Absolute value amplifier LM324
An absolute value amplifier is a device from which we can obtain the absolute value of a voltage whether it is positive or negative - supplied to it.
The absolute value amplifier is used for full-wave rectification of AC signals.
Take a look at the schematic and see how it works. U1 is used as an absolute value amplifier. U2 works as an astable multivibrator that generates the AC signal used in this experiment. This astable multivibrator is a square-wave-oscillator.
Figure 1 shows the relationship between its waveforms and the LEDs:
waveform B is for LED 1, C for LED 3 and F for LED 2.
When you finish wiring, turn power ON. You'll see the three LEDs begin flickering synchronously. Each time LED 1 and LED 3 take turns lighting, LED 2 also lights up, and this how the absolute value amplifier works.