Easyvibe Led Voltage oscillating issue

[djibrilliant]

I created a simulation of the easyvibe's oscillating circuit. It didn't seem to oscillate at all until i changed the value of the 47k resistance to 10k and less.
One simple explanation would be that i didn't have very precise simulation models for the low current leds. Do these have a smaller forward voltage drop than the 914 diodes? If so, can anyone tell me what that value is approximately?

Thanks in advance

[PRR]

> a simulation of the easyvibe's oscillating circuit

Oscillators usually do not simulate corectly without extra tricks. Many oscillators, if built with "PERFECT" parts, balance on the razor's-edge and never oscillate (but work fine with real-world parts and universal random noise). Others (many) will take hundreds or thousands of periods to build-up.

> leds. Do these have a smaller forward voltage drop than the 914 diodes?

LEDs are usualy 1.5V-1.7V, much higher than simple diodes.

The bias string may be replaced, for simulation, with a 3.8V battery. (0.6V+1.6V+1.6V.)

This plan?  --  http://www.hollis.co.uk/john/easyvibe.jpg

Yeah, that will "balance forever" in simple simulation. Try "Skip initial transient solution" so that all voltages start from zero (reality), not from their final average value at the end of time (un-real). If you have a time-switch, break some node and tell the switch to close the break after 1mS.

If you are going to build it, don't worry what the silly simulator says. This is a very reliable function generator.

[djibrilliant]

Funny thing, i did what you suggested about the bias point before i read your reply. Put it to 3.5V and things worked fine. Didn't know the low current led have bigger voltage drop than the diodes. Thank you very much for all the info. One last question though: do you know if these leds will work? http://www.banzaimusic.com/LED-3mm-red-low-current.html I hope they will...

[PRR]

LEDs are not ordinary diodes.

A detector/rectifier diode ideally has "no" voltage drop. It is only a oneway valve, and any forward drop is a waste. In real life, 0.5V-0.8V is what you get with Silicon.

An LED must "eat" real power, and significant voltage. We turn to GaAsP and other semiconductors with higher intrinsic voltage, bigger hop between electron orbitals, enough hop to excite a photon of light.

All cheap red LEDs are "the same", near enough. 1.5V-1.8V. Different enough that you can't parallel them and expect equal sharing, but similar enough that, as a bias-source, any red LED should work. (i.e. steal one from an old PC or toy, don't buy/ship one!)

Various sizes and leads, but that does not affect the electricals.

"Low current" seems to just mean they get bright with a few mA, not like the older ones which needed a dozen or more mA. Semiconductor technology gets better.

Red: 1.63-2.03 V
Yellow: 2.10-2.18
Green: 1.9-4.0