Thursday, July 17, 2014

Hacking Together a Replacement Switch for an LED Lantern

My sister recently asked me to have a look at an LED lantern she uses around the house, mainly to lock up her chickens at night.  It had been dropped and the switch mechanism damaged.  She wanted to see if anything could be done without spending too much.

Lantern
Arlec CL100 Lantern
The switch is a rotary type and is on the top.  It's designed to be removed to replace the light.

Switch
Lantern Switch
Removing the switch is done by unscrewing the top.  This is where the damaged occurred.  The light was dropped on this corner, causing a large piece of the plastic thread to break off.  I considered glueing it back together, but after playing around with for a bit I got the impression that if I didn't get it exactly right it would easily break again.  So I decided to replace the top entirely.

Switch
Switch Damage
So how do I go about doing that?  Let's take a look inside.  The first thing to notice is the four terminals on the white plate.  The inner two are connected to the LEDs, while the outer two are threaded rods that are used to hold the light together and at the same time connect the LED's to the battery in the bottom of the lantern.

Contacts
Battery and LED Light Contacts
Normally the switch on top electrically connects these terminals to operate the light.  The functionality of this is what needs to replicated.

Contacts
Switch Contacts
I had an idea of how to replace the top, but I needed to remove the metal cowling on the top to make more room.  It seems to only be held on by a rubber retention ring.  I don't actually know the purpose of it.  It seems to be decorative.

Cowling and retention ring
The outer thread can be seen, and now that the cowling has been removed there is more room to work.

Outer Thread
Outer Thread
While we're at this point I'll take a minute to show the light bulb.  It's made of 4 PCBs soldered together to make a rectangular tube.  These boards hold the LEDs and current limiting resistors.  If you remove it, pay attention to polarity, like most LEDs it'll only work one way.

LEDs
LED assembly
The light is passed through a diffuser to create a more even spread of light.

Lantern Diffuser
Diffuser
Anyway, back to fixing the light.  I simply added an old switch I had in series with the battery and LED terminals.  I added as much insulation as I could in case a wire came loose, but everything seems fairly firm.

Wiring
New Wiring
That's all well and good, but you can't have the switch and wires hanging loose.  They need to be held rigidly to prevent damage.  After thinking about it for a bit I came up with the perfect replacement.  An end cap for PVC storm water pipe.  It's just the right size and is made of a relatively strong plastic.  The end cap was attached to the lantern by drilling some holes around the perimeter and threading cable ties through them.  I could have riveted it on, but it would have made it hard to fix anything if it breaks in the future, besides that, it could have cracked the plastic.  I could have also used self tapping screws, but after you insert and remove them a couple of times the thread in the plastic would be damaged.

Switch
PVC End Cap Cable Tied In Place
It's not the most elegant of repairs, but it was cheap and quick.  This is one of those occasions that having a 3D printer would have been handy, but it would have taken longer to design and print the part than my quick fix took.  All up, this cost about 3 bucks and about three hours of time while watching TV, so we'll say an hour of actual work.

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