Blinking on and off with the 74LS42

I want to make an edge-lit numeric display. These were a common technology before numeric LEDs were available. They use 10 illuminated slides to display individual numbers.

The 74LS42 logic chip (4-Line BCD to 10-Line Decimal Decoder) seems a likely candidate to drive such a display. You feed it a 4-bit binary-coded decimal input, and the chip activates one of ten outputs. Here’s what I got working as a demo driven by an Arduino:

(Video link for the iframe-averse:

From the video you can see that:

  1. The 7442 only passes inputs from 0 (0000b) to 9 (1001b). All other inputs result in no output.
  2. The outputs are really more like 1–10 than 0–9, as a zero input activates the first output.

Making a clean breadboard layout for this circuit was a little more work than I’d anticipated. It just fits on a half-sized breadboard:

Simple 7442 demo circuit, showing BCD inputs (green) and decimal outputs (orange)
Simple 7442 demo circuit, showing BCD inputs (green) and decimal outputs (orange)

Because the 7442 will only activate one output at a time, it’s okay to use a single current-limiting resistor for all ten output LEDs. The chip also uses active low outputs: the outputs go from high to low when activated. The negative side of each LED goes to an output pin, and the chip sinks current when an output is selected, lighting the LED.

The components get in the way of seeing the wiring, so here’s another picture from Fritzing with just the wires and the breadboard:

7442-no_componentsApart from the 74LS42 chip itself, the components I used were 10× 3mm orange LEDs on the outputs and 4× 3mm green LEDs on the inputs. I don’t have a spec for them (they were from a bargain selection box from so my use of ridiculously precise 332 Ω 1% resistors to limit current is a little unnecessary. (I have a bunch of these precision resistors from a project that didn’t go ahead, so using them is cheaper for me than digging about for a 5% one.)

Finally, here’s the Arduino sketch I wrote to drive the chip for the demo video. All it does is cycle through digital outputs 4–7, incrementing a bit every half second.

   SeventyfourFortytwo - Arduino demo of a
   74LS42 - 4-line BCD to 10-line decimal decoder
   Steps through 0-15, two steps per second
   Shows the 74LS42's blocking of values 10-15 as invalid

   Arduino  74LS42
   ======== =======
    4      - 15  Input A (bit 0)
    5      - 14  Input B (bit 1)
    6      - 13  Input C (bit 2)
    7      - 12  Input D (bit 3)

   scruss - 2016-09-23

int n = 0;

void setup() {
  pinMode(4, OUTPUT);
  pinMode(5, OUTPUT);
  pinMode(6, OUTPUT);
  pinMode(7, OUTPUT);
  digitalWrite(4, LOW);
  digitalWrite(5, LOW);
  digitalWrite(6, LOW);
  digitalWrite(7, LOW);

void loop() {
  digitalWrite(4, n & 1);
  digitalWrite(5, n & 2);
  digitalWrite(6, n & 4);
  digitalWrite(7, n & 8);
  if (n > 15) n = 0;

ESP8266 BASIC is seriously neat!

screenshot-from-2016-09-19-22-27-57That picture might not look much, but it’s doing something rather wonderful. It’s a tiny ESP8266 BASIC script running on a super-cheap ESP8266 wifi module. The code draws a clock that’s synced to an NTP server. ESP8266 BASIC graphic commands are built from SVG, so anything you can draw on the screen can also be saved as a vector graphic:

The runtime includes a simple textarea editor that saves code to the board’s flash:

screenshot-from-2016-09-19-22-28-53(and yes, that first line is all you need to set up NTP sync)

Among other features, ESP8266 BASIC has a simple but useful variable display:

screenshot-from-2016-09-19-22-30-17I’d picked up a (possible knock-off of a) WeMos D1 ESP8266 board in Arduino form factor a few months ago. The Software now supports ESP8266 directly, so it’s much easier to program. Flashing the BASIC code to the board was very simple, as I’d noticed that the Arduino IDE printed all of its commands to the console. All I needed to do was download an ESP8266 BASIC Binary, and then run a modified Arduino upload line from the terminal:

~/.arduino15/packages/esp8266/tools/esptool/0.4.9/esptool -vv -cd nodemcu -cb 921600 -cp /dev/ttyUSB2 -ca 0x00000 -cf ESP8266Basic.cpp.bin

ESP8266 BASIC starts in wireless access point mode, so you’ll have to connect to the network it provides initially. Under Settings you can enter your normal network details, and it will join your wifi network on next reboot. I just hope it doesn’t wander around my network looking for things to steal …

micro, a nice little text editor


micro – – is a terminal-based text editor. Unlike vi, emacs and nano, it has sensible default command keys: Ctrl+S saves, Ctrl+Q quits, Ctrl+X/C/V cuts/copies/pastes, etc. micro also supports full mouse control (even over ssh), Unicode and colour syntax highlighting.

micro is written in Go – – so is very easy to install:

go get -u

If you don’t already have Go installed, it’s pretty simple, even on a Raspberry Pi:

If your running under Linux, you probably want to have xclip installed for better cut/paste support.

Overall, I really like micro. It’s much easier to use than any of the standard Linux text editors. It uses key commands that most people expect. Creating code should not be made harder than it needs to be.

(I was about to suggest FTE, as it appears to be the closest thing to the old MS-DOS 6 editor that I’ve seen under Linux. While it’s a great plain text editor, its Unicode support isn’t where it needs to be in 2016.
micro suggestion came via MetaFilter’s Ctrl + Q to quit. Need I say more?

When you see what Japanese glaze artists had for source material

When you see what Japanese glaze artists had for source material

When you see what Japanese glaze artists had for source material

Instagram filter used: Normal

Photo taken at: Japanese Garden, Portland, Or

View in Instagram ⇒

I can’t see Toronto Hydro going for this in their enclosed space working guidelines

I can’t see Toronto Hydro going for this in their enclosed space working guidelines

I can't see Toronto Hydro going for this in their enclosed space working guidelines

Instagram filter used: Lo-fi

View in Instagram ⇒