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
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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.