Cathode Throbber


I have been meaning to publish this mod for a long, long time and now I have finally got around to actually doing it!

I call this circuit the "Cathode Throbber". It makes cold cathodes, (CCFLs), repeatedly fade up and down in brightness, giving a throbbing effect. With careful adjustment of the controls other unusual effects can be created. More of which later.

The circuit is based around a common quad op-amp chip, (LM324N), and two power transistors. It has two outputs, as one brightens the other dims. You don't have to use both outputs if you don't want or you only have one CCFL.

IC1a and IC1b form an oscillator whose frequency is set by VR1. The triangular waveform output is fed into IC1d which controls transistor T1, which in turn controls the first CCFL. VR2 is a level shift control which changes the output from IC1d, allowing control over the throbbing effect.

The output from IC1d is also fed into IC1c, which inverts the waveform which, along with T2 forms the second output for the other CCFL. IC1c also has its own level shift control, VR3. Note adjusting VR2 affects both outputs whereas VR3 only effects output 2. It follows that VR2 should be set first to achieve the desired CCFL1 effect and then VR3 to set up CCFL2.

If desired, a single pole changeover switch, (SPDT or SPCO), may be fitted to the outputs as shown below. This allows the CCFL to be "Throbbed" or run continuously. If the switch has a "centre off" position, the CCFL can also be turned off.

You're going to want a list of parts used so here they are along with Maplin stock codes in bold.

R1, 2, 4, 8, 9 = 47K (M47K)
R3 = 1K (M1K0)
R5 = 100K (M100K)
R6, 7 = 12K (M12K)
VR1 = 470K horizontal preset (UH08J)
VR2, 3 = 47K horizontal preset (UH05F)
C1 = 47uF 16V (VH11M)
IC1 = LM324N (UF26D)
T1, 2 = MJE3055 (QH56L) (This is actually a TIP3055)
Switch (optional) = (FH01B)
Various connectors to suit and wire to hook up the circuit.

I built the circuit on a small home made PCB but it can easily be built on stripboard. The PCB layout, top and bottom foils are shown below. The top foil is not necessary, I just used it because I had double sided board at hand and disliked the idea of wasting the copper.

The layouts are shown DOUBLE SIZE and need to be reduced to 50% when printing them. This helps to maintain a sharp image for exposing pre-sensitised PCBs.

PCB connectors

Once the board is connected and powered, adjust VR1, (speed), and then VR2, (output1), for the desired effect. Once VR2 is set, adjust VR3, (output2). The speed can be freely set without having to readjust either output controls.

Finished PCB close up

I have included a selection of videos to give an idea of some of the effects that can be achieved with the circuit. Unfortunately I do not have a real digital video camera and so have resorted to using my digi-cam in "movie-mode". The results, whilst better than nothing, are really rather poor, especially those showing the "Macro Black" case. You will just have to take my word for it that the effects are a million times better that the videos show.