Electronic Flame Flicker This device is strictly for incandescent lamps. It connects in series with the lamp and modulates the RMS voltage across the lamp. It can't increase the voltage above the input line voltage, so the average voltage at the lamp is of necessity less than the line input. It will not work with a lamp fixture that contains a built-in dimmer or any other electronic controller between it and the load. Flasher bulbs are allowed, but they probably won't flash because of the reduced RMS voltage. Transformer-powered lamps must not be used because this device may present a substantial DC or very low-frequency AC voltage component across the load during startup. How it works (see Schematic): BR1 converts the AC to 120 Hz pulsating DC on which the whole system operates. In Dimmer mode, S1 is open and the circuit is a fairly conventional lamp dimmer using an SCR instead of a Triac. R1 is at minimum resistance and Q1 & Q2 form a voltage regulator using Q1's Emitter Breakdown Voltage (BVebo) as the voltage reference. The voltage presented to PUT1's gate is nominally around 8v. C3 charges through R5, R6 and R7 until PUT1's gate-anode junction becomes forward-biased. PUT1 discharges C3, and SCR1 switches power ON to the lamp, shorting out the pulsating DC supply to the circuit and preventing C3 from beginning a new charge cycle until the SCR turns OFF at the next zero crossing of the AC line. Except for the voltage regulator, this is pretty conventional. D1 precharges C2 to about half the nominal operating voltage. In Dimmer mode R11 continues charging it to the average voltage at the junction of R5 and R6, to reduce the time required for settling if you switch from Dimmer to Flicker mode. After that, D1 never conducts again and has no further effect. In Flicker mode, S1 is closed and C2 is connected into the dimmer circuit. Now C3 doesn't wait for the next zero crossing to begin a new charge cycle - C2 filters out the pulsations, maintaining a fairly steady voltage at the junction of R5 and R6 to keep the process running continuously. PUT1 is now a free-running relaxation oscillator whose frequency of oscillation varies with the voltage across C2. This voltage is controlled by the R7 setting and the average voltage across SCR1 and thus by the phase of the triggering relative to the AC line cycle. If the oscillator runs slightly above twice the AC line frequency, the SCR triggers progressively earlier in each cycle, reducing the average voltage across it and thus the voltage across C2, which lowers the oscillator frequency. This feedback creates a phase-locked loop (PLL), locking the relaxation oscillator to the 120 Hz ripple frequency. C1 degrades the loop damping by making the voltage at PUT1's gate slightly dependent on the OFF time. Any phase disturbance causes the oscillator phase to wobble at the loop resonance frequency for some time before damping out. This is what makes the lamp flicker. The phase disturbances are created by applying noise to PUT1's gate. Q1's emitter junction generates random noise and Q2 amplifies it. C1 provides low-impedance AC coupling of noise source to noise amplifier. R1 adjusts the gain and thus the amplitude of noise injected into the PLL. The flicker comprises both the noise input and the PLL's response to it. The former is a small rapid random fluctuation in brightness, and the latter is a much slower oscillatory response. Since operation involves uncontrolled device characteristics, you might want to experiment with different transistors for Q1. Most silicon transistors have BVebo's in the 6-8v range, which is fine for this circuit. You can use a PNP, e.g. 2N3906, if you swap the Emitter and Collector leads, since the base is not connected. Make sure you have the polarity correct, however, because the voltage ratings of several components will be violated if no Zener action occurs. Customization:

