Neil’s Website

Dive Light Project

Details of my cheapskate attempt to build a dive light

The only power tools I have are a drill and a soldering iron, so I wanted to build something using mostly off the shelf components.

Most of the designs on the internet are based on modified maglites, so that was the way I decided to go.

I started by modifying a 3-D-cell maglite from ebay; basically following the instructions here, with the exception that I used a Cree XML emitter. This was to see whether I could do it, and to have a torch to test different reflectors in.

Parts used:-

  1. Maglite from ebay £12
  2. P7 Flat Top "D" cell heat sink £7.00
  3. XML-U2-1C LED with 16mm Base(3.7V) £8.00
  4. JB Weld Marine Epoxy Steel
  5. PTFE coated wire
For full instructions follow the link above, but basically I removed and disassembled the switch assembly (the hardest part), cut off the part that holds the bulb and put the switch back together again. Then I soldered wires to the positive and negative contacts. I glued the LED module onto the heatsink with the epoxy steel (a very thin layer) - it is about 1mm bigger than the platform on the heatsink, but I don't think that is a problem. After the glue had set I soldered the wires from the switch to the LED module (making sure to get them the right way round). Then I put some thermal paste on the heatsink and fitted it into the Maglite body. I had to cut the bottom of the reflector off and drill the hole a bit bigger to fit over the 16mm LED module, then I put it all back together again.

Notes

  1. This modification works well because the LED module works best at around 3.7v and three D-cell NiMH batteries will give 3.6v. It won't work with a four cell (4.8v) or a 2-cell (2.4v) maglite. 2.4v is too low to power the LED and 4.8v will burn it.
  2. The LED module specifications state "320 lumens", this is the maximum output at 700mA (details in this pdf file). When it's powered by 3.6v it will be using about 3A, so this will give a maximum output of about 1300 lumens.

Choosing the optics

I used the modified 3 D-cell light to try out a selection of reflectors and aspherical lenses, what I wanted was a tight narrow beam.

 

The aspherics gave the tightest beams - unfortunately the light head had to be unscrewed almost completely to focus and there were lots of ugly artefacts around the central spot, so they are unusable.

 

I would have liked a tighter beam, but the smooth aluminium reflector was the best. Hopefully someone will produce some better optics for the XML LED in the future.

 

Dive light

The dive light consists of two parts; the light head and the battery cannister. I decided to have the switch and all the electrics in the light head.

It would have been nice to have a break in the cable to be able to separate the head and the cannister, but I couldn't find a connector for a reasonable price that I was confident would be waterproof at depth.

Parts used in addition to those above:-

  1. Pololu Step-Down Voltage Regulator D15V35F5S3 £10
  2. Switch latching circuit £5
  3. Schurter Piezo switch £26
  4. Polycarbonate lens £5
  5. 2-core H07 cable £4
  6. Otterbox 8000 £18
  7. Epoxy putty
  8. Liquid gasket
  9. IP68 cable glands from ebay

Dive Light Electronics.

The dive light is a little more complicated. I want a light that will run for 2-3 hours, which I won't get from three D-cells. So I'm using a bigger battery pack. But if the voltage is higher, then I need to use a step-down voltage regulator to avoid burning out the LED. I'm using this one from here.

These voltage regulators use Pulse Width Modulation (PWM) to reduce the voltage. You can find an explanation of how this works here. It's tiny and can accept an input voltage between 4.5 and 24 V and efficiently reduces it to a lower, user-selectable voltage. You can set it to output 3.3V (the default is 5V) using a jumper and it can supply up to 3.5A. The voltage is a little low, but it's good enough for me. Basically the PWM switches the power on and off very quickly to reduce the voltage, for example if the battery pack is three times the voltage you need (ie 3 x 3.3V = 9.9V), the power will be on for 1/3 of the time. A nice side effect of this is that, as the power is only on for a third of the time, the current is also reduced to a third. The important thing for me is that this will increase the battery life, in this case three times.

There is a problem with this voltage converter - it does have a small amount of current leakage - if you leave the batteries connected with the torch turned off they will eventually go flat. I'm not sure how long it will take, but I make sure I connect the batteries just befroe I kit up.

