Lab 08: A Digital Watch Face

This watch face has giant numbers, 114 pixels tall, big enough to read from across the room. The date sits underneath, and a ring of 60 tick marks around the edge fills up as the seconds go by.

What You Will Learn

• how every digit from 0 to 9 is made of just seven bars
• how a number can store seven on/off switches
• how to change only the pixels that need to change

Run It

Open 08-digital-watch-face.py in Thonny and click Run. Like the analog face, it sets its own time from the internet first.

Seven Bars Make Every Digit

Look closely at a microwave, an alarm clock, or a calculator. Their numbers are built from seven bars, called segments. The segments have letters, a through g:

     aaa
    f   b
    f   b
     ggg
    e   c
    e   c
     ddd

Light up different segments to make different digits:

Digit	Lit segments
1	b, c
7	a, b, c
4	b, c, f, g
8	all seven

Look at the dim gray shapes behind the white digits in the picture. Those are the unlit segments, painted in a faint "ghost" color, like the segments you can just barely see on a real LCD watch.

Filling in the corners

If you only draw the seven bars, each digit has black gaps at its corners and joints, and the numbers are hard to read. So this face also draws the six little squares where the bars meet. A square lights up whenever any bar touching it is lit, so each digit looks like one solid shape.

A Number Full of Switches

Computers store numbers as bits, 1s and 0s. A 7-bit number has seven places, one for each segment: 1 means lit and 0 means dark. Here's the digit 7, which lights segments a, b, and c:

0b0000111   # 7: a b c

The 0b means "this number is written in binary." Reading from the right, the first three bits are for a, b, and c, and they're 1.

Change Only What Changes

When the time goes from 12:59 to 1:00, which segments need redrawing? Compare the old digit and the new digit, bit by bit. Where they're different, that segment changed. Python has an operator that does exactly this comparison, called XOR and written ^:

changed = old ^ new

The program repaints only the segments in changed. A segment that's lit in both digits isn't touched at all, so there's no flicker. In a normal second, this watch sends only 333 pixels to the display, out of 129,600.

Try This

1. Near the top, change BLINK_COLON = True to False. What's different?
2. Change TWELVE_HOUR = True to False for a 24-hour clock, like the ones used in hospitals and the military.
3. Change GHOST to config.BLACK. The unlit segments disappear. Which way do you like better?
4. What digit is 0b1100110? (Hint: bits are read from the right: a, b, c, d, e, f, g.)

Next: Lab 09: The Weather Clock