Strobe Clock is a pocket clock which displays the time when moved back-and-forth using only 5 LEDs aligned in a vertical column. It is an interesting twist on the everyday clock or watch.
Strobe Clock works on the strobe effect principle. By turning the LEDs on or off at the proper positions, any number of characters can be displayed. Using only 5 LEDs it achieves a vertical resolution of 5 and an unlimited horizontal resolution.
The main objectives are: small size, ease of use, low cost, low power consumption, and low voltage (2 AA batteries) operation. The P87LPC762 microcontroller from Philips was chosen, among other things, for the low cost, low power consumption, and low voltage operation.
Figure 1 shows the block diagram of the device. The main components are: processor, LEDs, acceleration switch, input buttons, and real time clock (RTC). The current prototype uses a mercury switch as the acceleration switch; many other options, such as acceleration ICs are available. The input buttons are used to set the time. They are placed on a remote connected to the main unit through a stereo jack for easy setting of the clock while it is moving. The PCF8583 RTC from Philips was chosen because of the standard I2C interface and low voltage operation.
The firmware is organized in a very simple fashion. Only a main program and a timer interrupt are used. A timer interrupt is set up to occur at a time base of 0.1ms. All timing is referred to this time base.
The main program constantly monitors the acceleration switch and looks for the reference point which synchronizes the screen for a steady display. Once it finds the reference point, it creates and synchronizes the screen, and once again starts looking for the reference point. Meanwhile, the interrupt displays the current screen and debounces the pushbuttons.
The screen columns are output at a rate determined by the time between two reference positions. Thus, if the user moves the Strobe Clock faster, the column output rate will be increased so that the display fills the whole range of motion. As a direct consequence, if the user covers a wider range of motion, the screen will stretch to fill that range.
The design goals have been met by this design. The design is small, works with two AA batteries, and uses little power.
If this project should become a real product however, several steps must be taken. First of all, the mercury switch should be replaced with a safer, but still low cost, acceleration switch. If an analog accelerometer IC is used, one can take advantage of the comparator inputs in the LPC family in order to set the acceleration thresholds.
Functionally, the project may be improved by adding code for date display, alarm, and automatic wake up. These functions could be added with little or no hardware modifications, as the LPC microprocessor and RTC already support this functionality.
Figure 1 Block diagram