Audio-Light Modulator

Audio-Light Mod

An audio-light modulator is a funny device that controls the intensity of one or more electric lamps in response to the audio signal input. Such a device is ideal to give the rhythm to any party or disco, and is very safe to use as it usually provides perfect galvanic isolation between the audio signal and the mains voltage (otherwise any fault in the high voltage section of the device could completely destroy the low voltage section and give a fatal shock to anyone operating it).

Nowadays, you can easily buy cheap and simple audio-light modulator kits (also known as music-to-light modulator kits) from many online electronics storefronts (see below). Almost all of them have a phototriac to drive a regular triac which further drives the high-voltage lamps wired through it.

AudioLight Kit
Following is the online schematic of a widely known and esteemed music-to-light modulator do-it-yourself kit. See it is utterly simple and self-explanatory!

Audio-Light Modulator Kit Circuit
Nevertheless, below you can see a bit different version of the audio-light modulator circuit developed recently by me. This oddball version obviously has a few more electronic components but that is with a purpose and intent – I will explain that in a new post.

Audio-Light Modulator v1

The first part of the circuit involves a low-current, low-voltage dc power supply comprising two resistors (R1-R2), one regular diode (D1), one zener diode (ZD1), and a buffer capacitor (C1). This part is directly connected to the AC230V input to derive a regulated 12VDC output.

The last part is a sensitive-gate power triac BT136-600D (T1) which is able to drive AC230V lamps rated up to 4A. For small lamps, it might be okay to operate the triac in free air, a suitable heatsink is recommended though.

Now to the middle part of the circuit i.e., to the core of this audio-light modulator design. Since I’ve plans to enhance this crude design later, an analog optocoupler is preferred here rather than following the traditional concept of using a phototriac. So, my quick pick is an available Vactrol (LCR-0202) which I already tried in a few little projects before. The point of the Vactrol is to receive audio signals from an external audio source as well as firing the triac through its photocoupler mechanism. Given that resistor R3 has a value of 1KΩ and the positive rail is 12V, VT can fire T1 aright.  When the LED inside VT gets illuminated by an audio signal input, the 12V goes through R3 and the internal photoresistor of VT to fire T1. This makes a flash in the connected AC230V lamp(s). It must be noted that gate trigger is characterized in terms of gate trigger voltage (VGT) and gate trigger current (IGT). The gate trigger current varies with the gate pulse width, and it is evident that there is a minimum gate charge needed to trigger the triac (more on this later).

Going to resistor R0, it ensures that the Vactrol won’t be wrecked by an accidental overdrive. The recommended input current of the LCR-0202 Vactrol is 0-20mA (40mA max), and the recommended input voltage is 1.60-2.5V. You have to do your homework well to set the correct value of R0, naturally grounded on your specific application. Although some research or testing may be required, I think you should be able to do that.

Even though it is a very bad practice, I rigged up my test circuit on a standard breadboard as shown below. More work will be done soon to refine this.

BT Music Test Breadboard
And then, simply tapped the “line-level” output (dc-coupled on purpose) from a cheap Bluetooth speaker to feed it (R0 = 680Ω). The electric lamp used is an AC230V refrigerator bulb I have in hand at that time. Believe me, the cute yellowish light reacts fabulously well to every music I feed (sorry for excluding the quick test movie)!

Test Lamp

Side Note: An increasing number of audio interfaces now provide dc-coupled inputs and outputs to enable them to be used with dc signal control voltages. The dc-coupling is a circuit arrangement that allows both ac (e.g., audio) and dc (e.g., control voltages) to pass into (or out) of an amplifier or other circuitry.

Going Further…

Yeah, at this time first phase of this little project is complete! Oh, I hear that question – what is next? Perhaps, the next idea is to build a mighty multi-channel music visualizer.  My analog idea so far is to separate the input audio signal into separate frequency bands and then light each lamp channel up correspondingly. The trickiest part is the design of suitable filters (for different frequency bands) to control the multi-channel lamp driver. There are many ways to do this, the easiest way is to employ op-amps together with some passive components. Looks interesting too?

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