"...BY THE FLICKERING FIRELIGHT..."
Animation on a model railroad layout can add lots of visual interest. Here's a circuit which simulates the "flicker" effect associated with an open flame; consider the possibilities on your layout:
- A cozy fire in the living room fireplace
- Getting ready to grill steaks on the old BBQ grill
- An open-hearth furnace in your steel mill
- Busy steel fabricators using a arc welder
- A tragic fire in a frame building -- just add fire trucks
- Burning leaves on a crisp fall day
- A campfire in the woods
- A hard-working commercial incinerator
THE CIRCUIT
An LM556 Dual Timer IC is the heart of the flicker circuit. The two timers are configured as low-frequency oscillators, operating at non-harmonically-related rates (this is important to the appearance of "randomness"): the timer on the left runs at approx. 1Hz, where its brother on the right races along at roughly 4.7Hz. We're using a Miniatronics 12 volt incandescent bulb to simulate a flame; incandescents just seem to "flicker" more convincingly that do LEDs. The bulb is biased slightly ON with a keep-alive current provided by resistor Rb. Each oscillator then adds to the bulbs intensity: the slow (left) unit increases intensity when its output is HIGH, causing the NPN transistor to conduct and placing resistor Rs in parallel with Rb; the fast unit adds when its output goes LOW, turning on the PNP and placing Rf in parallel with Rb. Hence, we have four discrete resistances controlling the current thru the bulb (producing four discrete brightness levels):
1. Rb only
2. Rb paralleled by Rs
3. Rb paralleled by Rf
4. Rb paralleled by both Rs and Rf
Since the oscillator frequencies are unrelated, this yields the appearance of a pseudo-randomly-occuring flicker. I've tried this circuit with one clear bulb, one yellow bulb, and one yellow plus one red bulb; the circuit will handle two bulbs in parallel, and the effect of mixing different colors is quite entertaining. Feel free to fiddle, but do start with a single clear bulb just to get an idea of what's going on. If you'd prefer, the circuit can certainly be implemented with two LM555 Timer ICs, rather that the dual unit shown here; no difference either way.
CAN I FIDDLE WITH THE FLICKER ?
As somebody used to say, "You can be a fiddlin' fool" with this circuit...and it's kinda' fun to do. I do suggest one ground rule for the fiddling: change only one component at a time and observe its effect. That said, there are many ways to change the nature of the flicker:
- You can change the extent to which each oscillator affects the bulb brightness by varying the bias resistors (Rb, Rs and Rf); start by doubling or halving a resistor value and see what happens; if the effect is too great, go back and make the change percentage smaller.
- You can change the frequency of the oscillator(s):
- To make the slower (left) unit run a bit faster, try removing Csb (the 0.047uf cap); to make it run even slower, add a 0.1uf cap in parallel with Csb.
- Run the faster (right) oscillator faster yet by adding a 0.22uf cap in series with Cf, or slower by adding a 0.047uf cap in parallel with Cf.
- If you're especially adventuresome, you can even fiddle with the timing resistors on one (or...gasp!...both) oscillators; these are the 1 Mohm and 3.3 Mohm units; again, change sparingly.
- Especially transistor-savvy folks may even want to invert the logic sense by swapping the PNP device for a 2nd NPN...or the NPN for another PNP -- the change in the flicker is quite obvious; just remember to swap the emitter and collector (as shown in the drawing) when going from NPN to PNP (or vice-versa).
- As mentioned previously, you can also try various combinations of two bulbs: different colors, or even different voltages -- eg, try one 12 volt bulb and one 14 volt bulb.
CIRCUIT HINTS & CONSTRUCTION TIPS
1. I urge you to socket the LM556 IC (in a 14-pin DIP socket); solder to the socket, carefully inserting the IC only when all soldering is done.
2. Keep in mind that semiconductor devices of any kind are very heat sensitive; you might want to socket the transistors as well as the IC; I might use an 8-pin DIP socket with one transistor on each side; again, complete all soldering before inserting the devices.
3. Try to locate the "decoupling capacitor" (the 0.01uf unit at the top) as close to the IC as possible; on the back of the socket between pins 7 and 14 would be ideal.
4. All resistors may be either ¼ or ½ watt; all capacitors are 25 volt or greater ceramic types.
5. If you use two 50 or 60mA bulbs, I suggest reducing the size of the two 10 Kohm resistors to 4.7 K ohms.
6. All components (except the Miniatronics bulbs) are available from Radio Shack and others.
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New 2-15-02