The flicker flame circuit presented here was adapted from the one designed by Fred at the Armadillo & Western Railroad Co. It uses a pair of 555 timers to generate two clocks oscillating at different frequencies. The output of these oscillators drive a set of transistors which switch in various current limiting resistors. The intensity of the lamp is dependent on the total resistance applied (ie: the lower the total resistance, the brighter the bulb will glow). Since both oscillators operate asynchronous to each other, the bulbs appear to flicker randomly, much like a fire light does.
Each of the four lamps shown in the schematic uses a different combination of transistors. This ensures that at any given time, the overall brightness of each lamp is unique, giving the illusion that each is controlled by a separate circuit. If not all of the lamps are required, the associated pair of transistors and related resistors can be omitted. The resistors for the timing and current limits and the timing capacitors can be changed as desired.
The timing diagram helps to illustrate how each lamp visually appears different. For simplicity, the diagram was drawn with both oscillators syncronized to each other. However, in reality, the two clocks would drift with respect to each other, making the resulting pattern appear more random.
I am currently using this circuit to drive two campfires and a cabin fireplace in the campground scene of my model railroad. Even though all three "fires" are in close proximity to each other, they appear to be unique.
Below is a table that provides the general formulas for the 555 timer circuit.
th = 0.693(RA + RB)C |
tl = 0.693(RB)C |
T = th + th |
F = 1/T |
where: | th = output high time in seconds |
| tl = output low time in seconds | |
| T = output high time seconds | |
| F = output frequency in hertz | |
| RA = resistor connecting discharge pin to rail in ohms | |
| RB = resistor connecting discharge pin to to threshold in ohms | |
| C = timing capacitor in Farads |
The actual parameters of the two oscillators as they are shown in the circuit is summarized below.
| th (sec) | tl (sec) | T (sec) | F (Hz) | |
| Oscillator 1 | 0.80 | 0.20 | 1.0 | 1.0 |
| Oscillator 2 | 0.14 | 0.07 | 0.21 | 4.8 |
The effective value of the limiting resistors for the values used:
| lefthand transistor | righthand transistor | total resistance | brightness level |
| off | off | 330 | lowest |
| on | off | 132 | 2nd lowest |
| off | on | 77 | 2nd highest |
| on | on | 57 | highest |
Note that the NPN transistor is turned "on" when the oscillator output is high, and the PNP transistor is "on" when the oscillator output is low.
This page was last updated on Dec 12, 2008
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