LED no resister question

Any chemical cell has some internal resistance that limits the current it can supply to a load. You are relying on this resistance to limit the current to your LED. Smaller cells have higher resistance.

(snip)

I doubt that you understand how small a resistor can be. Resistors are available that are difficult to pick up without fine pointed tweezers. They have an insignificant size, compared to either the LED or the cells.

Bluer LEDs use up more voltage (short wavelength photons have higher energy, and that energy is directly related to the voltage needed to produce them), and might be a better match to your two cell battery. However, you may get a reasonable output from a red LED with a single 1.5 volt cell and that would save a lot more size than eliminating a resistor.

If you had a problem with the short life of the air-zinc cells, you'll have a much bigger problem if you try to light an LED without a series resistor. Life = microseconds. OMU

snipped-for-privacy@tm.net wrote:

Depends on the led and one may work without failing on 3 volt from the battery you have while others will fail. Look at the specs for forward voltage of the led and pick one that has close to 3 volt fv. The high intensity ones can run with 3.5 volt and would do just fine on 3 volt.

_________________________________________________________________________________ Re; Just a thought, but have you considered using an audible indicator? There are some pretty small piezo buzzers with all sorts volume levels available that could easily fit the handle of a fishing rod...

Dan Akers

As John P said, the internal resistance of the batteries is limiting the current. You can buy commerical keychain lights that connect an LED directly to two lithium coin cells (2 x 2016). The lithium cells seem to have enough resistance to limit the current; I've measured up to 50 mA or so on fresh batteries, but as the batteries wear down so does the LED current.

As long as you're experimenting with different battery types, you might try measuring the current for fresh, half-way used, and nearly-spent batteries. Try to determine what the LED is going through during actual use. If you don't have a sensitive enough ammeter, add a 1 ohm or 1/2 ohm resistor to the circuit and measure its voltage (assumin you have a DMM that will measure to the nearest couple of mV or better). Of course, you'd remove the resistor in the final product.

Regards,

Mark

Thank you for your respose. No I don't know how small they can be. I'm no electronics whiz but I do learn. Where would I find the small type?

I have experimented with different types and I must use the clear lens high intensity or wide angle to have them emit enough light in daylight. I have not got one to light up with a single 1.5v cell

Thanks again Dennis

Use a 1/10 watt resistor or chip resistor they weigh very little - will increase the battery life and perhaps the led life.

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(snip)

Digikey and Mouser are distributors that carry thousands of different resistor types and sizes. Both have search engines that let you narrow the choices.

The smallest with wire leads would be 1/8th watt types, (about 1/4 inch long and 1/16th inch in diameter) and 1/4 watt (.1 inch diameter by .3 inch long) but there are surface mount (SMT) that are tiny rectangles with no leads as small as .02 by .01 inch. That ought be small enough.

The 01005 size is difficult to pick up *with* fine pointed tweezers. They're about 0.016" long and 0.008" wide:

(the IC in that photo is a TVSOP-14, with 0.4mm pitch)

On 18 Oct 2006 11:18:09 -0700, "Dan Akers" wrote: ________________________________________________________________________________

I do have an audible indicator model that fits on the rod tip only slightly larger than the LED unit. It uses the same 1.5v cells as the LED unit. It holds 2 to 6 cells allowing volume to be adjusted. The peizo beeper is 12 mm dia. A reviwer wrote about this piezo:

Reviewer: Jonathan Broadwell from BEECH GROVE, IN US

change from using led's to debug logic ports...

I found a smaller piezo thats 9mm dia but I have to travel to China/Tiawan to get one.

I must keep this deice as small and as light as I have it now (or smaller) or No one will ever buy them.

Thank You for your post. I respect every reply I get.

Dennis

This web site has a schematic for a white led from a 1.5 volt source that can fit in the base of a miniature screw bulb,

When it comes to visibility in daylight . . . look for "high flux leds" generally speaking . . . these are leds with high output and high beam angles - the half power points are 120 degrees wide unlike "normal" 13 billion millicandlella lamps with only a 10 degree beam spread.

I found some surplus ones for 10 cents each on line - they had huge quantities - the only problem I could see were some of them were yellow and some tended more towards orange but the difference was only visible with two adjacent leds and not a problem in the 45 LED matrix I was making.

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On Wed, 18 Oct 2006 15:27:41 GMT, in message , snipped-for-privacy@tm.net scribed:

Resistors can be had in really small packages, especially low Watt applications like this. It's a better design to use one, and it will actually save drain on the battery. I see that these points have already been addressed, so take this as a reinforcement of those opinions.

I want to thank everyone who replied. You all have been a great help I also like to give help on these newsgroups when I find a topic I know something about.

Look for The Snooze Master on sporting goods shelves by the fall of

Thanks again Dennis

Greetings Dennis, Lithium coin batteries were mentioned by another poster. You may find that only a single cell would work for your project. Lithium batteries may end up having a longer useful life while being lighter than batteries using different chemistries. Try this link for some info on lithium batteries:

For reliability's sakes, I do advise a resistor. Although survival rate of an LED on a series string of two or three smaller size smaller diameter 1.5V "button cells" (2 for red to yellow-green, 3 for non-yellowish green-blue-white) is high. Also, it is fairly common for LED "keychain"/"keyfob" lights to have a smaller size lithium cell with a red, orange, or yellow LED or two of such cells in series with a non-yellowish-green, white or blue one.

Keep in mind the size of a 1/8 watt resistor of a through-hole type (with leads) - slightly smaller than a 1N914/1N4148 diode!

If you want a really compact resistor to use with a really compact LED, how about a 1/10 watt surface mount resistor which is 0603 size, and an LED of same size? Of course, those are surface mount parts and lower cost mass production requires some sort of PCB to mount these parts onto - but

(I do advise against using an 0603 size LED without a resistor even where a 3 mm [T1] or T1-3/4 [5 mm] LED with leads does work without a resistor! Just keep in mind how inexpensive 0603 size 1/10 watt resistors are!)

- Don Klipstein ( snipped-for-privacy@misty.com)