- Overview
- Technical Details
- Opto-Couplers
- State Detection
- Hardware Notes
- A Hack
- LED Strip
- Lessons Learned
- Gotchas
- Wiring
- Parts List
- Pictures
This is a Raspberry Pi Pico project to turn on/off a strip of LEDs in a cabinet. The cabinet has double doors. If either door is opened, the LED strip is turned on. When both doors are closed, the LED strip is turned off. Opro-couplers are used to detect when the doors are open or closed.
Even though this is programmatically very simple, I'm going to the trouble of writing this because as far as I know, there are few documented examples of opto-couplers for the Pico.
Someone suggested I try opto-couplers instead of reed switches to sense when a door is open. That sounded like a good idea, so this project uses them exclusively. Here is the particular one I'm using.
Since there are 2 doors on the cabinet, there are 2 opto-couplers to detect their individual status. Each opto-coupler is mounted on the door frame in the upper left and right respectively (see pics below). On each door is a small plastic wedge which fits in the opto-coupler's slot. When the door is closed, the wedge interrupts the beam sent from one arm of the slot to the other. Conversely, when the door is open, the wedge is pulled away and the beam is uninterrupted. Sensing the couplers are unblocked, the LED strip is then illuminated.
the Pico assembly is housed in a small project box. The Pico itself sits on a breakout board. (Why a breakout board? Because it makes it easy to switch out the Pico if you need to. By using a breakout board, you can just remove and replace the Pico instead of uninstalling the whole thing) Tiny nylon screws and bolts to secure the breakout board to the project box.
There are 3 sets of wires on the Pico: one for the LED strip and two for the opto-couplers. THe LED strip connection uses a BTF male adapter (to match the LED strip's female connector). Both of the two opto-coupler connections terminate in a BTF female connector (to match the coupler's male connection). See pictures below.
On the back of the project box are strips of magnetic tape. There are also strips of magnetic tape on the wall of the cupboard. Contact between these two sets of strips is what secures the project box in place on the wall.
I spent a lot of time looking for something to be used for a wedge to interrupt the signal between the two arms of the slotted optical coupler. It needed to be something unobtrusive but sturdy and could be mounted on the cupboard door. After paddling far up the Amazon, I finally found what I was looking for. These are sliding door floor guides. They are exactly the sturdiness and size I was looking for. Mounted on the cupboard door, they worked out perfectly - well, almost perfectly. I'll cover that part below.
The LED strip is a ws2812b strip mounted on the inside of the cupboard. There are 2 things worth mentioning about it. One, I used a strip-to-wire connector (see pic below) to avoid soldering on the strip. Secondly, I used double-sided tape to mount the strip (pic below). On the strip is a female BTF connector.
Opto-couplers are a mixed blessing. On the one hand, they are simpler then a reed switch to program. With reed switches, you have to contend with Normally Open/Normally Closed logic that can be confusing (not to mention the one case I experienced where the vendor got it backwards). With opto-couplers that goes away. there are only two states: open and closed.
However, reed switches are simpler to set up mechanically. You just make sure the switch is in proximity to a magnet, and you're set. Opto-couplers, or at least the version I'm using, require a more precise approach. There is a beam between the 2 arms of the coupler and something has to break that beam in order to establish a state change. Cobbling that together takes some ingenuity. I experimented with several physical configurations before I found one I liked. Hence, the opto switch is physically more of a challenge.
There was one interesting gotcha. Everything seemed to be working after the initial installation. The LEDs came on as expected when I opened the doors. But the LEDs would NOT turn off when both doors were closed. "Ok", I thought, "the beams are not interrupted by the wedges I had screwed to the doors. Alignment problem of some kind, no doubt."
But after re-re-re-checking alignment, it was clear they were exactly where they should be: positioned between the beam emitter and receiver when the door was closed.
After much experimentation, I figured out the color of the wedge was key. In my case, the wedge was light colored
plastic. When I tried putting a white sheet of paper in the slot (between transmitter and receiver) I got the same
result. The coupler didn't consider itself "blocked". I then tried a dark sheet of paper and got a successful result.
Therefore, the coupler would only consider itself blocked if something dark was used to block the light beam.
On a hunch, I wrapped black electrical tape around the wedges (see pics below) and it worked! The coupler finally considered itself blocked with the doors closed. The LEDs dutifully turned off.
Bottom line: Whatever blocks the beam on the coupler needs to be dark. Odd.
No self-respecting Pico project would be complete without a Fritzing diagram. Here's mine.
raspberry pi Pico
Pico breakout board
optocouplers
project box
magnetic tape
double-sided tape
sliding door guides (hacked to create the wedges)
black electrical tape
wire
wire connectors
USB power supply
USB power cable
wood screws
drill
Phillips head screwdriver drill bit (comes with drill)
3/32" drill bit
nylon screws and bolts
wire cutters
BTF Lighting 3-Pin Connectors (1 Male/3 Female)
BTF Lighting WS2812B LED Strip
LED Strip Couplers (1)
headlamp (optional - comes in handy under a sink or in a cupboard)
Project box with all connections
Another view of project box with lid on
The back of the project box with magnetic strips
View of the opto-coupler mounted inside on cupboard door frame
View of the "wedge" mounted on the cupboard door
The wedge after being covered with electrical tape
The wire-to-strip solder-less connector
The connector with wire and ws2812b strip
Closeup of LED strip with double-sided tape on back
Closeup of opto-coupler mounted inside door frame
View of coupler, project box, and LED strip all connected