diode series or array?

You know, I'll probably just buy a cheap bag of surplus red led's, which will probably have a Vf a bit over 2 volts. They won't come with a data sheet. I'm wondering how much tempco varies among plain-vanilla red led's? I thought it would be about 3 mv/degC.

Maybe use some SMT LEDs. But there are many kinds of LEDs with different forward voltages at a given current, different curves of voltage vs. current and different tempcos. 7 x 3mm leds will probably be pretty cheap and small if you have the height.

Best regards, Spehro Pefhany

Well, aren't they generally made out of the same kind of stuff?

And isn't "band gap potential", or whatever you call it, kinda related to the type of material itself?

I've never done any tests like this, I just sort of ass-u-me-d that a bag of LEDs would have come from the same run, and be fairly consistent from one to the other. I wouldn't be so confident from batch to batch, but you get the idea.

Thanks! Rich

I haven't selected a part. That 3 mv figure is just something I saw somewhere on the net. It looks like I am going to have to do something ridiculous like dunk led's in boiling water or something to measure their tempco, or just give up on it, because I find basically no info about it.

How did you establish this tempco ?

If you've selected a part why not simply say what the part number is ?

Graham

Any reason even to expect it's consistent ?

Graham

a bunch of SMD leds soldered end-to-end ? a bargraph module?

Bye. Jasen

Since the OP won't reveal his real requirements I hardly know what to suggest. I know I wouldn't trust the results from a few leds out of a bag.

Graham

On 29 Dec 2005 09:28:40 -0800, snipped-for-privacy@billburg.com put finger to keyboard and composed:

What about an LED bargraph?

- Franc Zabkar

if it's just the tempco that you need, have a look at zeners. Their tempcos are usually documented, and varies with the breakdown voltage. From memory, the 5v1 units have a nearly zero tempco. IIRC it's negative below 5v1 rating and increasingly positive as you go up in voltage above.

Don't worry about it. I'll adopt a more effectual approach.

Oooh, that's interesting. I have been wanting a dash-mounted LED RPM & Speed display for my car, since the needles are obstructed by the steering wheel at my driving position. This looks like it would let me create a rev-counter bargraph off of the back of the analog needle, somehow, although my electronics knowledge is very poor. Do you have any tips for something that would let me drive a three-digit speed (MPH) display off of the back of the speedo (analog needle) voltage? I think calibrating the speed might be difficult, whereas I could do the rev-counter on my driveway, but something which would let me adjust for a scale of 0 - 200 over a voltage change of

thanks, Carl

not quite that simply, a typical analogue tacho measures the "pulse rate" of the cars ignition system externally.... I guess somewhere deep inside that could be convertted to a voltage (but it might not be - typical moving coil meters only give 90 degrees of motion, not the 300 degres seen on automotive dials) sop there's probably some other drive mechanism....

there are bargraph tacho kits available.

if there's a voltage there get a three digit milivoltmeter unit and scale the voltage apropriately.

Lift the driving wheels off the ground, calibrate to your existing instrumentation. or buy some time on a chassis dynomometer(sp?)

Mechanical car speedos are usually driven by a rotating cable from the gearbox, and are all mechanical.

Probably the simplest way to convert to digital would be to glue a pot track on the instrument face, with a pot wiper on the needle. Run it off a regulated 10v or whatever, and feed to a multimeter or voltmeter. Bit crude, but youre going to have fun trying to do it other ways.

NT

yes bargraphs are available, but pricey. Same goes for high power led arrays, with series paralell elements. If you want an array of leds, buy the leds... what else is there to say

NT

"All mechanical" is an interesting term. The one I took apart as a kid (many years ago) had a magnet rotating on the end of the cable. That was near a conducting disk on a spring loaded needle. Eddy currents and such.

The one I played with wasn't very strong. I don't think a scheme like that would work. Maybe newer ones are different.

If there's enough torque behind the needle to move the wiper that could work. but, most mechanical speedos work bt a rotating magnet driving an aluminium disc (or cup) by induction, opposed by the force of a very weak spring.

a carefully placed hall effect sensor could detect the passes of the magnet

the rate of these pulses would need to be translated into the speed... the sort of thing a small microcontroller excells at.

Bye. Jasen

So, you tear apart the speedo, and mount a coil or Hall thing next to the rotating magnet, and voila! PFM!

Cheers! Rich