# Using Control Voltages In a Circuit

Discussion in 'The Projects Forum' started by DanRilley, Jan 19, 2008.

1. ### DanRilley Thread Starter Senior Member

Jan 13, 2008
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Hi, I am trying to build a module for a synthesizer. I have it working so I can change the output voltage (for controlling a synth frequency) by turning a potentiometer, however, I would like to build it so that this variable can be determined by a Control Voltage ( an input 0 to +5V ). I'm just not sure how to use it.

To be more detailed. I have a 555 monostable timer circuit, which generates a voltage ramp, once a trigger is set. The speed of the voltage ramp is determined by the product of one of the resistors and the capacitor value. I can use a pot to change the resistor value, and thus, control the ramp length, however, I would like to use a control voltage to change this value. Any help would be appreciated. I noticed the 555 timer has a control voltage input at pin 5, however, when i inputed a voltage I didn't seem to get any results.

Thanks, I'm totally new to electronics so bear with me (kind of in that stage or really excited about getting something done, but totally overwhelmed by the weirdness of circuitry)

Dan

2. ### hgmjr Moderator

Jan 28, 2005
9,030
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Since you mentioned a ramp, it sounds like the control scheme you are aiming to implement is to desing a ramp generator that will ramp up from 0V to 5V when externally triggered. You also want to be able to set the speed at which the ramp travels from 0V to 5V.

Is this what you are trying to achieve or are you looking for something different.

hgmjr

3. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
Do you use MIDI and sequencing software? Ever planning on being able to sequence/playback your compositions?

If so, you should consider using DACs (Digital to Analog Converters)

An 8-bit DAC would give you up to 256 discrete output steps. Since MIDI uses a range of 0-127 for volume, you could use just the first 7 bits of the DAC. A microcontroller such as a PIC could be used to control it's output, and you would have a direct upgrade path to MIDI interfacing if you planned ahead like that. Many microcontrollers have DACs and ADCs (Analog to Digital Converters) built in.

Going back to your 555 timer experiment - I haven't fooled around much using the control voltage pin, but I've used it to adjust the frequency of an astable 555 timer.

4. ### Ron H AAC Fanatic!

Apr 14, 2005
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A voltage-controlled current source works really well to control the ramp slope (and frequency) of a 555.

5. ### DanRilley Thread Starter Senior Member

Jan 13, 2008
107
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hgmjr, yeah I want it to ramp up from 0 to some variable amount of volts (max of 5) over a variable amount of time. I have figured out how to do this, by changing resistance values, but I want to be able to control it with a Control Voltage so that I can send sequenced patterns to these variables. I just don't understand how to convert a Voltage into a resistance.

In response to Ron H, does the 555 control voltage input only work in astable mode?

6. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
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I mentioned a voltage-controlled current source. The 555 control voltage is different. It controls the amplitude of the (exponential) ramp and the frequency at the same time. In monostable mode, it will change the change the amplitude of the ramp and the width at the same time.
The voltage-controlled current source I mentioned creates a variable slope (and variable length) linear ramp. It will work with a monostable. You would change the amplitude by buffering the ramp with an op amp voltage follower, with a potentiometer after that.
A 555 may not be ideal for what you want. What ranges of time and amplitude do you want? Do you want a ramp that rapidly resets back to zero, as you get with a 555?

7. ### DanRilley Thread Starter Senior Member

Jan 13, 2008
107
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Hey thanks for the reply, unfortunately I'm new to this stuff so hope I'm not wasting your time, but I'm trying to follow you. Here's a total explanation of what I'm trying to get and maybe you can tell me a different way that is better.

I have an input voltage of 0-5V
I want to ramp that voltage up or down x number of volts
Over a time period that is variable from .01s-6s
at the end the ramp would jump back to 0 immediately
I want to control both the amplitude of the ramp and the time range of the ramp with control voltage inputs (can't really use potentiometers, it needs to be automatic)

the final result when plugged into a synthesizer would be a note gliding up over a certain amount of time

I realize it's a hefty task for a beginner and am by no means asking for a final answer but just maybe a nod in the right direction. The main thing I don't understand is how control voltages work. I've searched the web, but maybe am missing something. How can I convert a control voltage signal of 0-5V, for example, into something that can change the RC timing of the 555? In terms of buffering the ramp with an op amp voltage follower, does that mean take the control voltage to the - side of the op amp, and the main voltage to the + so that the voltage follows the control voltage?

