Originally published by Paul Stenning in Everyday Practical Electronics, October 1995

• Diagrams and Schematics

Recently, while rummaging in the loft, the author found the circuit diagram of one of his favourite home-made lighting effects, a large VU meter. It dated back to when he and a friend ran a small mobile disco. The friend was the DJ, while the author dealt with the technical side, building most of their equipment to keep costs down. Feeling prompted to rebuild it, bringing it up to date in the process, resulted in the Ginormous VU Meter!

Seven lamps were controlled by the original unit, using individual comparators, whereas the design presented here controls ten lamps and utilizes a bargraph driver IC. The effect is emphasized by reducing the difference between the bottom and top lamps, giving a rather more dramatic effect than a conventional VU meter. In addition the scaling is linear, rather than the logarithmic scaling normally used in real VU meters.

The Effect potentiometer on the controller sets the level in a similar manner to a recording level control. Normally this would be set so that the top light illuminates on the loudest peaks in the music, however this may be turned down to give a more romantic atmosphere when playing slower records. Once the desired effect is obtained it will be maintained by the automatic level control circuit, despite variations in volume and music style. This allows the DJ to concentrate on playing the music rather than fiddling with the lighting effects.

Low cost home "Disco" lighting effects use a built-in microphone to pick up the sound. This arrangement is prone to picking up extraneous noise unless it is placed very close to the speakers. This unit connects directly to the speaker connections of your amplifier, eliminating these problems.

A small mains transformer is used to safely isolate the amplifier from mains circuitry. The input impedance is about one Kilohm which will impose no significant additional load on the amplifier. A high power amplifier is not necessary - by changing one component value the output from a domestic stereo system at a sociable volume can be accommodated.

The lamp outputs can drive up to 250W of lights each, giving a total of 2.5KW! In practice much smaller lamps would be used, 25W per channel being typical. Suggestions for constructing a suitable light box are given later. Zero crossing control is used to minimize radio interference. Note that the outputs are only suitable for resistive loads such as normal light bulbs. Inductive loads such as pin-spots and similar lights containing transformers are not suitable.

WARNING. THIS PROJECT OPERATES AT LETHAL MAINS VOLTAGES. IF YOU ARE IN ANY DOUBT ABOUT YOUR ABILITY TO CONSTRUCT IT SAFELY, PLEASE SEEK ASSISTANCE FROM A SUITABLY QUALIFIED OR EXPERIENCED PERSON. THIS PROJECT IS NOT SUITABLE FOR BEGINNERS.

How it Works
X1 is a small mains transformer and is used to safely isolate this circuit from your audio amplifier. Audio matching transformers are available but these are not generally designed to isolate mains voltages, and are normally more expensive. Although a mains transformer does not have a particularly flat frequency response, it is good enough for this application.

In addition the transformer reduces the signal to a more manageable level. The signal from an amplifier delivering hundreds of watts could be about 30 to 60V RMS, which is excessive for op-amp circuitry. The transformer has a turns ratio of about 10:1, which reduces these voltage levels to a more manageable 3 to 6V. On a domestic stereo amplifier producing maybe between 2 and 10V RMS we would still have between 0.2 and 1V.

R1 and R2 form an attenuator to reduce the signal still further, to a level suitable for the input of IC1. The value of R1 can be adjusted to suit the audio power levels that may be encountered. 470K is ideal for domestic use if you wish to remain on speaking terms with the neighbours! If other members of the household do not appreciate your taste in music, you may need to reduce this to about 220K. On the other hand, 2M2 is about right for use with a 100W power amplifier at high volume.

If the value of R1 is too high there will be insufficient signal for the automatic level control to operate, and the effect will vary as the volume is adjusted. If the value is too low, the automatic level control will be saturated, and all the LED's will remain on with virtually no variation. The automatic level control has a wide acceptance range so the value of R1 is not at all critical.

IC1 (CA3080) is a transconductance amplifier. The current gain of this device is controlled by the current flowing into the control pin (pin 5). R4 is the output load, which converts the output current into an output voltage. C15 reduces the impedance of this load at higher frequencies, reducing noise and giving some high frequency roll-off. R2 and R3 bias the two inputs to the mid-rail supply.

If you are experimenting with the CA3080 device, please note that it can easily be damaged by overdriving the control input (pin 5). This input is connected directly to the base of an internal transistor, and connecting it to a voltage above about 0.6V or allowing more than about 5mA to pass into it will destroy that transistor leaving you with a dead IC. I killed two IC's while developing this circuit!