A special feature of the LED_Watch is its quasi-analog always-on display. This consists of 240 concentrically arranged SMD LEDs (Stanley VCDG1104P-5C63C). Two LEDs light up on the inner two rings in combination with two LEDs on the outer two rings as a "minute hand". On the outer two rings, two further LEDs light up as the "hour hand" and two LEDs flash for 20 ms as the "second hand".

 

The current through the LEDs, and therefore the brightness, is regulated by the centrally positioned LDR to ensure good readability even in bright sunlight.

 

The watch has a tilt switch which, when the watch is placed horizontally, reduces the voltage after 12 minutes at the latest by shutting down the LT3459 boost converter to such an extent that the LEDs no longer light up in order to save energy. The watch is powered by a Li-Ion button cell with 3.7 V / 74 mAh, which only needs to be charged after approx. 20 days with a daily wearing time of 10 hours. This is achieved by an average current consumption of approx. 0.15 mA.

 

More details on the circuit can be found under the menu item "Circuit/PCB".

 

The Attiny1614 microcontroller is generally in deep sleep and is woken up every second for approx. 1.4 ms by an interrupt from the real-time clock DS3231 in order to control the LEDs via the 10 shift-and-store registers HEF4094. After 20 ms, it is briefly woken up again by a timer interrupt to switch off the second LEDs. The Attiny is operated at 10 MHz so that it still functions at 3 V.

 

The watch is set by touch capacitively instead mechanical switch, to make the watch water resistant.