Build and test a common LED visual effect circuit.
The “Breathing Lamp” is commonly found from sellers on ebay and aliexpress as a cheap parts kit, for example: M126 Blue Led 5MM Light LM358 Breathing Lamp Parts Kit
So what exactly is a “Breathing Lamp”? That’s what I wanted to know! It turns out to be quite prosaic: the circuit is just an op-amp oscillator controlling the intensity of an array of LEDs.
Most of these kits are based on an identical circuit and use the two op-amps from an LM358. The circuit diagram is published by most sellers.
But at first glance, the way the op-amp oscillator is wired looks a bit strange - so the primary purpose of this build is to understand how the oscillator is designed.
Note that in this build, I’ve used 2 quarters of an LM324 op-amp, since I don’t have an LM358 on hand.
A sample trace confirms that the two op-amp units are operating as a square wave multivibrator integrated to a triangle wave. The triangle wave is used to control the LED array (via Q1 BJT). The duty cycle is far from 50%, but this probably enhances the effect.
The lower trace is the square-wave output of the first stage (pin 7), and the upper trace is the triangle-wave output of the second stage (pin 1).
What’s a little confusing is the fact that C1 is performing double-duty in both stages of the oscillator. I haven’t exactly untangled it all in my head yet, but its a curious thing that it works.
It also seems this could be simplified by reconfiguring as a comparator–based relaxation oscillator, and only require a single op-amp unit. I might try that later…
From a parts cost and count perspective, I’m not sure there’s much advantage in using an op-amp oscillator over, for example, a 555 timer-based triangle wave generator.