Simple PWM LED Driver than randomly dims/brightens?

[chopnhack]

Is there a simple way to construct a dimmer circuit for LED's that randomly dims and brightens without resorting to higher level digital products? I need to power three green LED's in series, preferably driven by a 9V battery. Current requirements of the LED's are 20mA with a 2-2.2Vf drop.

I came across some schematics, but wasn't familiar enough with them to determine if the oscillation provided by the 555 would be random, or if it was fixed. The first schematic is the proposed 555 PWM, the next schematic shows it in place with the LED.

Taken from: http://www.pcbheaven.com/userpages/LED_driving_and_controlling_methods/index.php?topic=worklog&p=4

My current basic setup is a current limiting resistor, 150 Ω's so as the voltage drops, so should the current thus protecting the LED's. (yes, I am still waiting on a shipment of perfboard

 

[chopnhack]

A rather enlightening (no pun intended) few hours of web browsing shows that the easiest way to achieve my goal is to actually purchase "flickering" leds! Who knew? Apparently the chip to control the flickering is self contained right inside the epoxy dome of the LED. Wiring other LED's in series gives the downstream LED's the same characteristics! Should provide for some interesting Halloween displays this year

 

[Gryd3]

A rather enlightening (no pun intended) few hours of web browsing shows that the easiest way to achieve my goal is to actually purchase "flickering" leds! Who knew? Apparently the chip to control the flickering is self contained right inside the epoxy dome of the LED. Wiring other LED's in series gives the downstream LED's the same characteristics! Should provide for some interesting Halloween displays this year

I've seen those around. I have also seen an article on reverse engineering the pattern that is used to flicker the LED. They monitored the current draw of the device to figure it out http://hackaday.com/2013/12/16/reverse-engineering-a-candle-flicker-led/

I have also seen some rather simple solutions using an 8 pin micro. One such solution used a second LED as a light source and momentarily as a light detector to turn the device on/off automatically. (Sadly, I cannot find the original post for this)

 

[chopnhack]

Linear feedback shift register, very clever! I think that would be a 74 series logic chip? I may have seen one of those in my travels on the web. Pretty cool to be able to use a LED to act as a light detector! I didn't find the link you were referring to, but I did find this one: http://picatout-jd.blogspot.ca/2013/12/led-scintillante.html
Unless you parlevous frances (or however its spelled) use Google Chrome with automatic translation

I am happy that there is a simple solution, I can over think things some times Thanks for the link.

 

[Gryd3]

Linear feedback shift register, very clever! I think that would be a 74 series logic chip? I may have seen one of those in my travels on the web. Pretty cool to be able to use a LED to act as a light detector! I didn't find the link you were referring to, but I did find this one: http://picatout-jd.blogspot.ca/2013/12/led-scintillante.html
Unless you parlevous frances (or however its spelled) use Google Chrome with automatic translation

I am happy that there is a simple solution, I can over think things some times Thanks for the link.

Here's a link. It's not exactly what I remember, but is close enough
http://tinkerlog.com/2009/12/12/geeky-advent/

 

[BobK]

That is interesting. I wanted to try to get the same effect, and I used a linear shift register random number generator to set the duty cycle of PWM in a PIC microcontroller. To my surprise, it looked just like the flickering candle LEDs. Now I know why!

Bob