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Clojure on the BeagleBone part 2 - Blinking an LED with Clojure

25 May 2014


This is inspired by an example from the excellent book Getting Started With BeagleBone by Matt Richardson.


Schematic above was generated using the open source Fritzing.

Note: The GPIO pins can only handle 3.3 volts, so be very careful that you do not accidentally connect a jumper to one of the 5 volt source pins. If you are unsure of what you are doing I would highly recommend reading the system reference manual to make sure you do not damage your board.

Wire up the LED

  1. Using a jumper wire connect Pin 2 on Header P9 (ground) on the BeagleBone to the negative rail on the breadboard.
  2. Place an LED in the breadboard.
  3. Using a jumper wire connect the cathode (shorter) wire of the LED to the negative rail.
  4. Connect one end of a 100 ohm resistor to the anode (longer) wire of the LED.
  5. Using another jumper wire connect the other end of the resistor to Pin 13 on Header P8

Clojure code

SSH into your BeagleBone and create a file named project.clj with the following contents:

(defproject clj-blink-led "0.1.0"
  :description "Blink a LED on a BeagleBone"
  :license {:name "Eclipse Public License"
            :url "http://www.eclipse.org/legal/epl-v10.html"}
  :dependencies [[org.clojure/clojure "1.6.0"]]
  :main clj-blink-led.core)

And create a file name src/clj_blink_led/core.clj with these contents:

(ns clj-blink-led.core
  (:require [clojure.java.io :as io]))

(defn write-to-filesystem
  "Helper function for setting pins"
  [location val]
  (with-open [f (io/output-stream location)]
    (.write f (.getBytes val))))

(defn cleanup
  "Tidy up when a user presses Ctrl-C to exit"
  (println "Shutting down...")
  (write-to-filesystem "/sys/class/gpio/unexport" "23"))

(defn setup
  "Export pin and set the direction"
  (.addShutdownHook (Runtime/getRuntime) (Thread. cleanup))
  (write-to-filesystem "/sys/class/gpio/export" "23")
  (write-to-filesystem "/sys/class/gpio/gpio23/direction" "out"))

; Pin states
(def HIGH "1")
(def LOW "0")

(defn -main []
  (println "Blinking LED, press Ctrl-C to exit")
  (loop [pin-state HIGH]
    (write-to-filesystem "/sys/class/gpio/gpio23/value" pin-state)
    (Thread/sleep 1000)
    (recur (if (= pin-state HIGH) LOW HIGH))))

Now run it on your BeagleBone as the root user.

$ lein run

Your LED should now blink on and off every second.

In the next part of this series we will see how to blink a LED using node.js and use ClojureScript instead of Clojure.

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