LcdClockEnglish

project overview
File:Lcdclock.jpg
Year	2020
Number of pixels	29
Grayscales / Colors	2
Illuminant	LCD
Power input	4.5 V, 100 µA
Contact person	Arne Rossius
Project website

The CMOS LCD Clock is a simple 24-hour clock made from 4000-series CMOS logic integrated circuits. The circuit was designed to require as few ICs as possible without making the clock hard to use. It displays the time (hours and minutes) on a large 7-segment liquid crystal display, provides two buttons to set the clock, and runs on battery power (3x AAA). A standard watch crystal is used as a timebase.

Hardware

The clock consists of 4 functional units:

• Quartz Oscillator
• Pre-Divider
• Minute and Hour Counters
• Display Drivers and LCD

Quartz Oscillator

The quartz oscillator is using a 4060 which provides most of the oscillator circuitry and a 14-stage binary divider. With a standard 32768 Hz watch crystal, this results in an output frequency of 2 Hz. Outputs from some of the other stages of the divider are available and are used for the LCD (see below). The frequency of the oscillator can be fine-tuned by adjusting the load capacitance on the crystal. To make the clock as accurate as possible, a trimmer capacitor is used to allow easy adjustment of the load capacitance, requiring only a screwdriver. It is important that the metal part of the trimmer which touches the screwdriver is connected to ground, otherwise a metal screwdriver will change the load capacitance and the final oscillator frequency can only be read after the screwdriver is removed again.

Pre-Divider

The pre-divider receives the 2 Hz signal from the 4060 and divides it down to 1/60 Hz, i.e. one pulse per minute. For this, the rather unusual 40102 IC is used. It contains a pre-settable 2-digit BCD down-counter. The actual count isn't available, but the CO/ZO (count out/zero out) pin goes low when the count is zero. As the counter is using BCD (binary coded decimal), the maximum preset value is usually 99, so a division by 120 (from 2 Hz to 1/60 Hz) seems impossible. However, loading a digit with a value higher than 9 appears to work fine, so loading the counter with the binary value 0xB9 results in a counter value of 119. After loading, 119 clock pulses are needed until the counter reaches zero. The ZO signal is fed back to the SPE input (synchronous programming enable), so at the next (120th) clock pulse, rather than rolling over to 99, the counter is loaded with the externally set value of 119 instead. This means the CO output is low for 0.5 s (one 2 Hz period) every minute, and can thus be used as an input to the minute counter.