underestimated, as the time would reside in a damp place.

Step 5: Putting Everything Together

Once the enclosure was ready I have put the circuit inside and connected all wires. Photographs attached to this step show how it all fits inside.

I would be happy to answer any questions and hear any comments about this project.

I also hope this device will inspire you to create some creasy contraptions of your own!