The earliest strobe tuners had used neon lamps to illuminate the strobe disk. In the 1950's they were the only electric source of light capable of being modulated with fast audio signals. Today, the obvious choice is a LED. As I planned to make the strobe disk quite large (~120mm diameter), I decided to use a 1W (350mA) power LED. Not to be confused with Peterson's reddish-orange, I picked my favourite colour: deep, InGaN green.

It simplifies things a lot that the LED intensity depends rather linearly on the junction current. To modulate the LED brightness all that we need is a current source, made out of an op-amp and a power transistor. Another two op-amps amplify the audio signal, and the remaining fourth op-amp is used to generate 1/2 Vcc voltage to bias the amplifying stages and the electret microphone.

Zener diode D1 limits the maximum instantaneous current source control voltage (rather non-linearly, but hey, it works).

VR3 potentiometer adjusts the level of gain, and VR4 controls the LED idle current. To be honest, after some testing I found I could do without VR4..

..but hey, one never has too many knobs and buttons.