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Using an Arduino and 3 pins to control 5 LED’s

So, I’ve been sitting on two shift-registers I got for my Arduino for a few weeks, without getting around to actually doing anything with them. I finally decided enough was enough, and I cooked up a binary counter which counted to 31. All I used was a shift-register, 3 I/O-pins, and a handfull of 1kΩ resistors. Using the provided example in the Arduino-IDE I gobbled together my own Arduino sketch which first cycles through the LED’s and then counts to 31 over and over. Currently I only have the video, but I’ll add the source code and a diagram later on, when I get home to the computer with the sketch on it.

The shift-register I got for my Arduino

The shift-registers

What is a shift-register?

The advantage to using a shift register compared to not using a shift-register is, that you can “extend” your 10 I/O pins on the Arduino to a practically unlimited number. You will be using only 3 pins on the Arduino (apart from 5V and GND). The three pins are used for a latch-pins, a timer pin and a data-pin. The shift-register takes a serial signal, and sends it to the 8 out-pins on the shift-register, which then outputs them, once the latch-pin goes to ‘HIGH’. If you need to control more than 8 outputs, you connect another shift-register to the overflow pin on the shift-register, and the remaining bits are sent to the extra shift-register and so on.

For example, if I send binary 153 (10011001) to the shift-register, the pins 8, 5,4, and 1, will be set to ‘HIGH’ once the shift-register gets the all-ok signal (the latch-pin going to ‘HIGH’). In my example I only used 5 LED’s, so the highest number I could show, was 11111 (16+8+4+2+1 = 31).

What does my arduino project do?

My counter/gadget starts out by cycling the LEDs a la KITT (of Knight Rider fame). Once that is done, the counter starts counting in binary. Once the highest available number is reached, it resets the counter and starts over.

Basically the sketch is just a bunch of for-loops. At the top of the sketch I define the number of LEDs connected to the shift-register. Throughout the sketch, I refer to that number when setting the range of the loops. The Arduino sends the appropriate number, formatted as binary, to the shift-register. When the number has been received, the latch-pin is set to HIGH and the shift-register sets the pins as instructed.

Where to go from here?

The possibilities of using a shift-register with an Arduino is almost limitless. Just the fact that it frees up I/O pins makes it a great tool for your projects. The next step for me, would be to either a) get it to work with an LCD, or b) get my hands on a shift-out register. That way I can do the same thing, just for inputs. Currently I have an idea of making a intervalometer with a menu-system, which’ll require at least 4 input-pins.

About the author

GregersBoye I'm 30 years old and has a vivid interest in everything electronics and programming - making machines do my bidding is exciting to me. Other than that, I'm a Geek with capital G.

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