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

kmesne

Joined Mar 13, 2019
13
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

BobTPH

Joined Jun 5, 2013
2,102
How does converting the output to a square wave help get you closer to the solution?

You have an amplified signal. That is good. What characteristic of the signal tells you the level of the sound?

Bob

kmesne

Joined Mar 13, 2019
13
How does converting the output to a square wave help get you closer to the solution?

You have an amplified signal. That is good. What characteristic of the signal tells you the level of the sound?

Bob
Isn't it amplitude ? Since I need square wave with different duty cycles I thought converting it to a square wave would be better ?

Delta prime

Joined Nov 15, 2019
48
you need a pulse modulation technique called Pulse Amplitude Modulation (PAM) In PAM the amplitude of the individual pulses are varied according to the amplitude of the modulating signals. The PAM modulator and demodulator circuits simple compared to other kind of modulation and demodulation techniques. There are two kinds of PAM one in which the pulses have the same polarity and the other in which the pulses can have both positive and negative polarity according to the amplitude of the modulating signal.The PAM modulation technique is widely used in high speed digital communications like telephone modems, Ethernet etc. They are used to drive LED lights more efficiently than using PWM method. Unlike the PPM the transmitter and receiver synchronization is not required for the PAM. .

DESCRIPTION:

The demodulator circuit explained in this article can extract the modulating signal from the Single Polarity PAM wave.

The message signal used here is a pure sine wave generated using Wien Bridge Oscillator (WBO) and an FET based modulator circuit is used for the PAM modulation. The demodulation of the PAM signals can be done easily using a Low-pass filter

BobTPH

Joined Jun 5, 2013
2,102
Isn't it amplitude ?
Yes. So you need something that gives you a signal proportional to the amplitude.
[/QUOTE]
Since I need square wave with different duty cycles I thought converting it to a square wave would be better ?
[/QUOTE]
Does the square wave coming out of you Schmitt trigger do that?

Bob

crutschow

Joined Mar 14, 2008
23,730
Isn't it amplitude ?
What is "it"?
Since I need square wave with different duty cycles I thought converting it to a square wave would be better ?
How better?
By converting the signal to a square-wave, you lose all amplitude information.
The resulting pulse-widths are mostly proportional to some aspect of the signal frequency, not amplitude.
You need to give a lot more thought about what you are actually trying to do.

You want the average value of the PWM signal to equal the amplitude of the signal, averaged over some time period.
So you need a circuit that converts signal amplitude to the duty-cycle of a PWM signal.
Look up "PWM modulator circuit".

ronsimpson

Joined Oct 7, 2019
324
I think you need to look at a block diagram of a PWM.
The input "Speed Control Voltage" controls the duty cycle. Low voltage = low duty cycle, high voltage = high duty cycle.
The triangle wave generator (or saw wave generator) can easy be built with a op-amp and some R & C. (you need 1hz)
The "comparator" is not on your list but a op-amp can do the job. Or make one from transistors.

A good place to learn is to look at something similar. Audio VU meter.
Hard to see this picture. It is a preamp to amplify the audio. D1,D2 changes the audio to a DC voltage. The output is at the emitter of Q1. The louder the audio the higher the output voltage.

You can look on the internet for more diagrams like these.

I think you should build some diagrams like these and come back with your ideas.

Tonyr1084

Joined Sep 24, 2015
3,878
Might you consider a varying flash rate? Slow blinking indicates a minor problem. Faster means more serious. And a flash rate that is going nuts should be warning you of eminent failure. Might be an easier approach.

Audioguru again

Joined Oct 21, 2019
505
The assignment says what to do. It does not say to use Pam and the LED should not go nuts. The amplitude output of the opamp must feed three comparators for the three sound levels. The outputs from the comparators drive the on-timer for the LED circuit.

ronsimpson

Joined Oct 7, 2019
324
three sound levels
I think you are right. 0%, 25%, 50%, 75% and no 100%.
So my thought of 0 to 100% is not correct.

kmesne

Joined Mar 13, 2019
13
The assignment says what to do. It does not say to use Pam and the LED should not go nuts. The amplitude output of the opamp must feed three comparators for the three sound levels. The outputs from the comparators drive the on-timer for the LED circuit.
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.

Audioguru again

Joined Oct 21, 2019
505
You will have two "window" comparators and one ordinary comparator and each drives its own timer:
1) For very low sound levels then none of the comparators are activated.
2) When a sound level is more than very low but less than medium then the first window comparator is activated.
3) When the sound level is more than medium but less than very high then the second window comparator is activated.
4) When the sound level is very high then the third comparator is activated.