With my years of travel to developing countries, especially Africa, I often wished I had a bathroom scale to monitor my weight. Way too bulky and heavy. I have searched and searched and there appears to be nothing on the market. Plenty of scales for weighing suitcases and fish.
Even at the age of 84, I am able to balance on one foot. So, I decided to build the “Pocket Bathroom Scale.” It is the size of an iPhone but a little thicker. It weighs only 10 ounces and is driven by two CR2032 batteries. It is capable of weighing up to 400 pounds and measures to .5 pounds.
The protype was made of Lucite. However, if manufactured the case should be mold injected with a window. Four screws would allow for the changing of the batteries every five years. All the electronic parts including the batteries are mounted on a small circuit board.
The weight sensors are four strain gauges in Wheatstone configuration. These are placed on the four corners of the case. A recessed switch activates the power to the microprocessor where the Firmware takes over. The power to the Wheatstone bridge is closely controlled and a 24-bit a/d chip monitors its output. Using I2C it transfers the data to a microprocessor capable of driving a three segment LCD for display. A series of algorithms determine if the LCD flashes indicating to add .5 lbs.
Once the switch is placed, time is allowed to place the scale on the floor. The scale then performs a tare weight and then sounds an audio annunciator to stand on the scale. After 5 seconds the audio indicates to get off the scale and to read your weight. Ten seconds later it turns its self-off.
The board has programming pins and can be programmed to measure in any weight scale e.g., pounds, kilograms, stones etc. A scaling factor is just added to the weighing algorithm. Cost for the electronics for a single unit was less than $10.50 including sensors.
Manufacturing is straight forward. A computer is needed to calibrate the scale and a simple lookup table is used to calculate the scaling factor.
Although it would increase cost, BlueTooth can be added to the board and it will communicate with an iPhone or similar device. The weight could be added to iPhone “Health application” or a new application can be developed graphing the results.
A prototype is available for examination along with schematic, firmware, manufacturing protocols, BOM and Eagle files for board production are available upon request.
This product can increase the quality of life while traveling.