Project by Calin D. Raszga
IMPORTANT NOTE: Please do not try to reproduce this project. In this project 100V AC power is used which, when misused, can lead to sever accident or death. This project just proves a point for the sake of novelty.
At some point in my life I have found myself to be the proud owner of one old and used 110V traffic light. Where this thing came from I have no idea. It had a single outlet plug installed inside it and it would light up all three bulbs when plugged in. Obviously, not very fun. So I started tinkering with ideas of how to make it actually work like a real traffic light.
Before starting a fairly complex project, it’s good to make a small list of considerations. These are things that may give us trouble that we should brainstorm on right now. If these were part of a real project, for an employer or a client, they would be very detailed and called requirements.
- The traffic light is equipped with three 110VAC full size light bulbs with two wires each. They do not have any common wires. Since we will be dealing with 110VAC lines, we will need to use one of two things to switch them: Relays or TRIACS. Personally, I’m not a fan of relays. They click and they don’t live a long life due to arcing and such. I went with TRIACS. If you want to lean more about triacs, Wikipedia has a good article on them.
- Since we will be plugging the Traffic Light into a regular 110VAC socket, we will need to develop a 5VDC supply voltage for our micro-controller. Here we have two options as well: A small transformer or a SMPS (switching mode power supply). Switching mode power supplies are a serious design challenge on their own, so I went with a small transformer. If this was a large-scale production project, the SMPS would definitely be worth designing since it would be so much cheaper to produce (parts-wise).
- When one has both 110VAC and small DC voltages on a circuit board, it is necessary to provide an isolation zone between the two. We will have to design the circuit board such that there is a clear barrier between these two systems.
- Finally, we need to keep in mind that in order to be compliant with CSA(UL/CE) standards, the enclosure of the Traffic Light has to be properly grounded and that one side of each secondary coil on the transformer has to be grounded as well. This will prevent surges and also protect anybody touching the case.
The Actual Build
The micro-controller I picked for this (mostly because we had a few lying around) is a lowly PIC12F615. This micro-controller will not win any competitions for speed, memory of peripherals but will suffice for what we are trying to accomplish here.
Below you will find downloadable resources for this project.
Operation and Installation Manual: PDF
Here’s a video of my first prototype in action.