Archive for April, 2010

Speed Trap! A GPS-Based Speeding Alert

Difficulty Level = 9 [What’s this?]

This was a fun project I built using a perf-board Arduino with GPS receiver, some LEDs, some long wire, some clever code, and a car!

I have this friend that has a speeding problem. So I built this device for my friend so he’ll know when he’s speeding. How will he know? Because when the small perf-board Arduino device with a GPS module detects that his vehicle’s speed is over the speed limit, it turns on a police lights display that is mounted on the inside of his car’s rear windshield. When he sees those flashing police lights in the rear-view mirror, he’ll know he needs to slow down!

Police in the rear view mirror!

OK, it’s me, but you already figured that out. If you weren’t smart, you wouldn’t be here. Here’s a rundown of this project’s features:

  • A simple perf-board Arduino circuit with an EM-406a GPS Receiver connected to the microcontroller
  • A pair of CAT-5 cables running from the circuit to the back of my Honda Civic
  • A small perf-board with red and blue LEDs and two white LEDs representing the headlights of a police car. This small board is mounted on the inside of the rear windshield using suction cups.
  • The software running on the microcontroller is programmed to know the speed limit in different locations near where I live and drive. I did this by specifying “speed zones” which are polygons and a speed limit. The polygons are defined as a list of latitude/longitude vertices.
  • Whenever the GPS module reports the current position and speed (every second) the code determines which bounding polygon or “speed zone” the car is located in. If the GPS receiver reports that the current speed is greater than the speed zone’s limit, the police lights are activated. If below the speed limit, the lights will be turned off.

The Hardware

Look at this nice clean circuit…

OMG don’t look at the bottom!

Read more…

Published by Michael, on April 5th, 2010 at 12:37 pm. Filed under: Arduino,Automotive,GPS,Level 9. | 25 Comments |

Tutorial: Reading a 12-Button Keypad

Difficulty Level = 1 [What’s this?]

Most keypads like this are wired so it makes it straightforward to figure out what button is being pressed. With 3 columns and 4 rows of buttons, you only need 7 wires. Typically all the buttons in a column are connected together with the same wire, and all the buttons in a row are connected together with the same wire. To determine which button is pressed, you apply a voltage to the wire attached to a column and then check the wires attached to each row to see if current is flowing through any of them. If so, then the switch for a particular button is closed (button pressed). Then you proceed to the next column and try each row again, etc. Not rocket science — just scanning a bunch of switches to see which one is closed. In fact, there is a keypad library in the Arduino Playground that makes it easy to do this.

Well, the keypad I bought here has 10 wires instead of 7, and it’s wired in a really goofy way. I’m not sure if this is common, but I thought keypads were generally wired as described above. Here’s the schematic that shows how this one is actually wired:

Schematic of my non-standard keypad

Notice that the gray wire is used only for the 9 key. And the orange wire is only used for the * key. And the brown wire is only used for the # key. Why is this built so inefficiently? I have no idea!

Regardless, here is Arduino code that I used to scan this keypad. You can do something similar if you have a keypad that is not wired in a straightforward way.

// Pins
#define BLACK 2
#define WHITE 3
#define GRAY 4
#define PURPLE 5
#define BLUE 6
#define GREEN 7
#define YELLOW 8
#define ORANGE 9
#define RED 10
#define BROWN 11

#define STAR 10
#define POUND 11

void setup() {

  // Rows
  pinMode(BLACK, INPUT);
  digitalWrite(BLACK, HIGH);  // set pull-up resistors for all inputs

  pinMode(WHITE, INPUT);
  digitalWrite(WHITE, HIGH);

  pinMode(GRAY, INPUT);
  digitalWrite(GRAY, HIGH);

  pinMode(PURPLE, INPUT);
  digitalWrite(PURPLE, HIGH);

  pinMode(ORANGE, INPUT);
  digitalWrite(ORANGE, HIGH);

  pinMode(BROWN, INPUT);
  digitalWrite(BROWN, HIGH);

  // Columns
  pinMode(BLUE, OUTPUT);
  digitalWrite(BLUE, HIGH);

  pinMode(GREEN, OUTPUT);
  digitalWrite(GREEN, HIGH);

  pinMode(YELLOW, OUTPUT);
  digitalWrite(YELLOW, HIGH);

  pinMode(RED, OUTPUT);
  digitalWrite(RED, HIGH);


void loop() {
  int key = scanKeypad();

  if (key != -1) {
    if (key == STAR) {
    } else {
      if (key == POUND) {
      } else {

int scanKeypad() {
  int key = -1;

  // Pull the first column low, then check each of the rows to see if a
  // button is pressed.
  digitalWrite(BLUE, LOW);
  if (digitalRead(BLACK) == LOW) {
    key = 1;
  if (digitalRead(WHITE) == LOW) {
    key = 4;
  if (digitalRead(PURPLE) == LOW) {
    key = 7;
  digitalWrite(BLUE, HIGH);

  // Moving on to the second column....
  digitalWrite(GREEN, LOW);
  if (digitalRead(BLACK) == LOW) {
    key = 2;
  if (digitalRead(WHITE) == LOW) {
    key = 5;
  if (digitalRead(PURPLE) == LOW) {
    key = 8;
  digitalWrite(GREEN, HIGH);

  // Third "column".  Note that the 0 key is wired to this column even though
  // the 0 is really in the second column.
  digitalWrite(YELLOW, LOW);
  if (digitalRead(BLACK) == LOW) {
    key = 3;
  if (digitalRead(WHITE) == LOW) {
    key = 6;
  if (digitalRead(GRAY) == LOW) {
    key = 9;
  if (digitalRead(PURPLE) == LOW) {
    key = 0;
  digitalWrite(YELLOW, HIGH);

  // Last "column" is not really it's own column.  Only wired to * and #
  digitalWrite(RED, LOW);
  if (digitalRead(ORANGE) == LOW) {
    key = STAR;
  if (digitalRead(BROWN) == LOW) {
    key = POUND;
  digitalWrite(RED, HIGH);

  return key;

Published by Michael, on April 3rd, 2010 at 1:29 pm. Filed under: Arduino,Level 1. | 3 Comments |