Lighted Fan Strobe

 

The mod(s) are based around a small circuit which basically flashes LEDs on and off. It is a stroboscope, somewhat similar of the type used to freeze movement in discos, night clubs etc. but designed to operate LEDs instead of high power xenon tubes. A myriad of lighting effects can be obtained simply by adjusting the LED flash rate. More on the circuit later, lets have a look at some of the effects it can produce. For demonstration, I am using the strobe circuit to operate the LEDs on the excellent PCMods Lighted fan. Of course any LED lighted fan can be used. The first picture is of the fan running normally without the circuit connected.

PCMods LED fan - normal running

Now with the circuit connected to the fans LEDs. These next few shots show just four of the almost infinite variety of effects possible. A still picture of a lit fan spinning at several thousand RPM cannot convey just how impressive and spectacular some of the effects really are. These stills are taken from a video made with my digi-cam using "movie-mode" so the quality does leave something to be desired. In reality the LEDs appear much brighter with no motion blur. (Due to slow shutter speed). The video is included later in the article.

Video stillVideo still
Video stillVideo still

Here is the strobe circuit. You will notice it is not just a simple "555" timer based flasher circuit. For a strobe' to freeze the action or in our case, produce the lighting effects the pulses of light must be very brief. In a normal xenon strobe, these light pulses are measured in milliseconds, (1/1000 S) or even less! Just using the usual "555" timer circuits to flash the LEDs would produce pulses which would be too long, producing "motion blur" and spoil the effect.

Unfortunately LED output falls with such very brief pulses and produces insufficient light for our purposes, so a compromise is needed between pulse duration and getting enough light to achieve the desired effect. After considerable experimentation a suitable combination of sufficiently brief pulse time and LED output was found. That's were the extra circuitry comes in...

The circuit is divided into four sections. An oscillator based on a "555" timer chip, (IC1), which has a variable frequency range of 1.4KHz - 50KHz, (measured). The output from this timer is fed into a divide by 100 stage, (IC2 & IC3), which produces two outputs at 1/10 and 1/100 of the frequency of the oscillator. These two outputs are "ANDed" by IC4 to produce a pulse train that is 1/100th frequency of the oscillator and has a off/on ratio of 99:1. This output is then fed into the LED driver, IC5 to flash the LED(s).

The unused inputs of IC4 are tied high to prevent spurious noise in the circuit. I recommend the use of a 10 turn or preferably a 20 turn preset for VR1 to make flash rate adjustment easier and more precise.

Click for bigger image

The output is a stream of very brief spikes, which produces short, high intensity flashes from the LED.

Showing brief output pulses

A point worth mentioning is the LED series resistor, R5. It has a very low value, much lower than would normally be expected. This produces a high peak LED current which is needed to achieve sufficient light from the LEDs during the brief flash time. Since the LED is on for such a short duration, the average current and power dissipation is within limits and no harm comes to the LED. Of course using such a low resistor value in normal use, the LED would fry itself very quickly!

When testing the circuit for the first time, I recommend adding a 150 Ohm resistor in series with the LED just in case there is a problem with the circuit causing the output to latch fully on. Without this resistor the LED would be instantly destroyed! Better safe than sorry! Once satisfied that the circuit is working correctly, this resistor can be dispensed with.

The circuit diagram shows only one LED but more LEDs can be added to the circuit as shown below to increase the light output. Alternatively, the single LED can be replaced with an LED cluster.

Adding more LEDs

Here is one possible circuit layout using stripboard. If you choose to copy it, take careful note of all the wire and solder bridge links and the breaks in the tracks. I used standard 4 pin PSU Molex connectors for connecting to the supply and to the fan LEDs.

Board top side - click for larger imageBoard underside - click for larger image

For those with PCB making facilities I have included a design for a single sided PCB.

Home made PCB

PCB component layout Bottom foil - Viewed from above.
PCB component layout - Click for larger imagePCB copper tracks - Click for larger image

NOTE: The PCB foil is quadruple size and needs to be reduced to 25% when printing. This ensures a sharp image for developing. Use Paint Shop Pro, Photoshop or your favorite picture editing package to resize the pic since it does need to be accurate.

The PCB circuit has been changed slightly to make the layout design easier, but electrically the the circuit is identical. The stripboard version was made first and I didn't feel it was necessary to rebuild it. If you build the PCB version, it doesn't matter anyway since all the components have their own place and there are no links to worry about.

Circuit modification for the PCB layout - Click for larger image

I suppose you are going to want a parts list with Maplin stock codes...

R1 = 10K = 10,000 Ohm (M10K)
R2 = 4K7 = 4,700 Ohm (M4K7)
R5 = 4R7 = 4.7 Ohm (M4R7)
VR1 = 500K or 470K 20 turn preset (UH28F) Maplin do not stock a 470K multi-turn so I have specified a 1M 22 turn type.
C1 = 100uF 16V (VH13P)
C2 = 100nF (DT98G)
C3 = 1nF (DT92A)
IC1 = 555 timer (QH66W)
IC2, IC3 = 4017B (QX09K)
IC4 = 4011 (QX05F)
IC5 = ULN2803 (QY79L)
LEDs to suit. The brighter the better.
Male Molex + Pins = (JW64U)
Female Molex + Pins = (JW65V)

Cost of above components is about £5 excluding LEDs, wire and board etc..

To use the PC mods fan involved a minor mod - shorting out the LED resistors. There are four LEDs and each have their own series resistor. NOTE: After doing this mod DO NOT try running the LEDs normally from a direct 12V supply. They WILL be destroyed! This mod is for use only with the strobe circuit.

Here I have treated all four LEDs as one, they are in fact now all connected in parallel and share the same single 4.7 ohm resistor shown in the circuit diagram above.