With the prevalence of Arduino and easy to use micro-controller platforms, people often overlook analog circuits. Our Delhi Interns Gursehaj and Karmanya wanted to make a completely analog version of a common digital circuit and given Karmanya’s predilection for LEDs, they decided to make a VuMeter.
A VU meter represents the amplitude of an incoming audio signal, mostly for aesthetic purposes. The heart of the circuit is the LM3914 display driver. The driver takes input in the form of a signal from 0-5 Volts and represents the amplitude of the signal in either bar or dot form. The chip also regulates current to the LEDs which reduces the number of resistors or other components in the circuit as the entire circuit can run off any 9-12V supply.
This this circuit the 3914 driver is connected to 30 leds, with 3 parallel leds connected to each pin. The leds themselves are of the ultra bright variety and their light gets diffused through the drinking straws. A potentiometer between pin 7 on the chip and ground controls the current supplied to each led.
We used an opamp (AD620) to amplify the signal from the audio jack (~200mV peak to peak) and bring it between 0-5V which the 3914 required. The advantage of using an AD620 or LM386 is that they both require only a gain resistor between pins 1 and 8 to manipulate your opamp gain. Going by the datasheet of the AD620, another potentiometer was used so that different sources (phones,laptops etc) could be accounted for.