My project is a sound meter shown by LEDs. It makes use of an electret microphone, op amp, and LM3914 LED Driver IC. How it’s used is the louder the environment around the sensor is, the more LEDs are turned on by the LM3914. It’s a fairly simple and interesting project that’s ideal for newcomers to electronics.
· Breadboard (Lee's #10516)
· Jumpers OR (Wire Stripper AND Breadboard Wire) (Lee's #10325 + various)
· 10 × Thru hole LEDs (Lee's #549 + various)
· Electret Microphone (Lee's #2272)
· Operational Amplifier (Lee's #7292 / #7293)
· LM3914 (Lee's #7304)
· 1 μF Capacitor (Lee's #8962 / #82)
· 3 × 2.2 kΩ resistors (Lee's #9223 / #9548)
· 2 × 1 kΩ resistors (Lee's #9190)
· 330 kΩ resistor (Lee's #9427)
· Potentiometer AND Screwdriver (Lee's #71044 + various)
· 2 × AA×3 battery packs (Lee's #21079)
Step 3: Amplification of the Mic Output
As is the case with most transducers, the signal is too small to be useful to us. AC signals can be amplified with operational amplifiers, transistors, or transformers. However I find op amps the simplest to work with so we’ll focus on op amps. Only one dual supply op amp is needed (I used the TL084 but you can use whichever one you want). All op amp ICs must have at least 5 pins which are Vs+ (positive supply voltage), Vs- (negative supply voltage), V+ (non-inverting input), V- (inverting input), and Vo (output). There is loads of theory behind op amps but the important things to us are that Vo can never go beyond V+ and V- (in reality, the real range is what’s known as +Vsat and -Vsat), the input terminals draw no current (theoretically not practically), and the non-inverting and inverting inputs are kept at the same voltage when the op amp is in negative feedback (Vo connected to V-).
See the schematic for the connections. Our configuration is what’s known as a non-inverting amplifier meaning that the gain is positive. The gain generically is Av = Vo / Vi. For a non-inverting amplifier, Av = Vo / Vi = 1 + Rf / Ri. We are using resistor values of 1 kΩ and 330 kΩ to achieve a gain of 331. Once everything is connected, there should be a pure amplified AC signal at Vo.