8. ### hgmjr Moderator

Jan 28, 2005
9,030
214
Provided you have a circuit which generates this ramp over which you have control of both the ramps duration and amplitude, what do you envision as the means by which the ramp is triggered?

Also how do you imagine that you will program this circuit for how long the ramp should be and what amplitude the ramp should have?

hgmjr

9. ### DanRilley Thread Starter Senior Member

Jan 13, 2008
107
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An analog sequencer will generate control voltages to trigger this thing and set its values. Basically I will have a sequencer set up that has 4 CV outputs.
1) a trigger output feeds in to the 2 pin of the 555 to trigger the ramp
2) a note voltage is fed in to the input of the box that will set the starting note (or initial voltage)
3) an amplitude voltage is fed into some input on the box to set the amplitude of the ramp
4) a length voltage is fed into some input on the box to set the length of the ramp

the sequencer will be stepping through and sending these 4 voltages at each timestep, so a different type of ramp can be played at each timestep

does that make sense?

as for your second question: how do you imagine that you will program this circuit for how long the ramp should be and what amplitude the ramp should have?

this is basically what I don't know. I can currently program the length and amplitude of the ramp with pots, but I want to use these control voltages that my sequencer is sending.

10. ### DanRilley Thread Starter Senior Member

Jan 13, 2008
107
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I guess people are stumped on this one. If anyone else has a similar problem, I found th is article which seems to do the trick, although I have not implemented it yet. Only problem is that AD734 costs \$25 bucks!

http://www.avtechpulse.com/papers/vres/

11. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
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I have no problem showing you how to make a voltage-controlled current source to in turn control the timing of a 555. I started to post a schematic, but I got bogged down in the plethora of controls that you need. They are so poorly defined that I didn't know where to start.

12. ### DanRilley Thread Starter Senior Member

Jan 13, 2008
107
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Hey Ron H, didn't mean to say I stumped you just meant to put out an answer I had found. I realize the stuff I put up there was ambiguous because there's so many sides to it and I wasn't sure which of the sides would affect the answer, so I just put it all out there in a rather lopsided sequence, sorry about that. I am very grateful that there's guys like you out there that take time to help out beginners like me, it's amazing.

But now that I have narrowed down, over the course of 20 messages, what I wanted to learn, I would love to hear how you would solve the problem when you get a chance. Right now I'm ordering the parts for that other solution I posted just to see if it works, but it's too expensive to implement over the long run. I require no full schematic of the device I want to build, but simply solutions to just controlling the R/C timing of the 555 timer with a voltage (i.e. a voltage controlled resistance of some kind).

Thanks

13. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
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Here are two different approaches on one schematic. The schematic is annotated to explain the differences. The control voltage is the same in both, stepping from zero to 5V in 1V steps. I don't know if either approach would be a useful seed for your design.
As you can see, the right hand schematic doesn't use a 555, but it is still a monostable.
I built in a small starting current into the 555 circuit (with R4), because otherwise, at zero control voltage, you get no current for the timing cap, so the ramp is flat, and the monostable never times out.

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14. ### DanRilley Thread Starter Senior Member

Jan 13, 2008
107
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WOW that's amazing that you made the whole circuit. It's going to take me a few days to follow the paths and fully understand it, but once I get it going I'll come back with my results. Thank you so much!

15. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
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Do you Spice? I can provide the "source file" for LTspice, if you can use it.
BTW, the current source in the right hand circuit actually has to be made with real parts, and could also be voltage-controlled, providing another "knob".

16. ### DanRilley Thread Starter Senior Member

Jan 13, 2008
107
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I don't currently have LTSpice, I'm guessing its software for prototyping circuits that you used to layout the schematics and tests? Seems like a great way to save time, I"ll check it out, no need for the files just yet though. Still looking over the schematic. For the version using the 555 do the two transistors have to be complimentary? I noticed you noted the name of the PNP but not the NPN, just trying to figure out which parts to use.

17. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
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NPNs and PNPs are not interchangeable. I forgot to pick a model for the NPN, so the simulator used a generic model. I would use a 2N3904. There are lots of acceptable substitutes for either transistor, if you can't get those.
If you're going to build these, you will need help in selecting suitable op amps, and some info on what to do with unused sections of your ICs.