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Light Control – the DMX system

From LED flashers to DMX Lighting

Programmers starts with “hell world”. Beginners in electronics start with light flashers.  Arduino does too! Although we boast about this being an analogue sight be mindful of the fact that digital is with us and won’t go away. It has made things smaller, faster and more accurate.  Audiophiles don’t like the nasty little bits (pun intended) added in between but high end ADC and DAC chips really make a difference.  So we have this great sound system and now we want to party. Ever party without lights?

For the younger generation

Amplifiers, loudspeaker and loud music has always been around,  but lighting was primitive. Lights were manually controlled, sometimes by servo but the local church hall definitely did not have this – we had the coffee can approach, lowering, tilting and panning all done manually by possibly using draw wires or some or other trick. Digital?  Bah, that was for sissy’s.  Along came the heady 80’s and of course this was also around the time that manufacturers realised that a computer could be used for other things besides arithmetic and nuclear tests. This also around the time that United States Institute for Theatre Technology (USITT) wised up and cracked the code.

Chauvet Helicopter Q6
The popular Chauvet Helicopter Q6 – DXM controlled lighting display.

Before moving on I suggest you have a look at the Grateful Dead’s “Wall of Sound”, a marvel at the time and boasting the best sound system in the world. Have a look at some of the image files and think of what this might have looked like at midnight without any stage lighting?

Great sound is one thing but lighting is imperative to create the “mood”.  A quick word of caution here though – the rapid flashing on and off of light or lights,  called strobing can cause convulsions in people prone to epilepsy.  This is usually around the frequency of 8Hz to 20Hz. Dedicated strobe lights are usually high intensity Xenon tubes. Large disco halls and live bands rarely warn their audience of the potential harm – so be warned.

LED and Bulb Flashers
LED and Bulb Flashers

The two circuits above are very common. The NE555 was included because it is one of the most versatile timing chips ever invented (which excludes to 556 which is a dual 555 timer IC).  The NE555 application notes can be found here.

The LED flasher is not as versatile because it is fixed frequency, timing being set by capacitor 6.8uF and the 1K resistor.  The NPN will switch on when power is supplied pulling the base of the PNP down causing the transistor to switch on causing a rise in voltage across the 390 collector resistor switching off the NPN though the RC network.  I had this circuit jotted down somewhere and believe the 9.1V zener was used to stabilise the oscillation frequency due to auto use. The more common circuit however is that of the 555 timer IC and driver transistor which can also be used to switch a relay. (note that a diode would be needed reverse biased across the relay energising coil to prevent back EMF from damaging the transistor).

Low Voltage SCR sound to light
Low Voltage SCR sound to light

The circuit above is a very popular one and shows the most basic of audio modulated SCR light controllers. Mains controlled circuits should not be built by a novice. Many of the circuits on the internet are downright dangerous. The circuit above was originally a 220V powered light flasher with the audio secondary connected to the 10k pot without any indication of which part of the circuit was Live and Neutral.  The cathode side was also grounded. In some areas Neutral is bonded to ground which defeats the object of the circuit and what makes matters worse is if the anode was to be connected to Live then the SCR would trigger into a mains short.  An electrician will also remind you of the hazards that if the circuit were to be connected properly what would happen if the Neutral lifts. Absolutely insane!

The circuit above, in it’s primitive form will work fine with an audio input strong enough to drive the gate of the SCR positive to the cathode. The variable resistor will control the sensitivity and the 2.2uF capacitor will control delay.

A circuit such as this will work best with low wattage 12V bulbs – the lower the wattage the quicker the response.

Important note:  An SCR, once triggered will remain on. As the mains cycle (or from transformer secondary) is fluctuating through zero it will force the SCR to switch off. The circuit above unfortunately will cause noise because in a properly designed circuit the SCRs trigger in synch with the mains voltage cycle starting a positive going excursion.  Sounds intimidating – it is. For that reason we have zero voltage switching, a technique used to reduce switching interference and enhances lamp life.

SCR Sound to Light - colour organ
SCR Sound to Light – colour organ

The above circuit is more in line with what one use at home for parties or after smoking a joint. 🙂  There are some  things one needs to know of course.  The audio isolation transformer was originally a 1:1 transformer but I had doubts about the insulation as these were designed as driver transformers.  I had safer results with a 500mA 220V/12V transformer using the secondary in the emitter circuit of the BD139.  You will need to add a potential divider chain across the output to reduce the trigger voltage – this is an old circuit copied from the net somewhere so more or less serves as a guideline only because most of us will want to increase the load capacity.  The TIC106C is known as a Silicon reverse blocking triode SCR and trigger at very low voltages hence the caution given to the isolation transformer. I will not advocate any direct coupling between amplifier and colour organ unless there is a double insulated transformer or reputed mains transformer used as the isolator. Ground the case.   These circuits can be deadly if proper electrical isolation practices as not followed.

The circuit can be modified to use the BTA41/600. I no longer have the circuit so please don’t ask for it – these circuits are for learning only. Experiment at your own risk.

DMX and Intelligent Control

Over the last few years lighting, or rather the intelligent means to control lighting through what is termed the DMX controller. The most common DMX or Digital Multiplex control system is the DMX512 which is an asynchronous system communicating in 8 Bit data packages at up to 250 000 times per second.  Common usage is for intelligent light control at clubs and live events although as the price has dropped it has become popular even in the consumer space.

Most DMX lighting systems can be purchased at the professional audio houses – lighting for supergroups and large capacity crowds cost upwards of a million Rand.

Continued – how DMX Lighting works


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