555 cv

[turing]

can i just shove an lfo output into the cv input of the 555 chip? or is some kind of intermediary circuit needed to regulate it?
i have a ken stone psycho lfo, and a little 555 based dub siren hacked out  of velleman kit, but don't want to fry it. so any help appreciated as always.

[sta63bmx]

That sounds kind of cool.  What would you be modulating?  The modulation input lists an 11V max for Vcc=15V and 4V for Vcc=5V.  It doesn't list a current number, so I'm guessing that's a real high impedance input that won't need hardly any current.  I don't see anything in the datasheet about current limits. *scratching head*  But hey, 555's are cheap!   You might put a resistor in front of it to ground to make sure that not all the current goes to the chip?  I'm not sure.

[Peter Snowberg]

As long as you do not feed the CV a voltage higher than Vcc, go for it! 

You might want to add a couple of resistors or a transistor and couple of resistors depending on wheat you want in the end, but for experimentation, play away! http://www.fairchildsemi.com/ds/NE/NE555.pdf

[turing]

hmmmm.just hooked up the psycho lfo, started jabbing the output around on the 555 board. no real business resulting. a light buzz introduced into the steady tone, not the lfo sweep i was hoping for. then again tweaking the psycho (?) didn't produce any change in the buzz freq, so maybe the lfo is to blame. damn, got nothing else to test it on.
oh well at least the siren still works. should the voltage output from the lfo change as i sweep the controls? never used cv before so i am being a bit dense about the whole thing.

[Coriolis]

A control voltage, like that coming from your LFO, should vary, yes. CGS stuff is probably 10 V peak-to-peak, that is; -5v to +5v.
Don't know about negative voltages on the cv-input of the 555, but the datasheet says cv should be 9-11 v if you're running 15V, and 2.6-4.0 if running on 5V. Hmm...can that be right? Not much of a swing methinks...
Maybe you need to create som offset on the CV-output, and maybe you need to restrain how much it swings too, but I think I've seen 555's used in VCO's for synths using the 10V PP standard, so it must be possible.

C

[gez]

If I recall (my brain is a little ropey these days), the control pin sits at half supply.  If the threshold/trigger pins sit at 1/3rd and 2/3rd of supply (can't remember which is which...told you things were ropey) then your LFO should swing within these limits...I think...hmmm, need to check my books/schematics (sorry!).

I've used the control pin for modulating in a synth sounding phaser circuit I designed (for guitar), using not much more than 555 timers...so it can be done.  I say phaser, but 'micro-flanger' would be more appropriate as there was real-time delay.  Great idea on paper, but ended up trading phase-shift for range.  Sounded like a comparator fuzz thru a phase 45, except the octave down which was lush (sounded like a pro-synth).  No schematic unfortunately...

[gez]

Just checked block diagram for 555.   Ignore thresholds stated in above post (total rubbish).

The control pin sits at around 2/3rds supply, so you might want the centre axis of your LFO to be around that voltage...probably not critical.  Anyway, if I recall it doesn't take much amplitude to get things going.

[gez]

The control pin sits at around 2/3rds supply, so you might want the centre axis of your LFO to be around that voltage...probably not critical. 

Ah, it's starting to come back to me now.  Because the control pin is at 2/3rds supply and because there's not much upward swing, going by the figures Coriolis noted, the centre axis of the LFO should be below this.

Just checked by notes and I ran the 555 chips at 5V and the LFO at 3V (so it swung either side of 1.5V...though not by much).  So the control pin got pulled down a fair bit.  Probably doesn't help...I don't know why I bother...sorry!

[Sir H C]

For the 555 the sawtooth wave in uses for timing is 1/3 to 2/3 scale.  That way the shape which is set by the charging/discharging of a cap with a resistor will not be too ugly at the ends.  That said, if you want to control this with a CV, I would guess you need to have some resistor in series to convert the voltage to a current that will affect the charge/discharge cycle. 

[turing]

thanks for all the tips, just back from doing live sound at festival, so i'll get the little guy up again and see if can't make that pitch wobble a bit.