VU Meter only with 1LED (PWM) no IC except for OPAMP

[kmesne]

First of all yes it is homework but I worked on it and stuck at only this part. Maybe I am approaching on wrong side I am open to every advice.

I need to design a environmental noise checker circuit. I am not allowed to use any IC except OPAMPS! Passive elements (resistors, capacitors, inductors, diodes), LEDs, Analog Microphone, DC Power Supply allowed.

They are expected us to show sound level with only 1 LED by using time. To be specified; very low sound its off every time. above very low it should be open for 0.25 second and 0.75 off, above that 0.5 on 0.5 off and finally when its at highest it should be 0.75 on 0.25 off.

So I managed to have a decent output from microphone by using non-inverting opamp. After that I obtained a square wave by using Schmitt trigger principle.

Now I am thinking about how to use that time varying with respect to input amplitude. I know its somehow related with Pulse Width Modulation but have no idea how to construct something like that.

Thanks

[KlausST]

Hi,

so you did some research...
Really research about PWM? If so, you should already now the most PWM use a triangle shape waveform to compare with....

Is this "hint" enough?
--> go on this way.

***
Show your ideas - even if not correct or not perfect.

We help, but we won´t give you a ready to use solution.

Klaus

[betwixt]

Not PWM, what you need is an op-amp as a gain stage, a rectifier (either diode or better still an op-amp based precision rectifier) to give you a DC level proportional to the sound level. Then you need a monostable where the delay is set by that voltage. The monostable output feeds the LED.

I think it should be possible to do it with a single quad op-amp IC but beware of the pitfall that you must reset the output completely when the voltage is zero or risk it staying on if it was already on and the sound level drops to zero.

Brian.

[KlausST]

Hi,

I agree with Brian.
He thinks about an "analog" solution.

But I followed your idea with PWM. This is the digital solution where the brightness is controlled via the duty cycle of the PWM.

Both ways are possible (and a lot more). There is not the single one and only perfect solution.
I think it´s the meaning of the homework to find "your own" solution.

Klaus

[FvM]

According to post #1, you want 4 discrete display levels (duty cycles 0, 0.25, 0.5, 0.75), no continuous variation of duty cycle. If so you need comparators (can be implemented with OPs) after the input amplifier and rectifier to distinguish the levels.

[kmesne]

Yes I know how to implement comparators with opamps. But I didn't understand how they discrete levels of microphone output ? Aren't they work as giving output of their Vcc or -Vcc so doesn't it mean that I lose my microphone information, and still I couldn't understand how it seperates.

[betwixt]

You can't use the outputs of comparators directly because all you get is an on/off indication of the instantaneous level. You would see the output flash at the frequency of the audio instead of its amplitude.

You have to convert the audio level to an analog voltage level (amplitude envelope) with an appropriate time constant before using the comparators.

Brian.