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 Forum index » DIY Hardware and Software » Circuit Bending
Running circuits on voltage control
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Dr. K



Joined: Jan 15, 2020
Posts: 27
Location: wisconsin

PostPosted: Wed Jan 15, 2020 7:36 am    Post subject: Running circuits on voltage control
Subject description: Running a 4093 on CV?
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Hello, first post on this forum.

I'm curious if someone can answer this. I've built a lot of individual "noise" circuits--the classic APC, this Instructable for a 4093 based noise maker : https://www.instructables.com/id/4093-Weird-Noise-Generator/

Recently, I started working on an 8 step sequencer, that I had planned to use with 555/556 based oscillator circuits. It seems easy--the chips have a "control" pin.

But what about using a sequencer for chips that don't have a control voltage pin? I am sorry for my super primitive understanding of this! I'm trying to go from hand-operated noise makers, into creating something actually musical. It would be cool if I could sequence whatever devices I wanted.
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cslammy



Joined: Apr 27, 2018
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PostPosted: Wed Jan 15, 2020 8:30 am    Post subject: Re: Running circuits on voltage control
Subject description: Running a 4093 on CV?
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Dr. K wrote:
Hello, first post on this forum.

I'm curious if someone can answer this. I've built a lot of individual "noise" circuits--the classic APC, this Instructable for a 4093 based noise maker : https://www.instructables.com/id/4093-Weird-Noise-Generator/

Recently, I started working on an 8 step sequencer, that I had planned to use with 555/556 based oscillator circuits. It seems easy--the chips have a "control" pin.

But what about using a sequencer for chips that don't have a control voltage pin? I am sorry for my super primitive understanding of this! I'm trying to go from hand-operated noise makers, into creating something actually musical. It would be cool if I could sequence whatever devices I wanted.


Removing my response, since Embarassed I completely misunderstood what you were asking--Phobos got it, he has a good answer.

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PHOBoS



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PostPosted: Wed Jan 15, 2020 9:26 am    Post subject: Re: Running circuits on voltage control
Subject description: Running a 4093 on CV?
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welcome party!

It depends a lot on the circuit but for simply controlling the pitch of something that uses a potentiometer you could use a vactrol.
In case you are unfamiliar with them: a vactrol consists of a light depended resistor (LDR) and a light source usually an LED.
By varying the brightness of the LED you can vary the resistance. The LDR can than be used in place of the potentiometer to control
the pitch. You could use also use a switch to select between pot. or vactrol or you could also place it in parallel with the pot. which
gives you some extra control over it.

The brightness of the LED can be controlled with the sequencer but you might need some extra circuitry to have some good control
over it. Ideally a LED is controlled with current and a sequencer usually sends a out a voltage but there are ways to convert those.
It is also possible to use a multiplexer to create a different kind of sequencer that creates a varying resistance directly which can
replace the pot. Another option is a digital potentiometer but they are not so easy to control without the use of a microprocessor.

I know this is the circuit bending section but for modular synths the circuits are designed with a control voltage input to communicate
with other modules. This is by far the best option but it involves a bit more than just hacking an existing circuit.

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Dr. K



Joined: Jan 15, 2020
Posts: 27
Location: wisconsin

PostPosted: Wed Jan 15, 2020 1:23 pm    Post subject: Reply with quote  Mark this post and the followings unread

thanks for that idea PHOBoS. I never considered optical coupling. Just use the sequencer to modulate the brightness of an LED, then use some photoresistors, transistors, or diodes to actually control the oscillator.

I've played with photoresistors--but musically they are very difficult to modulate by hand. But inside a light-tight container, under good time and voltage control, they would work great!

i'll have to tinker with this a little bit. One of hte oscillators has a pot to set the gate, and 3 photoresistors to set the tone. It should be straight forward to build a 3-resistor/led device so they are all under the same drive/intensity. Perhaps use some trimmers to tweak the final response.

Thanks again. Cool idea!
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JovianPyx



Joined: Nov 20, 2007
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PostPosted: Thu Jan 16, 2020 9:51 am    Post subject: Re: Running circuits on voltage control
Subject description: Running a 4093 on CV?
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Dr. K wrote:
Recently, I started working on an 8 step sequencer, that I had planned to use with 555/556 based oscillator circuits. It seems easy--the chips have a "control" pin.


From personal experience, there are two things you want to know about 555/556:

1) The bipolar parts (i.e. not CMOS) exhibit a PSU crowbar effect (shorting) when the timer resets (each cycle of sawtooth output). This is a rather large current pulse that propagates to the rest of your circuit through the power rails. This won't damage the PSU, but it causes problems elsewhere in the circuit. To avoid this, use a CMOS version such as 7555. If you don't know if the ones you have are CMOS or bipolar, get the datasheet for the exact number on the package, that information should be on the first page.

2) The pin (555 pin 5) labeled CONTROL VOLTAGE is not really what you think it is. Yes it can change the pitch of the oscilllator, but over a very limited range. This input will not do FM, as it doesn't tolerate an audio rate signal on this pin. It can modulate pitch if an LFO output is sent to the CONTROL VOLTAGE pin. I do this with my Fatman which has two 555 (7555 now) based VCOs.

If you want wide pitch range with voltage control, you can look at Thomas Henry's design (for expo control) or you can look at the schematic of the PAiA Fatman (for linear voltage control also called V/Hz) which is on their website at www.paia.com

Have fun!

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