How to make a Voltage Controlled Oscillator

[Cem Uzunoglu]

Hello all,

What is the simplest VCO that can be done with off-the-shelf components?
It doesn't have to be very fast. Audio range is fine.

Cheers,
Cem

 

[Tim Williams]

There are simpler, but this works out well enough, and it's a familiar
circuit.

Tim

 

[Cem Uzunoglu]

There are simpler, but this works out well enough, and it's a familiar
circuit.

Tim

Hello Tim,

Either you have forgotten to paste the link, or somehow my newsreader
does not show it.

Cheers,
Cem

 

[Tim Williams]

Derp derp...

It's one thing to forget an attachment, but a link, that's just
embarassing
http://myweb.msoe.edu/williamstm/Images/Cheap_VCO.png

Tim

 

[F. Bertolazzi]

Tim Williams:

Derp derp...

It's one thing to forget an attachment, but a link, that's just
embarassing

It may be a byproduct of bad quoting.

 

[Cem Uzunoglu]

You can make it linear by replacing the 100k resistors by PNP current
sources.

Wow! A modified RC flip flop. Never thought of that. Thank you very much
Tim!

Jan: Will this thing work logarithmically this way? That would be must
more suited to my application than linear then? I'm going to use it to
generate tones.

 

[Cem Uzunoglu]

On Oct 26, 4:16 am, Cem Uzunoglu


I like this one : TL084 + 2N3819
both triangle and rectangle waveforms output :
http://cjoint.com/?0kAogzIgPf

Looks cool. There are no resistor values on the schematics though. Are
they important?

 

[Tim Williams]

Wow! A modified RC flip flop. Never thought of that. Thank you very much
Tim!

Jan: Will this thing work logarithmically this way? That would be must
more suited to my application than linear then? I'm going to use it to
generate tones.

Resistors will give you RC time constants, which have an exponential sort
of thing. For voltages close to Vbe, time goes to infinity (or the gain
is too low to turn on the transistors anymore and it stops completely).
For large voltages, it looks linear (in which case, the resistors
approximate current sources).

If you require linearity, the good old hysteretic comparator with current
sources is hard to beat.
http://webpages.charter.net/dawill/tmoranwms/Circuits_2008/Triangle.gif
This particular circuit has a 4.5 million times range of frequency in a
single range. For simplicity, the discrete comparator could be replaced
with an IC like so:
http://webpages.charter.net/dawill/tmoranwms/Circuits_2010/Pulse_Width_Modulator.png
using a current source to discharge the capacitor, instead of a resistor.

Tim

 

[Tim Williams]

A 74HC05, capacitor and three resistors:

http://www.abvolt.com/misc/vco_74hc05.jpg

Pffbt. Too many resistors, and way too many pins:

+Vin
o |
\
/ 100k
\
| NE-2
+--(||)--+
| |
=== 0.01 |
| |
+--------+
|
--- GND

If you'll accept a control current, instead of a control voltage, it can
be done in two components

Worked fine in the toob days. Back then, they had a hundred volts to play
with, so the required headroom wasn't a problem. It is quite drifty and
noisy, and should be kept away from external light or radiation.

Tim

 

[Clifford Heath]

The VCO in a 4046. See figures 14B and D on page 11
http://www.onsemi.com/pub_link/Collateral/MC74HC4046A-D.PDF

The 74HC4046 has problems when the control voltage is low.
Many people find they can't get better than a 10:1 ratio.

Use a 4000 series 4046 instead, they work over a 1000:1 range.

Clifford Heath.

 

[Cem Uzunoglu]

Question: Does OP want a sine wave (tone) or square pulses? Ken

Square wave or sawtooth is fine. This is for a chiptune synthesizer.

 

[Cem Uzunoglu]

If you'll accept a control current, instead of a control voltage, it can
be done in two components

Two components plus a high voltage power supply and shielding

 

[Cem Uzunoglu]

A 74HC05, capacitor and three resistors:

http://www.abvolt.com/misc/vco_74hc05.jpg

Wow this one is pretty easy. I'm going to breadboard it to see what kind
of output I can get from it. Hope I still have those 74 series chips.

 

[Cem Uzunoglu]

Tuning range? (fmin...fmax)

About hearing range. 30hz - 16khz would do.

 

[Cem Uzunoglu]

http://www.4qdtec.com/index.html
several oscillators there including a woode range VCO.

This one seems to require a password.

 

[Cem Uzunoglu]

A 555, diddle the voltage on pin 5 (threshold?).

Does it work that way? I remember having to change a resistor value to
change the frequency. I remember threshold pin as used for PWM. I maybe
wrong though. It was a long time ago.

 

[F. Bertolazzi]

Cem Uzunoglu:

About hearing range. 30hz - 16khz would do.

You can't hear notes that high. 66 Hz - 5 kHz will do.

 

[F. Bertolazzi]

Vladimir Vassilevsky:

For ~25 years of my experience with electronics, I have never had any
use for four things:

1. Unijuction transistors
2. Microchip PICs
3. Linux
4. 555 timer

ROTFL. Well, maybe the last two...

Could you tell WTF is going on France? Looks like idiots are on the
rampage, are they?

By all means.

 

[Tim Williams]

Been there, done that. That NE schematic sucks unless you use a
thyratron designed to work that way; or add a thyristor so the NE lamp
latches it into the open state.

Thyratron has variable breakdown (when grid is biased to the "linear"
region), but it's just as noisy as the neon. They make bad sweeps, but
they're easy to synchronize thanks to the gain.

They do tend to discharge the capacitor more completely, for a wider range
of capacitances. Neons tend to turn off in the 20-60V range, lower for
bigger C (~1uF makes bright blue-orange flashes ).

How do you "latch it into the open state"?

Tim

 

[Rich Grise]

Hello all,

What is the simplest VCO that can be done with off-the-shelf components?
It doesn't have to be very fast. Audio range is fine.

555.

Cheers!
Rich