WHAT?

A rechargeable, battery-free LED torch which gets it's power from USB and stores it in a capacitor.

WHY?

I reckon USB stuff is cool. I also reckon LED stuff is cool. I HATE paying for batteries. I love my shaky torch. Putting all this together, I thought I'd make a LED torch, simmilar in form to those keychain types, which stores it's power in a capacitor. The capacitor is charged from the USB port.

HOW?

This is only a "proof of concept", and is intended to show it works. I doubt it will have the best performace, but that's not the point. I think I should be able to do it fairly cheaply, for about the cost of a keychain coin-cell LED light.

Step 1: Gather Parts

Here are the parts I used. The link directs to the product page where I purchased them to give you an idea if you are unsure what you are looking for (or to see how much/if I got ripped off). All prices in AU$ at time of posting.

Obviously substitute parts and suppliers where applicable for your location. I think you can get pretty much everything here from SparkFun, but I listed Altronics as they are a good source for parts in Australia.

- 1 x 5.5V 1F Supercap - Altronics - $4.25
- 1 x 5mm Blue High Intensity LED - LSDiodes $0.75ish depending on exchange rate
- 1 x 18 Ohm resistor - DSE - $0.04
- 1 x mini push button toggle switch - SparkFun - $0.30ish
- 1 x Verobaord, 310mm x 95mm, 0.1" spacing - DSE - $7.99 - OK I didn't actually buy this, I used leftover scraps, but this is what you'd buy. You only need a small bit, so I'll count 1/10th of the price (even that's way too much)

So total is AU$6.13. Definately acheived goal.

Yes the capacitor looks like a coin-cell battery. Trust me, I'm not trying to cheat, that's what it looks like.

Step 2: Resistor Sizing

Last step I called out an 18 Ohm resistor - why 18 Ohm?

The product page gives the following MAXIMUM specs for the LED. You could use the NOMINAL specs, but hey, I live on the edge. The effect of using the maximum values will be to increase brightness at the expense of life. AS the life is quoted at 100,000 hours, even a 50% decrease would still be 5.7 years continuous use - I'll take that deal.

Vmax = 4.5 V
Imax = 30 mA (=30/1000 = 0.03A)

The capacitor can supply 5.5V, but since we will be feeding it from a USB port the voltage should only reach 5.0V.

The purpose of a resistor in series with an LED is to limit current, preventing thermal runaway. Thermal runaway occurs when an LED draws too much current, therefore heating up a little, which in turn causes a little bit more current to be drawn, which causes an increase in temperature in a viscous cycle.

So to determine the size of the resistor, we use Ohm's Law, where:

I = current
R = resistance
V = voltage

V= I x R

This can be arranged for each of the terms, here we want to know the resistor required for the maximum voltage and current.

R = V / I

The 'left over' voltage after the LED has taken it's 4.5V need to be paired with a 30mA current (this is how I think of it anyways - other people teach/learn it different ways).

(5.0 - 4.5)
R = ------------
0.030

= 16.66666666666...6

(Instructables won't let me format that properly - hopefully you get the idea, (5.0-4.5) is the nominator, 0.030 the denominator)

Resistor come in standard values... rounded UP (I'm turning soft!) the next standard values is... you guessed it... 18 Ohm.

Of course you could have just used a calculator like the one here

The colour code for an 18 Ohm resistor is brown-grey-black

Step 3: Concept Sketch

Here's my concept sketch, done one boring work day.

The capacitor is charged by putting it in across the power source, in this case the USB port. The capacitor stores the charge, acting as a low capacity battery.

The capacitor is then used as the power source for the LED. When it discharges, the LED lights.

Step 4: Circuit Diagram

Heres the Eagle file... pretty damn compicated hey... (actual .sch attached)

torch.sch
Download

Step 5: Layout Sketch

So I've claimed that this would be USB powered - but where's a USB connector? While looking around for a male type A PCB connector, I thought it'd be pretty cool to copy those "integrated plug" connector used on mini SD type cards.

