I just wasted some time trying to use a very simple but powerful product.

It's a proximity sensor ([Link: led-lighting-systems.net]). It's made to be used with a power supply and LED strip(s) by the same manufacturer. Every product comes with connectors on the wires and no documentation. The manufacturer could not even come up with an ID on the connectors, though I located a suitably close set of connectors by spending a half hour chasing it over the phone with a technical rep from DigiKey, once I sent him photographs. From that, I should have seen it coming.

This thing is made to turn on a set of LED cabinet lights when the cabinet door is opened, off when it's closed. I checked its operation. It's always off when something is less than two inches from it and always on when there's nothing within about six inches of it. It switches at some intermediate distance dependent on how good of an IR reflector the moving thing is. "On" means the voltage into it is present at its output. "Off" means no output voltage.

Operation and wiring are incredibly straightforward, so straightforward that the manufacturer needs, and has, almost zero technical information on the thing. You just plug their parts together and they work.


So... we have a need to turn lights on when any one of four drawers is opened. This will work GREAT! I wired up a little board with input and output connectors for four sensors. The only technical advice the manufacturer had was that maybe I should avoid interconnecting the outputs of these things, so I added steering diodes. I powered up the whole thing.

It did not switch. It was on all the time. What?

An hour passes while I fiddle and check my layout and wiring. I take a break. I realize something during dinner: maybe I don't have continuity on the ground somewhere. So I go back to it. The connectors are tiny TE connectors, a bit flaky, and I can't reliably get probes into them. I clip the connectors off one of these (good thing I bought extras!) and solder it in place, bypassing some of the wiring, and I can't figure out anything.

Another half hour; no progress.

Suddenly it occurs to me that the product has two wires in and two wires out, and maybe something screwy is going on with the ground. It takes another half hour to realize that these things are switching on and off by disconnecting THE GROUND on the output! And, of course, I assumed the + side of the power was being disconnected, so when I wired up the little board, the first thing I did was lay out all the ins and outs and connect all the grounds together... thus ensuring that they will never turn off.


It was ASSumptions all the way.

Now my nice little board has input and output ground interconnections snipped out and other grounds added by wire along the top of the board. It's looking pretty ugly. At least the damn thing works!

People sometimes ask why not to make some small little thing in a box when a custom need arises. The classic has been the stereo to mono adaptor. I've answered in the past that it's not worthwhile because there are just too many details to attend to, even more so if it's something that requires a power supply. I pay $24 plus shipping to have these ready-made and in stock. This proximity sensor example now screams out above all the others as the one that wasted time making something simple work!

You couldn't use a small switch?

Highfigh,
this is actually the second theater for which we've provided multi-drawer 3D glassses charging. We built the first one because we outfitted the entire theater from a concrete shell into a fully acoustically treated etc etc etc. This one is an existing theater where we're retrofitting existing drawers for this use.

A switch requires pressure. The room we built has touch latches, which require more motion on the push than a switch would allow. We used reed switches and magnets to control the lights. It worked but it took a lot of fiddling to get the positions right.

With the retrofit, we can't get to the back of the drawers with the drawers in position. We'd have to do one drawer at a time, messing with positioning until we got it right. And the drawers simply push shut, so back pressure from a switch would keep them from staying closed.

These sensors don't require a particular target (such as a magnet). All you have to do is get them within two inches of the back side of the closed drawer. They come in a small enclosure that lets you screw them onto a surface that's at 90 degrees to the back of the drawer, or you can take them out of the enclosure and drill a hole in a surface parallel to the back of the drawer.

Basically it's messing with electronics out in the open in order to make the clumsy part, sensor installation, easier.

Is time wasted getting to know a new product really time wasted (well maybe it is for the customer)? The first time we set a new product up there is always a learning curve. In this case it took a couple of hours. But next time you use this product it'll be done in minutes.

One more tool in the tool box. At least you had the knowledge to figure it out.

Craig

If you will ever do it again it may not be time wasted. If it is the first of many, then it is good.

If it is a one off, and you were on the clock, all is good.

If it is a one off, and you bid it for x number of hours, and it came in at 5x, not good. The client has no idea all you went through to be the hero. It happens more often than we would like, I suppose.

It was worth doing for this customer. This is a design and build where, for instance, we have two projectors in a hushbox behind the about 18 foot 16:9 screen... we've had some challenges but ultimately it's being done right every step of the way.

I just received the two $24 stereo unbalanced to mono balanced or unbalanced boxes (+ $8 shipping for each). I'm happy to have them.