If you wish to make an array of electric candles flicker independently, you need a controller per bulb. Since the the most expensive components are potentiometers R1 and R7 (R12 is not needed for candles), you'll probably want to replace them with fixed resistors once you've determined how you want your settings. Unless you pick settings that cause very slow PLL lock-in, you can omit D1 (no connection). Also leave out R11 (since S1 becomes a jumper) and R10 (leave the bottom of R7 open) and increase R6 if necessary to get the desired setting within range of R7. Verify that you get proper startup with your chosen settings. Then measure R6 + R7 and replace them with one resistor. If you purchase all the transistors at once, chances are good they'll all come from one production lot, which improves the chance that the same resistor values will give like results on all units. Although the candles flicker independently, they all respond to power line events – a bit like real candles to air movement. See photos for construction of a kerosene lamp simulator for a porch light or lamppost. Aesthetics: The most enjoyable effect of the flickering light is in the appearance of things illuminated by it. A translucent lampshade or a nearby wall enhances the visual effect by creating a halo that expands and contracts with the varying light intensity. When I use the flicker circuit on Christmas candles, I make sure there is something behind the candle to show the halo. A translucent curtain will show it on both sides. A piece of paper in front of the bulb also makes adjustment easier. If used in a lamppost, some kind of decorative glass would enhance the visual effect. For higher power, several small bulbs are better than one big one because the small filaments are more responsive. Operation: Rapid flicker is the direct contribution of the random noise generator. Slow flicker is the oscillatory response of the PLL to the random noise. It increases with decreasing damping. R1 in all versions affects both the rapid flicker and the damping. With R12 (if present) fully CW, full CCW position of R1 yields minimal damping but also very little noise input to the PLL, so there is little flicker unless the PLL becomes unstable or responds to a power line event. Advancing R1 a quarter-turn yields significant noise disturbance and still fairly low damping, so lots of slow flickering. At the fully CW position, there is a lot of noise input and significant damping, so the light shows a lot of rapid flicker and much less slow flicker. In the Campfire version, R1 is designated Flicker Balance and there is a separate Flicker Level control, R12. Both R1 and R12 affect both the flicker level and the damping. A candle's or kerosene lamp's flicker is mostly at one frequency, modeled as “slow flicker” although it is rather fast. A campfire shows more rapid flicker than a candle because of air turbulence, and the slow flicker is slower. A campfire also has more range of expression in its appearance. To maximize the slow flicker and minimize the rapid flicker, set R12 fully CW and adjust R1 to set the level. To maximize the rapid flicker and minimize the slow flicker, set R1 fully CW and adjust R12 to set the level. If you will be turning the system off and back on, make sure it starts up and settles with the settings you've chosen. Settling, even with the precharge circuit, can take a while (see Quirks). You can change C2 to alter the slow flicker frequency. It also varies with R7 (Brightness).

Brightness control R7 also affects the dynamics. Damping is minimum and flicker is maximum near half-brightness because the SCR is triggering near peak line voltage. The RMS voltage to the load and the feedback voltage are thus most sensitive to a given timing change. Quirks: In Flicker mode, the PLL may go out of lock (oscillate at the wrong frequency) if R7 is adjusted very far from half-brightness when damping is very low. When out of lock, the lamp will flash on and off rapidly instead of flickering. As you adjust R7 back towards the half-brightness position, a point will be reached where lock-in occurs. The flashing decreases in frequency (slowly in the campfire version) and then stops and and the brightness wobbles around an average level. In all versions, there is a brightness setting below which the PLL will not lock-in at startup, although it will stay locked if brightness is slowly turned down past this point after startup. The closer the brightness setting is to this point, the longer it takes to lock-in. If damping is very low, there is also a maximum brightness setting for lock-in at startup. Even with the precharging, startup of the campfire version involves several seconds of out-of-lock behavior if power is switched on with the device in Flicker mode. The lamp may not even light for a few seconds. In an application such as theater where it is desired to switch the lamp on at a particular time without seeing the effects of the startup process, let the system stabilize with a small out-of-sight lamp load and leave it running, dimmed if necessary. Connect the final load on cue, leaving the small load connected. The disturbance when the additional load is connected is small unless the damping is very low. Make all adjustments slowly to avoid overshoot or instability in the brightness. R11 lets C2 charge to nearly its final voltage with S1 open. Thus if the system has been operating in Dimmer mode for a minute or so, it will take less time to settle out when switched to Flicker mode. Parts List Notes: R10 is a “select on test” component. In Dimmer mode, you may find that as R7 is turned CCW the lamp gradually dims out and then suddenly jumps to about half-brightness. The jump occurs when the trigger delay becomes so long as to skip alternate half-cycles. If this happens, R10 should be increased. If the lamp isn't nearly extinguished with R7 fully CCW, R10 should be decreased. “Lamp Load” refers to the total bulb rating in watts. Actual power in operation will be less than that due to the reduced RMS voltage. The SCR should be heatsinked for Lamp Loads above 100W. If you want to build a version that can be switched between Candlelight, Kerosene Lamp and Campfire modes, build the Campfire version and provide a means of selecting C2. For controlling a single Christmas candle, a small TO-92 packaged SCR is used. Since it costs less than the fuse required to protect it (even without the fuse holder), F1 is noted as optional. The fuse is mainly to protect SCR1 and BR1 when a filament fails and a short piece of it bridges the terminals, briefly drawing many times the normal current.

