This VU meter is made up of 7 rows of parallel-wired voltage-matched leds , these leds are driven by 7 switching mosfet transistors by using a constant-voltage ~1.7v source. This risky arrangement (which can potentially overdrive the leds) avoids the need to connect them serially, avoiding the use of current-limiting resistors, which not only waste current, but also add complexity to the circuit while adding more parts to the bill of materials.
The gallery below shows some of the pictures taken while building the circuit, which is separated into two boards: The vertical board contains the leds and the driver transistors, while the bottom board contains the microphone, the op-amps and some passive elements.
In order to capture the ambient audio, a condenser microphone is used. This microphone is wired to an op-amp in a 100x amplifier configuration. The amplified signal runs through 7 comparator op-amps which compare it with various voltage levels, if there is a loud enough ambient sound -like some background music is playing- the op-amps progressively switch the 7 transistors, lighting up the led rows.
Two notable properties:
- The whole circuit is powered using a single 5v 1A power supply.
- The sensibility of the VU meter can be adjusted by varying the comparator voltages, lower voltages make the VU meter become more sensible to sound, and vice versa.
- The brignthess of the led’s can be adjusted, this is done by changing the voltage that the led’s are being driven at, by taking advantage of the diode properties of the leds.