The Flicker Circuit

A while back I posted about a circuit I had devised that would make a lightbulb flicker such that the light it cast would resemble a candle flame. You see, last Halloween I was living in my apartment on RIT, where fire is not permitted. So I began to comb around a look for a circuit online that would make a normal light bulb flicker in this way. There are some floating around, but after a while I decided I wanted to build my own. I did, and it works pretty well. I was going to post a schematic, but it was something I kind of put together on a breadboard, and didn’t have a diagram handy. I was going to make one and post it, but I forgot about it. For a while. Well, now I do indeed have a schematic, having just thrown one together. I am posting it here, and you may use it for personal or educational, non-commercial use. Here it is:

The Pseudorandom Flicker Circuit

The circuit is meant to run on 12 volts; if you’ve checked out some other posts you’ll find I’m a bit of a solar power nut.   I figured low voltage is relatively safe for this sort of thing, and I had 12 volts handy anyway from a gel cell, so it works out.  (A wall cube supply would also be great.)  The light also must be 12 volts; I’m just using a small 1/4 amp bayonette bulb I found at an electronics store (it works well in a Jack ‘O Lantern).  The bulb is driven by MOSFET Q1, an IRF720.  It can handle a lot more, probably a couple amps, but you’ll need a heatsink for it.  The bulb I use doesn’t make it heat up all that much. Basically this circuit is kind of like a light dimmer.  You’ll notice it uses three 555 timers, that tried-and-true timer IC that’s been around since forever (the 1970s).  These have lots of pages associated with them scattered around the Internet, so check Google for the fine details.  But basically timer U1 is a PWM light dimmer – we use it to turn Q1 on and off really, really, fast (around 1 kHz), and we change the duty cycle (the percentage of the time it’s on) according to how bright or dim we want the light.

To control the dimmer, we use two other 555 timer driver circuits.  These control it via Q2 and Q3, which add resistance and thus change the brightness.  Q2 and Q3 are controlled by U2 and U3, which turn them on and off.  By doing this at much slower rates (they must not be exactly the same, but close), different levels of brightness are acheived.  Because the rates are different the light cycles through the pattern.  It’s not truely random, but it looks close enough to the casual observer, making it pseudorandom.

Now, you could play with R5, R6, R7, and R8 to get different rates, and maybe make it a little more realistic.  You could even stick potentiometers in there if you wanted to adjust it.  But you’ll probably be fine with just experimenting with different values until you find something you like.  (Again, I won’t go into it too much, as plenty of info is available on the Web.  Also, it’s late, I’m tired, and I don’t feel like thinking about it too hard.)  I suppose you could also build upon this concept and use it to drive something like a thyristor and control an AC bulb.  (Be careful when working with high voltages; you do it at your own risk.)  If the one bulb’s not bright enough, you could also add more in parallel.  They would all flash the same, so it would work.  You could add more dimmers and more driver circuits for different combinations, if you wanted to drive a bunch of pumpkins and not have them all mysteriously flicker in unison.

Or, you could just use a real candle.

Flicker Circuit Preview

Well, the flicker circuit I alluded to earlier is done.  It’s on its own project board, and it does in deed make a light bulb flicker like a candle flame.  Unfortunately, I don’t have a schematic to post at the moment.  I’m not sure if it will make a difference at the moment, especially with Halloween in a few days.  Yes, I’m sure it would be useful for other decorations or whatever, but a pumpkin light is a good application for it.

Well, for those of you a bit more electronically inclined, I’ll explain how it works.  It’s probably not the best way to make a light flicker, but it works for me and is pretty convincing if you mess with it.  First, consider a circuit that dims a 12 volt bulb.  A way this could be done is using a 555 timer to switch a MOSFET on and off really fast – PWM.  So, you change the duty cycle to change the brightness of the bulb (ie, the bulb is dimmer when it is off more than it is on, but it’s still switching on and off so fast you only perceive it as being dim).  I based this part of a circuit off of a light dimmer I found in Home Power Magazine Issue 34 (I think), though you should be able to find something like it online.  (If you are not familiar with Home Power, check it out.  It’s great magazine on renewable energy, though a lot of the juicy electronics type stuff is in the back issues.)

The circuit I mentioned runs at around 5 KHz (I recall; I don’t have the specs in front of me).  Now, think of another 555 timer circuit, this one with a much smaller frequency, maybe a few Hz.  In other words, imagine an LED blinking noticeably.  Now, think of a nother one, but with a slightly higher or lower frequency.

My circuit uses the two 555 timer circuits I just mentioned, as well as the dimmer circuit.  Basically, the two circuits flashing at a few Hz switch resistors on and off on the dimmer circuit, causing the brightness of the bulb to change.  With both circuits going at once, there are a few different levels of brightness the bulb can be at.  Technically the patter is not random, but it does look that way to the casual passerby.  I believe the term for this is pseudorandomness.

Right now I have this working for a 12 volt DC bulb.  For something like a 60 watt 120 V bulb on AC, search around.  There are some other ciruits I found in the quest for finding a nice one for a low voltage bulb.  The MOSFET I used can tolerate I think around 20 amps, though I only use it with a bulb that pulls about 200 milliamps.  If you want to use a bigger bulb a heatsink would be a good idea.

I’ll try to get a schematic up soon.  Until then, maybe someone can come up with a similar circuit.