Category Archives: Technology

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Stepping up from traffic lights last time, I decided this time to have a crack at making a Pelican crossing.

A Pelican crossing is a pedestrian crossing consisting of two sets of traffic lights, a button, and a signal to indicate that it is safe for pedestrians to cross. The sequence that the lights follow is slightly more complex than before:

  • Initially, the traffic signal lights are green and the pedestrian “red man” is on red.
  • When the button is pressed, the traffic signal lights cycle to red.
  • Once the traffic signals are red, the pedestrian signal is set to green and a buzzer sounds for a period of time.
  • When the buzzer has finished beeping, the traffic signals are set on flashing amber and the pedestrian green signal also flashes.
  • After a little while, the traffic signal is reset to green and the pedestrian signal to red.

The Circuit

The circuit for this is slightly more complex.

The traffic signals are now linked, so each red, amber, and green light can be linked in parallel. We introduce two new lights for the pedestrian signal, together with a buzzer.

experiment 2 - traffic lights crossing

I know there are two red/green men in real life, but this is a slight simplification. Just connect red/green leds in parallel for the second set, as we did for the main linked traffic signals.

The Software

The code is fairly similar to before, we extend the class to control three additional outputs – the red and green man and a buzzer, as well as to listen to a given input button press.

The main loop waits for a button press and when detected it toggles the lights, sounds the buzzer, then toggles the lights back.

Here it is in action…


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experiment 2 - traffic lights In the last experiment I wired up LEDs to each one of the PiFace’s outputs and cycled through them. In this one, I decided to try a slightly more “real world” application, and build a set of traffic lights.

Since I live in the UK, these are the UK three light traffic lights (red/amber/green) and follow the UK light sequence – RED, RED/AMBER, GREEN, GREEN, AMBER, RED.

The Circuit

The circuit here is very similar to the one used for the previous experiment, but will only use six of the available eight control outputs. Each light contains a set of three LEDs connected to one of the pins of the PiFace’s output interface, one red, one amber and one green.

See the attached diagram.

The Software

The code for this experiment is where the extra complexity resides, since we must drive each light in the correct sequence and obeying certain rules:

  • The lights must transition in the correct, UK, sequence, i.e. RED, RED/AMBER, GREEN, and then GREEN, AMBER, RED.
  • For traffic safety, the green light must transition to red before the red light transitions to green.

To avoid repeating myself, I created a simple class to drive the lights.

Each class is initialised with the pin number of each light, and an initial status. The Toggle method will transition the light; RED, RED/AMBER, GREEN if status is RED, and GREEN, AMBER, RED if status is GREEN.

The main loop of the program waits for a period of time, then toggles the lights, starting with the green one.

Here we have it in action…


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So, a few weeks ago I got a PiFace for my Raspberry Pi.

The PiFace is a clip on extension board for the Raspberry Pi which, together with the free library software, makes it super easy to control various real-world inputs and outputs – turning things on and off, and sensing whether switches have been pressed.

Pretty cool, but unfortunately due to my work load, and an ever growing to-do list, I had so far not had the time to play with it at all. However, a recent sports injury forced me to stay at home for a few days, and since I couldn’t really move (or indeed, put on my own socks), I decided to use this as an opportunity to play.

My first bit of tinkering was to see if I could wire up and take advantage of the 8 software controllable outputs exposed by the PiFace. These outputs consist of 9 connectors, the first of which is a 5 volt power line, and the other 8 being the ends of an open collector.

The Circuit

I am not an electrical engineer, so this is all brand new ground for me, and that is enough of a reason for me to find it all terribly exciting. These first bits of hardware hacking giving me the same thrill of figuring something out and getting something to work as when I wrote my first software programs all those years ago!

Anyway, the circuit is a simple one, and consisted of a breadboard with 8 leds connected in parallel with the 5 volt power rail on one side and the open collector terminal on the other. The back of the packet my LEDs in also noted that a resistor of 330 ohm should be used in series with the led when connected to a 5 volt power supply in order to avoid burning them out.

Experiment1

The Software

As a proof of concept, I wrote a bit of software to cycle through and turn each light on for a second, before turning it off and turning the next one on. Nothing too fancy right now!

The Finished Article

So, here’s my first experiment in action…


Like I said, I’m not an electrical engineer so this is hardly the Starship Enterprise, but I learnt a lot!