Select One From Several Inputs

Yesterday, we selected one of many outputs with only 4 pins from an Adafruit Trinket microcontroller.  Here is the mirror project.  Say you have 8 digital data lines in and only few microcontroller lines (Adafruit Trinket only has 5 I/O pins).  You can use a circuit like the one below:

The chip used is the 74XX151 8 input multiplexer.  The binary address of the line you want is sent on Trinket Pins #0, #1, and #2.  Then the data line selected is read in on Trinket Pin #3.  This leaves Pin #4 as an output for serial data or other use.  The code would similar to the sample code below if we only wanted to read the 74151 input pin 3:

void setup() {
   pinMode(0,OUTPUT);
   pinMode(1,OUTPUT);
   pinMode(2,OUTPUT);
   pinMode(3,INPUT);
}

void loop() {
   digitalWrite(0, HIGH);
   digitalWrite(1, HIGH);
   digitalWrite(2, LOW);
   int mydata = digitalRead(3);
}

For a robust program, you'd probably cycle through each 74151 pin address and read the values you want. 

If you only want to multiplex 4 inputs, you can use a smaller multiplexer or just ground S2 low, freeing Trinket Pin #2.

Triggering Ten Pins with Four Outputs

A common issue in microcontrollers: what do you do when the pins on your controller do not match well with the outputs to your controlled items?  This is something I wanted to write about in Getting Started with Adafruit Trinket but I was over on page count as it was, so I'll post it here for you.

What the designer may not normally think about is using TTL logic chips.  Often called 74 series after the original numbering by Texas Instruments, there are hundreds of pre-existing integrated circuits to help make outstanding projects.  A number of years back it was one of the only ways to make circuits.... anyway...

Take Trinket.  There are only 5 I/O pins.  What if you want to use some sensor or serial input to output one of ten outputs (say to the delicious Adafruit Audio FX Sound Board that just came out).  You could be satisfied triggering only 4 of the sound board's trigger pins. But we want more!

So 4 binary digits can actually represent 16 different outputs (0 to 15 or 0 to F in hexadecimal).  Would one create a bunch of 7400 series logic (AND, OR, NAND) gates to change a number to one of several outputs?  You could, but it takes alot of gates;

Gates to decode 4 digital signals to 10 outputs

There is a way to reduce this down to one 16-pin DIP chip - use the 7442 BCD to decimal decoder IC.  All the logic above is already pre-wired into one easy to use chip!  So how would we wire Trinket to use the 7442 to the Adafruit Audio FX Sound Board which wants one of many pins to trigger a file to be played?  See the wiring diagram below:

If you set the right 4 bit number on Trinket pins 0 to 3, it will trigger one of ten outputs to the sound board, triggering the playing of up to ten sounds.  The code to trigger output 5 would include:

void setup() {
   pinMode(0,OUTPUT);
   pinMode(1,OUTPUT);
   pinMode(2,OUTPUT);
   pinMode(3,OUTPUT);
}

void loop() {
   digitalWrite(0, HIGH);
   digitalWrite(1, LOW);
   digitalWrite(2, HIGH);
   digitalWrite(3, LOW);
}

If you hooked a transmitter like an Adafruit Bluefruit or other receiver to Trinket Pin #4, you could use Trinket to set the right combination of 4 states on Pins #0-3 to trigger the right sound output. (Note: if you connect to pins #3 and #4 on Trinket, be sure to disconnect these when you use the USB programming connection, then reconnect when programming is done). And the Sound Board can hold alot of WAV file sound compared to a tiny bit on Trinket itself.

Note the data sheet I link to is the 74HC42.  The letters in the middle (if any) designate different types of logic (some improvements have been made in these chips over the years.  I'm partial to LS logic, although nearly any will do that I'm aware of, just be sure if you use 5 volt logic you are consistent).

You can see the usefulness of older chips in making the most of newer microcontrollers and modules.  With a bit of study of the 74 series chips, I'm sure you can find some that will help you in projects on your drawing board.