I looked up the USB specification here, which is a fairly boring document. The specification though shows that the contacts on a male connector are 1mm wide, with about 2.5mm between them, and the carrier is 2mm thick. Hmmm... pretty much the same dimensions as the strips on the veroboard. Awesome.

Doing a quick test showed that but cutting a section of four strips, and inserting it into a USB port, I got 5V on the two out rails. Bam.

I'm not a massive fan of veroboard, because I think you have to spend too much timecutting traces and installing jumpers. I prefer to use matrix board where each pad is isolated, but the USB-compatability is too convienient here.

Using the schematic I layed out the components on a peice of board. Because the bottom terminal of the capacitor is exposed, I'm putting some of the jumpers on the underside.

Here is my combined layout/circuit board sketch. Dotted components on other side. I tried various vero/stripboard CAD software, but nothing worked the way I liked. Any suggestions - let me know in the comments.

Step 6: Performance Calculations

At this stage lets look at how we'd expect the torch to perform. Performance wasn't part of my goals, so this is for interests sake only. Of course if the performance of the torch were critical, these calc's would have been done as a first step to determine the feasability of the project, and the component values.

My discalimer: I'm a scructural, not electrical engineer, so I may have made multiple mistakes here. Please let me know if I did!

The energy stored in a charged capacitor is given by

E = 0.5 x C x V²

where C is the capacitance and V is the voltage

Here

E = 0.5 x 1 x 5²
= 12.5 joule

1 joule is one watt of power for one second.
We've got 12.5 joules to play with, that would get us 1 watt for 12.5 seconds

Ignoring the internal resistance of the capacitor (not insignificant, in reality will affect the result), the LED/resistor is consuming power

P = V x I
= 5.0 x 0.03
= 0.15 watt

Therefore 12.5 joule will power 0.15 watt for 12.5/0.15 = 83.3333333 seconds.

Hmmm... not exactly inspiring stuff, and will get worse when you included the capacitor internal resistance... but OK to muck around with. Of course the LED will continue to give off light as the voltage fades, gaining a bit of time, but it won't be as bright. On the plus side, recharges are (virtually) free and quick.

AHA! But how quick?

The charging profile of a capacitor is generally calculated in terms of a "time constant". The capacitor will increase in charge by 63.8% every time constant. The attached images shows the cycles and came from here

eg. Assuming starting at zero, after one time constant, charge will be 63.8% of max, after two will be 63.2% + 63.2% of remaining 36.8% = 86.5% and so on, 3 time constants = 95%, 4 = 98%, 5 = 99+%.

the time constant is given by

s = R x C

Here the internal resistance of the capacitor is 30 ohms.

s = 30 x 1 = 30 seconds

So it will take 2.5 minutes or so to be pretty much fully charged... and for an 83.3 second use time thats a 35% duty cycle...

Step 7: Assembling It

Soldering all the components... not much to say. None of the components are particularly heat sensitive... maybe the LED. Bend the LED 90 degrees so it sits against the board pointing forward.

I ended up putting the link from the LED to the negative terminal of the capacitor on the top side, rather than the bottom as I had on the sketch in step 5.

Step 8: Finished!

Insert into USB port. Wait 2 or so minutes. Remove and push button to use!

It's never going to photograph great, but as you can see it throws off a useful light. In comparison to my brand-name keychain light I'd say it gives maybe 50% of the spread, but greater brightness in the centre of the beam. Maybe using a wide-angle LED would be a good idea.

Endurance wise: I left it to charge for a long time, then quickly removed it and timed how long it ran for. Of course there is no 'cut off', it just gets dimmer, but it definately retains a useful amount of brightness for well over 2 minutes.

TO DO: A custom housing. I'm thinking clear expoxy or silicon. Epoxy I would need to relocate/raise the button, silicon I'd need to look at how much it would gum it up.

In conclusion, a fun project with a pretty cool result.