Shift the voltmeter to pin 6 of IC4 and trim VR2 such that the voltmeter reads 100 mV. At this instance, the tenth LED begins to glow without any flicker along with all the preceding nine LEDs. Your null detector is now ready for use.
The minimum input voltage Vi (min) to which the null detector can respond obviously corresponds to the voltage that just turns on the first LED, viz, Vo, DC =10 mV.
For DC signal, Vi, DC (min) = 10 mV/10A=10 mV/400=25 µV. For AC signal, Vi, rms (min) = 10 mV/9A=10 mV/360=27 µV.
This circuit costs around Rs 100.