triggering a capacitor to discharge even after trigger is finished

Thread Starter

jacopo1919

Joined Apr 12, 2020
80
Hello everybody!

excuse me if the title is not very intuitive.

I wish to trigger a circuit in order to feed the Vin of a 555 timer with the decreasing voltage coming from a capacitor discharging (in the picture attached is 4.7uF).
My main issue here is that I need the capacitor to continue discharging even after the trigger is finished. At the moment, it stops when the trig is over.


I would like to start discharging the cap even with a very short pulse.




This will allow to have a decreasing trigger signal that can be time-adjusted by changing the value of the 27k resistor on the 555 section.


the initial pulse should both give the first trigger at the output, and push the capacitor to discharge.



Unfortunately I can’t find a solution for this. It might exist a very simple solution.

In the picture attached, each division is 1V
this is the link for the falstad simulation https://tinyurl.com/yxk8exws



Any help appreciated.
MN
Mod;lightened image.ESchermata 2021-01-02 alle 18.32.07.png
 
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Thread Starter

jacopo1919

Joined Apr 12, 2020
80
i found a very simple workaround that allows me to do what i was asking in this thread. It still has some issues with very short pulses (it almost istantaneously drops from 11.52V to 7.5V) and the triggers out of the 555 timer are unequal (but in this case it could be the simulation program itself.. will need to start using LTspice)

Simulation : https://tinyurl.com/y4dko2pe
 

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Alec_t

Joined Sep 17, 2013
12,744
Unless you use the CMOS version of the 555, the current draw of your 555 circuit will drain the little 4.7uF cap in no time.
 

AnalogKid

Joined Aug 1, 2013
9,965
Your question needs a lot of work. What is it you are trying to do? And please use the word "trigger" only to refer to 555 pin 2.

Are you using the 555 as a monostable or as an oscillator?

Which capacitor is the one (?) you want to discharge completely to 0 V?

Please add reference designators to each component so we are talking about the same things.

Which 555 are you using?

ak
 

AnalogKid

Joined Aug 1, 2013
9,965
Don't forget the base resistor.
BUZZZZ - Incorrectomundo

The transistor is acting as an emitter follower. Not only does it not need a resistor to limit base current, adding such resistor makes things worse by reducing the peak voltage available to the IC and increasing the source impedance at the 3904 emitter.

ak
 
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Thread Starter

jacopo1919

Joined Apr 12, 2020
80
I understand that when there is a 555, the word trigger should be used for pin 2 only but the whole circuit i’m trying to create aims to be used as drum sound “triggering”, therefore i don’t know what other word should i use instead!

Given an anitial pulse, i want to generate a series of decaying pulses that are decreasing in Voltage and can be time-adjusted. 

This allows me to make a sort of instrument delay “triggering” system. Moreover, some drums (as the roland 808 or 909 kickdrum as an example) have their Accent varying depending on the Voltage of the “pulse” that they receive.

On the circuit above, the “Pulses” generated are delayed and decreasing depending from C1 and R2 (please don’t mind the resistor values, i was just trying)

On the circuit below , a comparator gives an HIGH to the 555 untill the C3 capacitor discharges and the voltage at the non-inverting input of the comparator decreases.


By changing the comparator Vref, this changes state quicker or slower; therefore the whole delay time change.


When the comparator gives the LOW, the Mosfet sees a LOW on the Gate and allows current to flow through R7 and discharge quickly (in this way the mechanism can be re”activated” straight away.

R10 and rR8 determine the interval between the pulses generated by the 555 ( used as a monostable. I’m relatively new to 555, i need to check out what are the different types).

By tweaking it, the delay between the Pulses is adjustable.



From the initial question, i reached some points, but there are still many issues :


- maybe because of the poor simulation engine of falstad, the resulting pulses are not equal in lenght and timing.
- maybe for the same reason of above, the resulting “pulses” of both circuits don’t hit at the same time, there is some weirdness.

- in the circuit above, there is an initial voltage drop befare the decay.

- overall the circuit above has a also decaying voltage on the pulse itself, and i want to reach flat pulses as i poorly drew on green in the picture.
 

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sghioto

Joined Dec 31, 2017
3,061

- maybe because of the poor simulation engine of falstad, the resulting pulses are not equal in length and timing.
- maybe for the same reason of above, the resulting “pulses” of both circuits don’t hit at the same time, there is some weirdness.
That is normal when using a decaying supply voltage.
I would leave the 555 free running and use an attenuator circuit on the output. The attenuator would be voltage controlled.
Also agree with ak, I don't see any need for those diodes and extra resistors. ( R1-D1-D2 ), ( R5-D4-D5 )
 
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MisterBill2

Joined Jan 23, 2018
11,621
There is another, more digital, but still analog controlled, way of generating a string of decaying pulses. Unfortunately we don't know how many pulses are required, or if that number varies. And do those pulses need to be flat-topped or have a slope? A CD4017 can provide ten outputs in sequence, each feeding a common point through a resistor, different for each pulse in the string. And every trigger to advance the count can also trigger a one-shot toset the width of the pulse. That scheme can simply provide a string of pulses, each different amplitude in any height sequence. Actually, it takes a second IC to serve as the transmission gate to set the pulse width. sorry about that. But the pulses will have flat tops and be a constant width and I just realized that this can be extended to a lot more steps with just a few more parts.
So how many pulses in a given burst are required??
 

Thread Starter

jacopo1919

Joined Apr 12, 2020
80
R1-C1-D1-D2 are indeed not useful.
@sghioto @ElectricSpidey thanks a lot.
I will need to give all this a proper look into this!
@MisterBill2
I didn't start with the idea of a fixed number of pulses for a given busrt but instead, a repetition of pulses for a certain period of time (which is given by the size of C1 or C3 in post#8)

So, from the initial idea, the number of pulses was depending on how fast C1 is discharging and how short/long was the interval between the pulses.

From what I understood, this is a different approach from the nice idea of using a CD4017 which can output a fixed and maximum of 10 pulses in sequence
 
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MisterBill2

Joined Jan 23, 2018
11,621
OK, but it is seeming to be getting quite complicated.
And now I am wondering about the purpose of the whole scheme. Understanding the purpose may help somebody propose an alternate way to achieve the goal.
 

Thread Starter

jacopo1919

Joined Apr 12, 2020
80
Given an initial 12V pulse (which at first was intended to activate a percussion sound on a different circuit) I want to take that pulse and generate other subsequential delayed pulses. Of this following pulses, I intend to control the period and the whole time frame of the pulses.
This is already quite nice since it would recreate a "TRIG" delay which i think is going to be musically interesting.
Now about the decaying part:
Some percussion circuits have their Volume presence (Accent?) depending from their "TRIGGER LOGIC". Pulses of greater voltage determine a louder sound level and viceversa. I attach an example.
If decaying pulses are used as the "Trigger Logic " of those particular circuits, then the result is a decaying delay.
 

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