Introduction: Op-Amp Music LED Simulations

In this instructable, you will be simulating operational amplifier music LED. This LED is turning on and off with the incoming music signal.


PSpice Simulation Software

Step 1: Calculations

The gain of the first amplifier is -(Rf1 + Rf2max) / R1 = -(2.01 Megohm / 100 kohm) = -20.1. The minimum gain is: -(Rf1 + Rf2max) / R1 = -(10 kohm / 100 kohm) = -0.1.

The input resistance of the LM324 Op-Amp is very high.

The Rp1 variable resistor controls the brightness of the LED.

R4 and C2 time constant is equal to: R4 * C2 = 10 ms or 50 ms for five-time constants. You can replace the R4 with a variable resistor. However, never set R4 to zero. You will damage the C2 capacitor. A good idea is to put a 1 kohm resistor in series with R4.

The minimum frequency cut off is for low pass filter is: fmin = 1/(2*pi*(Rf1+Rf2max)*Cf) = 7.92 kHz.

The maximum frequency cuff off for high pass filter is: fmax = 1/(2*pi*(R1+C1)) = 3.39 Hz. Using a pillow capacitor is always a better idea than electrolytic or bipolar. Pillow capacitors are more reliable. However, their maximum values do not usually exceed 470 nF.

The current across the LED is: (Vs - Vled) / Rl1 = (9 V - 2 V) / 820 ohms = 8.54 mA. I chosen a higher resistance value although the LEDs are supposed to operate at 10 mA because the Vs might increase to 12 V.

Step 2: Simulations

The blue signal V2 is slowly increasing as the filtered DC voltage builds up.

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