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Op-Amp Help Answered

Hi- So, I made the circuit below to make sound-reactive blinky LEDs for a halloween costume (the round thing on the left is a condenser mic). I think I just got lucky that it worked, because I'm no genius when it comes to op-amp usage or design. Now I want to adapt it to control something else that needs a slightly higher voltage than the LEDs. Basically, I'd like the output to swing as high as V+ when the mic input is "loud" (a relative and possibly adjustable level). I've messed around with various resistor values as well as adding or removing resistors in places it seemed like it would help (according to the LM741 datasheet). All to no avail. Anyone out there who might know what to do? Again, I want the mic input to cause the output to go as hight as V+ (9V in this case) when it's loud, and be more or less at 0V when it's "quiet." Thanks in advance, _mykle



9 years ago

A couple observations: --Keep the load together on the collector side of the NPN (move the LEDs.) --Your low-pass filter (the 100uF and 15K resistor) seems off. Your corner freq is something like 0.1 Hz--way low for audio. A better combination would be 0.1uF and 47K, with a cutoff of 33Hz. --It's a bipolar transistor, I'd eliminate the 47K bias res and tune the response with a series resistor (between the filter and the NPN base) or POT. It's current switching the NPN, not voltage. With an NPN, ultimately the response will depend on the Hfe (gain) of the transistor, and the amount of current the transistor is switching. I.E., adding or subtracting from the load will have a big effect. -- No values for the bias on the non-inverting input? Maybe try a 100K POT in a divider setup (ends to V+ and GND, center to non-inverting input.) That would be one way of adjusting the peak voltage.

Ya know, on second thought... Ignore what I wrote about the filter--bipolar transistors have relatively low input impedance, so a cap value between 10uF and 1uF is probably best...

Apologies: that's a high-pass filter, not low-pass...

I think the bias resistors were 5.6k, but I'm not 100% certain. Thanks for the tips. I'll take them into account and play around with it.

If they are equal, that's fine.

if the existing circuit works ok and stable you may reuse it i dont like to place stuff between emitter of transistor and earth but you did so i'd try to leave it as is


It works, but I was hoping to use the same supply. In the current version, the output goes from 0V to right around 3V depending on the volume. I was just wondering if it was possible somehow to get it to go to 9V (or close). The circuit is already powered by a 9V battery. I figure it has something to do with properly setting the gain and feedback on the op amp, but I'm not quite sure how, exactly. Thanks for your reply.

is the chip a 358 ? if yes use the second amp in it to amplify the volts before the 1st transistor base (add a resistor between amp and base)

when you connect opamp with input to + and R1 from out to - and R2 from - to earth then

vout = in * (1 + R1/R2)

the amplification is higher when the feedback is weaker (thru bigger resistor)

or you can try to redesign the existing part (i can help if you ask)