Scale

The Construction of the Scale includes the construction of 4 main parts: 2 boards, the load cell and the case.

Now let's start with the boards. You can find the parts list (including all the necessary parts for the boards), as well as a schematic draw and a circuit layout at the bottom of this article.

The main and the display board are constructed similar to the boards we constructed for other Genesis projects. If this is the first Genesis project you're doing you should start by reading our Construction Tutorial section. Otherwise you can assemble the board the same way as you did for the other projects.

You will have to add a cable to connect the two boards after you finished the construction. The type is specified in the parts list again and you can gather the information how to connect the boards from the picture to the right. This picture also illustrates how the board should look like after the construction process, with all the devices for the display and the board attached.

The "load cell" picture to the right shows the completely assembled load cell. You have to build the load cell as illustrated by the picture. We have simply used a cable crimp and solder, as well as polyesther tape to attach the strain gauge strips and the other parts. Once you're finished you have to connect the load cell connection cable to the appropriate pins on the main board.

Using the Mechanical drawings (Printed Circuit Board and overlay layout) the case can be constructed according to the original design in parallel to the construction of the electronic circuits. The types of materials required to construct the casing are indicated on each of the drawings. However, there are some minor modifications to make to the drill holes for mounting the electronics board and the drill holes on the back plate for access to the power input socket and the stereo jack serial I/O socket. These can be accommodated by measuring the board mounting holes from the board itself and then the other drill holes on the back panel can be done after the board has been constructed.

Put the two boards and the load cell now carefully into the case and attach it with the screws. Tighten them hand-tight.

For the power supply you will require a power adapter pack with 12V AC and 0.5 Amps. Connect it to the power supply plug at the board.

Testing the scale

Finally you should test the scale before using it and wondering of occuring errors. Turn the power on and you're running! (Note: the load cell is a delicate instrument and may require up to 10 minutes to warm up. You can already use the scale before it is warmed up, but you may experience problems with the accuracy and the auto-calibration function).

To test the scale, you will require a multimeter with frequency display or an oscilloscope (20 MHz). Once the scale is built and the load cell installed, you need to check the voltage between TP1 (the test point) and TP2 (ground). With nothing on the scales except the proper load tray (the full zero weight), check the voltage TP1-TP2. It must be between 7V and 8V. If lower or higher, adjust R1. As an example, a value of 9V (too high) may require R1 to be raised to 150 Ohms or even 200 Ohms. Note that the more weight placed on the load cell, the lower the voltage reading will be (this is why it is important to have the proper zero weight on the scale when testing). As an exercise, you might like to find out by how much the voltage reduces with more weight, and therefore the maximum measurable weight. Also check the frequency between TP2-TP3. It should be approximately 5 kHz and no less than 4 kHz. The higher the frequency, the less the maximum measurable weight will be. In testing our prototype, we observed a frequency of 5.6 kHz, which was fine.