Swap  The guard on a four-digit display

Band 7's core build puts the parking guard on an LCD. On the bare sixteen-pin kind that is twelve wires and a contrast knob, and it is the longest single build in the course.

This is the same device on a four-digit display. Two Arduino pins instead of six, and four wires to the screen instead of twelve. Twelve wires on the whole board rather than twenty-two. And digits bright enough to read from across a room, which is a thing an LCD genuinely cannot do.

If the twelve-wire screen defeated you, come here instead. Be honest with yourself about the trade, though. You do not get the padding bug, you do not get the contrast knob, you do not get the experience of finding one wrong wire among twelve, and you do not get the sixteen-character writing exercise, which is the best design problem in the band and cannot be done on four digits. Those are real losses. The Look closer section turns two of them into something you can write up, and the Done list below makes you do the writing exercise on paper anyway.

Band 7 Part 7 spends a long time on two faults: a screen that flickers, and a screen that leaves old digits behind when the number gets shorter. Going from 100 to 99 showed 990, and the fix was to pad every write to a fixed width.

Neither problem exists on this display.

void showDistance(long cm) {
  if (cm == lastShown) {
    return;
  }
  lastShown = cm;
  screen.showNumberDec(cm, false);
}

No padding. No spaces. No counting to sixteen. showNumberDec draws all four digits every time, so there is nothing left over to leave behind.

The colon, and the two numbers that are easy to confuse

The two dots in the middle live on the second digit, and there are two ways to switch them on. They use different numbers, and swapping them is the usual way people lose an evening.

0x80 is the colon bit inside a digit. 0x40 is what showNumberDecEx wants you to hand it, and the library turns it into 0x80 on your behalf. Two numbers for one job, because two different people wrote the two functions. This is normal, and reading the library's own header file is how you settle arguments like this.

This sketch leaves the colon off. Turn it on with showNumberDecEx(cm, 0x40) if you want to see it. Blinking it once a second is what makes a four-digit display look like a clock rather than a number, and it is a few lines with a millis check if you ever build one.

Bronze

Show ---- instead of 999 when nothing is in range. Use setSegments, which takes four bytes, one for each digit:

uint8_t dashes[] = {0x40, 0x40, 0x40, 0x40};
screen.setSegments(dashes);

In a setSegments byte, 0x40 is the middle bar of a digit. That is the same number that means “colon” when you hand it to showNumberDecEx, and it is exactly the confusion the table above is about. Then say in your log, in one sentence, why dashes are better than 999 for the person using the thing.

Silver

Use the four digits to show two numbers at once: the current distance on the left pair and the closest you have got on the right pair, with the colon lit to separate them. You have four digits and two things to say, so both have to fit in two digits each, and deciding what to do about 100 or more is the interesting part.

Gold

Make the brightness follow the room, using the light sensor from Band 4: dim at night, bright in daylight. Then measure your own thresholds properly, as Band 4 taught you, and write them in your log. A display that is blinding at night is a display people unplug.

This Swap replaces the LCD guard. To claim Band 7 you still need:

• A video of somebody using it, and a close photo of the display with a real reading on it
• Your ruler-accuracy table from Part 3, all five rows, plus your notes on the three awkward surfaces
• Your beep timing table from Part 9b. Part 9b walks through the LCD sketch, but its gapFor function is character for character the same as the one in your sketch, so do the table on your own file and write in your log that you did
• Your four sixteen-character messages, written out and counted on paper, even though this display cannot show them. This is the band's best design exercise and you do not get to skip it for having a shorter screen
• The two distances you decided you would trust, and why
• Your build log
• Five or six correct answers in Part 11, the Check yourself questions, including question 2

Part 11's questions are set on an LCD sketch you have not built, and one of them is about the padding problem this display does not have. Answer them anyway, and use the comparison above to do it. Knowing why a bug cannot happen on your hardware is a better answer than never having met it.