Advise needed for 555 timer/envelope generator design.....

Hi guys,
I am trying to figure how to use the 555 chip to generate pulses to turn on the VCA as an envelope generator, as you can see I have included a voltage divider at the trigger input and AC coupled it to the half wave rectified inverted audio input.

Aiming to use the 100K resistor as the attack control to set the voltage/delay timing for turning on the pulse and the capacitor & resistor at the reset input to set the time for switching off the pulse.

Being a newbie to electronics, can anyone check it out and see if it will actually work as proposed?

That won't work. The 555 triggers when pin 2 goes below VCC/3. It must return to above VCC/3 before the 555 times out. The width of the pulse is controlled by the product RA*C. The width of the pulse on pin 2 has nothing to do with the time that it triggers. You also cannot set the release time with reset (pin 4).
If you are trying to generate a pulse with variable delay and variable width, you need cascaded monostables (two 555's or a 556). The first one adjusts the delay, the second one adjusts the width. If you need a schematic (I think you do), post your requirements for delay range and pulse width range here.

Ron H said:

That won't work. The 555 triggers when pin 2 goes below VCC/3. It must return to above VCC/3 before the 555 times out.

Click to expand...

Hi Ron,
Thanks for your help!!!!! Really appreciated!!!!
Just got a question about the attack adjustment on pin 2, since I'll be rectifying the positive peak of the audio signal and invert it wouldn't the voltage at the trigger input return to 1/3 of the VCC at half the period of the input frequency?

Ron H said:

The width of the pulse is controlled by the product RA*C. The width of the pulse on pin 2 has nothing to do with the time that it triggers. You also cannot set the release time with reset (pin 4).
If you are trying to generate a pulse with variable delay and variable width, you need cascaded monostables (two 555's or a 556). The first one adjusts the delay, the second one adjusts the width. If you need a schematic (I think you do), post your requirements for delay range and pulse width range here.

Click to expand...

Would it be possible to do it all with one 555 timer chip?

This is actually where I got my idea from and I am just trying to simplify it, especially trying to avoid the transistors at the input and may be try to incorporate Sustain and Release into 1 pot. Any ideas on how I can do that?



Thanks!

OK, I missed the half-wave rectified audio part. I was fooled by the shape of the waveform you drew. I didn't realize that the amplitude could vary.
I think I understand what you want to do, but I doubt that this can be done with a single 555. I'll think about it, and maybe some of the other trolls here can help out.

Ron H said:

OK, I missed the half-wave rectified audio part. I was fooled by the shape of the waveform you drew. I didn't realize that the amplitude could vary.

Click to expand...

Hi Ron,
I have made a minor update to the proposed schematic, after giving it some more thought I think I should just use an inverted op amp at the input instead.

I think I understand what you want to do, but I doubt that this can be done with a single 555. I'll think about it, and maybe some of the other trolls here can help out.

Click to expand...

As you can see, I have also moved the Attack control to the top of the 1/3 VCC voltage divider resistor so I can use that to increase the voltage above 1/3 of the Vcc in order to cancel out the input inverted signal to create a delay in triggering the pulse. Any thoughts on this new adjustment?

What about the time constant cap at the Reset pin 4? It is there to set the timing of the pulse, as it charges and discharge from 5V Vcc it will vary the voltage at pin 4 and when it rises up to 0.7V it will reset cause the pulse at the output to set low.

Any thoughts?

Can anyone advise on the revised plan?

Thanks!