I made this table to help me work out what size battery pack I needed. Power supplies For 3.6v @ 3A with PWM controller

Battery Configurations.
Capacity (Ah)
Current draw
Battery life
7.2V Battery
6 D-Cell
9
1.50
6.00
6 C-Cell
5
1.50
3.33
6 Sub C
3.3
1.50
2.20
8.4V Battery
7 D-Cell
9
1.29
7.00
7 C-Cell
5
1.29
3.89
7 Sub C cell
3.3
1.29
2.57
9.6V Battery
8 D-Cell
9
1.13
8.00
8 C-Cell
5
1.13
4.44
8 Sub C cell
3.3
1.13
2.93
8 AA Cells
2.1
1.13
1.87
10.8V Battery
9 Sub C cell
3.3
1.00
3.30
12V Battery
10 D-Cell
9
0.90
10.00
10 C-Cell
5
0.90
5.56
10 Sub C cell
3.3
0.90
3.67
10 AA batteries
2.1
0.90
2.33
14.4V Battery
12 AA cell
2.1
0.75
2.80

The other big problem with the light is the switch - it needs to be waterproof to 4-5 bar (at least). I'm using a piezo switch, I got the idea from a few sites, I think this is the original. Piezo switches are momentary switches, meaning that when you press them they send a pulse, they don't change from on to off, and, in this case, they don't stay on as long as the button is pressed either. So I need to use a latching circuit which converts that pulse into a on-off action - I'm using this one ordered from here. As you can see it's even smaller than the voltage regulator

The contacts from the piezo switch are connected to the two holes on the left-middle labelled with lollipops.

Light Head

The light head is made from a two D-Cell Maglite. The first step is the same - take it apart and remove the switch and bulb holder.

I soldered wires to the Piezo switch and stuck it into the original switch hole using epoxy steel. I added several more layers to make sure it was strong and watertight. I thought at first thet the switch wasn't working, but the action to turn it on is more of a tap than pressing.

For the umbilical cable I drilled a hole and tapped it with an M16 thread to screw in an IP68 cable gland, I put Aquasure under the gland as I screwed it in to seal it, then filled the inside with epoxy steel. After I put the cable through the gland I sealed the inside with liquid gasket (basically a soft silicon sealant) - Aquasure would probably work even better, but it would be harder to take the cable out again if I wanted to change anything.

The LED module is just glued onto the heatsink with epoxy steel, hopefully it will provide good enough thermal contact.

 

Be careful wiring up the LED - it does have a positive and negative end!

I wasn't convinced about using a glass lens, even if I could get one thick enough, so I had some stepped polycarbonate lenses made. These are 5mm thick with a 2mm rebate around the edge so that the Maglite bezel can screw in all the way (without the O-ring). I used epoxy putty to glue the lens onto the bezel of the light head.

I had to get a batch of ten made, so I have a few for sale at cost price (£5.50 inc. postage) - email me if you want one.

The lens fits very neatly inside the bezel, the 2mm rebate means that the front of the lens does not protrude past the bezel.

As well as the epoxy putty the lens will be held in place by the reflector when it's screwed into the bezel.

After I'd wired everything up and put it all inside the Maglite body I screwed it all back together. I sealed the bezel and the end-cap with epoxy steel, and the light head with Aquasure (making sure the light beam was as focused as possible. If I need to take it apart I might be able to take the light head off, but I think it's probably permanently fixed now.

Canister

I wanted to use something like an OMS cannister, but they don't seem to be available any more. In the end I followed the suggestion here and used an Otterbox 8000.

I drilled a hole and tapped it with an M16 thread. I screwed in an IP68 cable gland and sealed it with epoxy steel.

 

For the umbilical cable I used 2-core H07 cable.

 

The advantage of this design is that I can put a wide range of battery packs in the cannister - I'll be starting using an 8 x AA pack (9.6v ˜2 hours), but the Otterbox can take up 15 C-cells which would last for about 7.5 hours. The step-down voltage adaptor will take up to 24v input and reduce it to 3.3v.

 

Final Thoughts

So far the torch has been tested to 15m - no leaks and everything works fine!

What would I change? I would have liked the light head to be smaller, but Maglite don't do a single D-cell model and I wasn't confident about being able to make a square cut to shorten the body. It would be nice to have a Goodman handle on it, but it's a bit too long. I really don't like the cannister. It just about fits in my thigh pocket and would probably fit in a BC pocket, but I really would have prefered a smaller cannister - I had an OMS one on order for over a month before I lost patience. In the end though the Otterbox was cheaper, available and offers more choice of battery pack.

It would be nice to have a voltage converter without a current leak, but it's not really a problem.

The beam in the water is not bad, but it would have been nice to have a better choice of reflector or collimator. It is possible that there may be better optics available in the future, but unfortunately the kind of beam that is desirable in a dive light is much narrower than most reflectors are designed for.

© Neil 2015