My most recent project is an electronic to/from card for presents. A small device you can attach to presents showing "To" and "From" on a backlit 16 x 2 LCD display.
In order to save battery power, an accelerometer is used to detect motion and the LCD backlight is turned off after a couple of seconds with no motion and the MCU is put to sleep ("deep power-down", actually). When motion is detected, an interrupt is signalled by the accelerometer which wakes up the MCU which in turn turns on the LCD backlight.
The text shown on the LCD can be programmed using a USB connection and a Mac OS X application. The texts are stored in EEPROM.
There are two main design goals for this project:
- Make it cheap. These devices are meant to be attached to presents and then they are gone. Unless the recipient wants to use them, of course.
- Low power. The device should be able to run for a couple of days off two series-connected CR2032 batteries.
The low power consumption appears to be fairly easy to achieve: the LCD with backlight off, the accelerometer and the MCU in power-down mode all use very, very little power.
The low price, however, is more tricky. We'll see how it goes…
These are the main parts:
- MCU: LPC11U24 Arm Cortex M0
- Accelerometer: Freescale MMA8452Q I2C 3-axis accelerometer
- Display: Midas MCCOG21605C6W-BNMLWI 16 x 2 white-on-blue I2C display
The PCBs are designed in EAGLE and I will get them made at ITead. The top side will be hot-plate reflow soldered (mainly due to the 0.5 mm pitch QFN accelerometer) and the bottom side will then be hand-soldered.
Here's a rendering of the PCB courtesy of Mayhew Labs: