This seems a fair enough request, I'll have a lash or two at it. I'm going on the assumption that this is intended to be both a practical project and a learning experience and that we should start out simple.
I went to my local RS to see what was offered. Hopefully, you'll have the same parts or something similar. I went looking for high output ('very bright') red LEDs and bought one (part number 276-0086) a 5000 MCD (at 20 mA) "jumbo" (therefore narrow beam). A bit pricey at $2.59 but knowledge ain't cheap, right?
The idea behind starting with red is the low forward (operating) voltage, typically 2.0 Volts more or less. This means we can drive it from a two cell battery (say two D cells for long life). Using whites would need another cell (raising the battery total voltage to 4.5) to deal with the 3 plus volt Vf.
First off, don't buy into the 'you don't need resistance' idea. It's simply not so. Resistance is what sets the current, without it current would 'run away' at voltages above Vf and the LED would be destroyed. Those who 'direct drive' are in fact still using resistance, the internal resistance of the device (one to several ohms), battery (a dozen to a few hundred miliohms per cell depending) and the various contact resistances (often into the few ohm range) alone. Such systems are characterized by very large (read 'wasteful' in this use) currents on fresh batteries, quick rundown to a long period of dim light as the last bits of energy are drained. Adding the correct resistance allows us to limit the initial current (adding greatly to battery life) and control it's 'mid life performance' as well. We need to decide how much to add.
A good place to start is to pick a number that will give low current and see what we get (pun intended). Let's say 1 mA (one miliamp, a thousandth of an amp, 5% of the typical 20 mA rating. First a bit of subtraction. The total battery voltage (3 Volts for the case of two 1.5 Volt cells) less the forward voltage (operating voltage of the LED at that current, say 2 Volts), giving us 1 Volt 'across the resistor'. Now for the division. Ohms law tells us to divide this voltage in volts (1) by the current we want in Amps (.001) to get an answer in ohms (in this case 1000). This resistor is so much larger than the internal and contact resistances that they can be safely ignored.
So, if we take our two cell battery (holder advised) and put our LED and resistor in series with it (short LED lead to the more negative side) we should be Jake. Order doesn't matter, the battery, resistor and LED can be hooked up in any order as long as the battery and LED polarity are watched. Hooking the LED up 'backwards' is safe as the total voltage is under the typical 5 Volts the device will tolerate reversed. In the example I built with these parts today I got what I think is a usable amount of light (easily enough to find your way about IMO), but you'll need to judge that for yourself.
We can expect nearly full capacity from our battery at such a low rate, more over as the cells discharge and their voltage goes down the current will follow as the 'voltage across (and therefore current through) the resistor goes down'. A quick estimate (conservative) gives over 2,000 hours from AA cells, some 18,000 from D cells. Roughly 3 months and two years respectively. If you want more light, lower the resistor. Using 500 ohms (actually 510 the nearest standard value) will double the current (and light output) and cut the lifetime in half.
You might consider a 'turbo switch' to boost the light when you need it (say like when you're trying to navigate in the bathroom). Simply put a second resistor in for the higher current. Adding a switch and say 51 ohms across the 1000 ohms already there will raise the current to 20 mA when needed. This resistor could also be placed in series with the 1000 ohms (making the total 1051 ohms for dim) and the switch used to short out the 1000 ohms (bringing the total to 51) for 'turbo mode'. You might consider a micro switch mounted to the battery pack and pressing down when the light is normally rested on the counter. The contacts (we want to use the 'normally closed' contacts which would be open, no connection, when the switch was actuated as when hitting the counter) to automatically switch to bright when the unit is picked up or turned on it's side. That's what I plan at any rate on the one I'm building. Makes it easy to find in the dark and a 'no brainer' to pick up and use.
That should at least get you started.
If you're serious about fiddling about with this sort of stuff, 'we should talk'. How you fixed for mail order where you are? Jameco Electronics <
www.Jameco.com>, might be just the place to stock up on cheap electronic bits. Let me know if you're interested and I'll see if I can cobble up a list of suggested parts.
Cheers,
Doug Owen
