UPDATE 15 August 2008
Did some circuit tuning tonight. Initially the 1-turn trimpot was 50k ohm, but it wasn't providing enough resistance for a low-low. So I tried 500k, it works nicely but there's too much resistance the LEDs turn off at about 300k. The trimpot has a total rotational angle of 240degress. Which means roughly ~2k change with every 1 degree of adjustment. It's a pain to micro-adjust the 500k. The next step down from 500kOhm I could source is 200kOhm. It's 0.8k change every 1 degree of adjustment. Which is a good balance between low-low and easy micro-tuning.
UPDATE 26 July 2008
Okay, I have received the spring steel sheets few weeks ago, seems nice but it's coated with some sort of blue coat making it unsolderable and non conductive. It would be a lot of work to polish and cut, so $25 down the drain. I guess I shall stick with plan B, K&S 0.015" spring steel wire.
The thin spring steel wire only requires a small solder pad for attachment. Which means I have more space. I think I can do a slight redesign to the position of the components so I can include a hole for the trimpot access. I felt that this is important because I was getting really tired of removing the screws and circuit board from the bezel to do a slight adjustment to the trimpot.
Once I am satisfied and confident with the final design I will make the final prototype for testing and then send the circuit for pcb production. I hope that you understand that a simple screwup means the end product(circuit board) will become useless and that will result in a lot of wasted cost. Once again I am sorry for the delay I have lots to do(many distractions too ), don't wish to rush this out.
I made the rings a few weeks ago but hesitated to post because I was having a couple of issues such as alignment problems and etc which I will discuss later tonight.
Firstly, I need to know how many people are interested in the Onion Rings. It's not going to cost a lot so I can make them first but I need to know the numbers so I won't have surplus lying around my tiny shack.
A2 dumb LED rings (Onion Ring)
1) Linear constant current regulation, 3 individual channel.
2) Adjustable output 2-38mA per LED via trimpot.
3) Accepts Rechargeable Lithium-Ion.
4) Direct replacement, no mods required.
A2 dumb LED rings (Onion Ring)
Basically this is a A2 Ring with a LED driver which limits the current to 2-38mA per LED. The LEDs are plugged into the gold plated machine sockets which makes them user replaceable. This means Onion Ring owners can buy whatever LEDs you wish regardless the color/forward voltage and plug it in without frying them. No soldering required!
The LED driver has a current control pin and it has 3 separate channels at max 38mA each. It has a max input voltage of 6v which is perfect for the A2. Rechargeable users fear not, because I have included a beefy low dropout voltage regulator which limits the voltage to 4.8volts. The LDO regulator has a typical dropout voltage of 0.29v at 150mA. On top of that, it will work even when the voltage drops below 4.8volts. So the LEDs will still shine and suck the battery quite dry. At this point, the current to the LEDs will start to drop even though the input voltage is higher than the forward voltage of the LEDs. The reason is because the source voltage of the trimpot(current control) is tied to the output voltage of the LDO regulator.
The orientation of the trimpot seems normal but for it's really pain for someone without a power supply to make random adjustments. Initially I wanted to make a hole in the circuit board big enough for a philips screwdriver to go through. This makes current adjustment easy, without having to remove the Onion Ring but with the introduction of leaf springs it's no longer possible. Unfortunately there isn't enough space, the leaf spring will cover the hole and thus prevent access. So be it, the trimpot will stay that way.
Down side of this system is the max 38mA per LED drive current. This is the limitation(120mA total) of the LED driver. I've seen some high power 5mms claim to take 150mA so it will be under driven. If required I will just build a clone of the original A2 ring with resistor limited to 150mA per LED rings if anyone need them.
So why Onion Ring? Well it makes me cry when I look at it. It looks so cool but yet so much technical issues to deal with
The number #1 issue is with the alignment. I think I have not got it perfectly right. The original A2 rings has larger screw holes which is good because the holes on the circuit board don't need to be precise. The problem with the Onion Ring is that there's not a lot of space for a big screw hole. I think I could thin the tracks and increase the size of the screw holes.
Secondly, the machine sockets raise the total height of the LEDs by 1-2mm. This may cause problems with non Nichia LEDs. I have not investigated thoroughly yet I will have to look into it later.
Finally, I haven't quite found a solution to the leaf springs on the underside of the circuit board. I am thinking of brass sheets. Easily soldered and quite flexible. I tried some thin ones from my stash but the spring rate is too soft. I'll see what I can get and post them as soon as I found a good one.
Oh, If you are getting confused from reading these that's because I speak mainly broken English.
I love this holder, it's perfect for holding PCB and heavy enough not to move around when I poke it with my solder iron.
I begin to understand that stuffing in additional components is not an easy task.
Shiny stuff is lacquer to prevent shorts when installed. Note the current adjust trimpot at 2 o'clock .
The LED driver(above the resistor marked 203) has a lead pitch of 0.65mm.
The body is about 2x1.25mm. Its' dimension is perfect for such application.
Lets play "Spot the led driver"
The three pairs of gold plated machine sockets(grey) on the right are actually DIP machine sockets, reduce from 40 pins sockets.
The LEDs in the machine sockets are slightly taller than the original A2 ring.
Not done with the leaf springs yet. I'm thinking brass.