R6

220 K

R5

270 K

Q2

2N3904

C3

39 nF 25 v

R1

5 K

R2

47 K

SCR1*

See P/L

R4

470 R

cw

cw

PUT1

NTE6402

R8

100 R

R9

47 R

F1*

See P/L

_

+BR1*

See P/L

SO1

PL1

C2*

See P/L

25 v

R3100K

D1

1N4148

* Campfire version shown

C11.5 uF 10 v

S1

Open @ R1 CCW

R11

1 Meg

R12*500 R

CW

Q1

2N3904

R7

1 Meg

R10

390 K (nom.)

D evice P inouts

N TE6402 FS 0102D A

C 1 0 6 D G

K B P C 1 0 0 2 P

A K G A K A GKG

GG

K A

(P U T)(S C R )

Inside view of the model in the title photo. R12 is under the perf board. This was the Campfire version before I tried the NTE6402, and it's different from the schematic. The clip-on heat sink is on a TO-39 packaged SCR rated 1 amp. The socketed device is Q1. Even with the lid on, this model handles a 100 watt lamp load comfortably.

Kerosene Lantern model made for front porch or lamppost light (outdoors but enclosed):

Junkbox parts include the base from a defunct compact fluorescent lamp and part of an old-fashioned fuse socket. Other old lamp sockets may also be suitable. The center terminal of the socket connects to the center contact on the base, and the circuitry is in the path from the shell of the base to the shell of the socket. There's no fuse, but the SCR is way overrated for the lamp. Ignore resistor values in these photos. Some are non-critical, but the values in the Parts List are appropriate for the PUT specified, which also has a different pinout from the one shown here.

Disassembly of the CFL, and an old fuse socket similar to the one used. The CFL comes apart with the aid of a hacksaw, wire cutters, pliers and a soldering iron. This was a MaxLight (TM) unit - others may differ. Leave the fluorescent tube attached to the top of the base and cut the leads from the tube to the printed circuit board. The PCB was retained by a cylindrical extension of the top, now cut, which was glued inside the bottom. This must be removed to extract the PCB. I chipped part of the bottom off along with it, but if you can separate it from the bottom you'll preserve more shell to hide the circuitry. Don't do what I did and pull the white wire loose from the screw base! Instead locate the place where the wire is crimped under the side of the screw base – there will be a slight distortion of the aluminum there, visible in another photo. The wire is soldered to the PCB near the edge. You probably won't have to heat too many solder pads before finding the end of the wire. If possible, unsolder the other wire from the tip of the screw base first. Soldering a “handle” to the PCB might help. The PCB is keyed to prevent rotation.

Socket with wire attached for center terminal, wrapped with electrical tape except where the bridge rectifier lead will be soldered to it; “haywired” electronics assembly; CFL base with drilled hole. The dark spots on the inside the white base are evidence of how I learned that a faucet washer is a really bad insulator. I suspect it's filled with graphite.

Partially assembled; top bridge rectifier lead will be soldered to the side of the lamp socket. Bottom lead is inserted through the drilled hole...

...and it comes out here. Connecting it to the aluminum base must be accomplished without melting the plastic. The wire was secured with a rubber band for the last photo following. I'm experimenting now with conductive adhesives.

Note the slight distortion of the aluminum threads just below the wire, where the original white wire from the electronic ballast was crimped. Preserving this wire would have avoided the need for an adhesive.

Socket in place, other bridge rectifier lead soldered to it. The socket is glued in place and wrapped with electrical tape except where the bridge rectifier lead is soldered. The electronics assembly fits rather snugly in the plastic shell, but is really anchored only by the rectifier leads. It should be secured with a dab of hot glue if there's any possibility of movement. Remember that although the socket is wrapped with electrical tape, the base of the light bulb won't be. Avoid having any conductors in the C2, C3, R5, R6, PUT1 anode circuit touch the plastic base. Condensation there could prevent the PLL from starting properly.

A 40 watt appliance bulb makes a nice lamp and reduces the excess length compared to regular bulbs. Brightness is set for a somewhat yellowish glow, like a kerosene lamp. A plastic cover over the exposed parts will be the finishing touch.