I'm going to show you how to create a turn indicator for bicyclists that lights up automatically when you extend your arm to indicate a turn.
It's based on a small commercially available EL panel, to which we'll add a simple tilt switch and a band so it can be worn on an arm. If you prefer you could alternatively sew it into clothing or attach it to clothing with velcro.
This can be built either as a beginner-level project without soldering or as a medium-level project with soldering. This instructable describes the no-solder version that is really just a proof of concept prototype. Soldering will make for a more robust unit and it'll be brighter because there will be less resistance in the circuit.
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Step 1: What You'll Need
- EL Panel with AA-powered AC inverter (X 2)
- tilt rolling ball switch (X 2)
- 1.5V button cell, of same approximate width as AA battery eg AG13, LR44 (X 2)
- armband (reflective LED bands are good but plain cloth would work just as well) (X 2)
- tinfoil (aka silver paper)
- plastic drinking straw
- insulating tape (or regular tape)
I used these particular parts because they were what I happened to have available. Once you see the general principle of this build, you'll know what you can substitute from your own collection of bits and pieces...
Step 2: Overview
The EL panel is pretty much ready to use as an indicator light. It's a big bright flashing arrow that you can strap to your arm. We want to turn it on when you raise your arm, and to do that we'll use a tilt switch. The trick here is that we're going to substitute a button cell for the AA battery, and use the space freed up by the smaller battery to house the tilt switch. (Because this will only be powered up when turning a corner, the smaller AR13 battery is quite sufficient and should provide for around an hour of continuous use which will equate to months of intermittent use. The AA cell would only be needed for continuous use.)
A slicker way to do this than the one I'll show you would be to build a module the same size as an AA battery, containing a tilt-switch plus button-cell battery, so the modification to the EL kit could be done simply by plugging in the battery-substitute... But that's an extension for another day. For now we're just going to shoe-horn the parts into place...
Step 3: Insert the Battery
First, cut a thin sliver of electrical tape and attach it to the side of the button cell. Put the button cell in the AA battery compartment and use the tape to hold it roughly in place. The tape isn't necessary for the final config, we're just using it as a 'third hand' otherwise the button cell tends to jump out when you're busy fitting the tilt switch later.
Insert it at the spring end (-ve terminal) of the compartment. Make sure the battery is the right way round. On an AA cell, the flat end is the negative end. On a button cell, the flat end is positive. So you want the narrower 'head' against the spring, and the flat end facing out.
Next, cut a length of the plastic straw that's just long enough to fill the gap between the button cell and the other terminal in the AA compartment. It has to be long enough that there's some pressure on the spring when you insert it in the compartment.
Step 4: Add the Tilt Switch
The tilt switch I'm using has legs at each end like a resistor; there are other tilt switches where the legs are parallel, like a capacitor. If you can only find that style you'll have to work out your own method to install it. (Mine came with an arduino kit from Maker Shed by the way, an expensive way to buy a tilt switch :-) )
Ideally the tilt switch would have legs that were long enough to attach to the battery and the +ve terminal directly, but if you got one like the one I have it'll be far too short. And for this proof of concept build I'm not going to do any soldering - I'm actually going to create "wires" out of tinfoil, and hold them in place with spring pressure. This is a pretty crude hack and is the first thing you'll want to improve when you build this yourself, but for now go with it as it'll let you prototype this device in minutes. (If you go the solder route, try to find a solution that avoids soldering directly to the battery as that is quite dangerous. FYI I did first try using that conductive glue but it was absolutely useless...)
So, cut a strip of tinfoil whose width is the same as the length of the straw that you cut, and roll that up and twist it to give it some body. Then cut it in half across the middle. Slide the pins of the tilt sensor into the 2 rolls of silver paper and perhaps attach a small piece of electrical tape just to hold these together.
When you insert this monstrosity into the straw, it will stick out a little at each end. Bend these ends over to hold the tilt switch in place and also to create a larger contact area at each end of the straw.
Wrap the straw in electrical tape until it is as wide as an AA battery, just to give the assembly some bulk so that it won't roll around or pop out when it's inserted in the battery holder - which is the next step. It should fit snugly and push the button cell up against the spring. Close the battery compartment case and turn on the switch.
Hold the case pointing upwards and then invert it 180 degrees to confirm that the tilt switch turns the EL panel off and on. Once you confirm it works, switch the battery pack from 'always on' to flashing mode.
Step 5: Adjust the Angle
Attach velcro strips to the front and back of the battery compartment. Attach the complementary strips to the armband and the underside of the EL panel.
Wear the armband and extend your arm as if signalling. Rotate the battery compartment so that it just comes on with maybe 15 degrees extra to ensure a good switching angle. Velcro the compartment to the strap at this angle. Velcro the EL panel to the compartment so that it is pointing appropriately for that arm.
That's pretty much it done. Repeat for the other arm.
Despite the crude assembly technique, if you did it right, it works quite reliably, although due to resistance from poor contacts with the tinfoil, it may not be as bright as it was with the stock AA battery fitted. I'll leave improving the mechanics as the traditional 'exercise for the reader' - personally I plan to create an AA-sized plug-in battery substitute tlt switch using a 3D printer as soon as I've finished assembling my new Printrbot :-)
This project also should lend itself to creating a 'soft' or sewable version, but beware of the 1000V AC inverter embedded in the battery pack. It's low current but could be dangerous for instance if you have a heart condition or a pacemaker (which I do, which is why I didn't cut the battery compartment off with a dremel and make a smaller and more better packaged version)
Another improvement you might consider is using the battery and switch from the reflective LED armband to power the inverter. I didn't do that because this inverter uses 1.5V and the armband uses a CR2032 which is 3V.
Finally, you might be interested to know that when I started this project, I designed a solution based on the Arduino. My first draft had been around a Memsic accelerometer, but after about an hour I had a forehead-thumping moment when I realised that a tilt switch was an actual mechanical switch and not a passive sensor, and that the Arduino was not even necessary!
Have fun with the electronics and cycle safely!