De-ticking a 555 oscillator

[jim68000]

I started building a Shin ei FY-2 with the scoop mod, but the scoop being controlled by an LDR rather than a rheostat. I found that let you play with the texture of the fuzz.

I then thought, aha! What this needs is an LFO that changes the light falling on the LDR. So I added a simple flasher to the circuit. It sounds great at reasonably fast speeds, but the 555 is ticking away in the mix. What's the best way to get rid of the ticks and whines from the 555? I thought a big cap where I've written a ? would do it but it's not happening. Mr SmallBear smooths the signal from the 555 at 3 by adding a low pass, but I think that at the speeds I want to go that's just going to make the interference smoother, not eliminate it. What's best practise here?

[nick d]

           Try turning the LED round and connecting to +9V instead of Ground , so the LED
            is in when the output at pin 3 is low . 
           Also , are you sure that 330K on the LED is right ? Looks a bit big to me .

[smallbearelec]

A couple of other possibilities:

--Are you using the CMOS version of the 555? The low operating current makes a big difference.

--Physical layout and grounding! I learned this from R. G.: Create separate grounds for the LFO and effect sections, and join the grounds only at the point where the negative power supply lead enters the board.

[jim68000]

>>  Also , are you sure that 330K on the LED is right ? Looks a bit big to me .

Oh yeah, that should be 330 ohms. I don't think I'd get much light out of 330K

I'm not sure what my 555 is. It's just one I had left over from a noisemaker project (ironically).

The grounding might be an issue: it's on a breadboard at the moment and everything grounded to one of the side rails. I'll try keeping it discrete as possible.

[Jdansti]

Place your big cap + side as close to pin 8 as you can. The cap might need to be as large as 4700μF.

Something I've noticed with the 555 as an LFO is the ticking you're hearing could be caused by the behavior of the LED/LDR as the square wave abruptly goes to zero. I used a smoothing filter to reduce this effect, but I found that it's hard to completely eliminate it when playing through a high gain fuzz or overdrive pedal.

Check out my take on it here:

http://www.diystompboxes.com/smfforum/index.php?topic=102964.0

Edit:
Some have reported success using the CMOS version of the 555:

Custom Silicon Solutions   CSS555/CSS555C
GoldStar   GSC555
IK Semicon   ILC555
Maxim/NXP Semiconductors/Intersil   ICM7555
National Semiconductor   LMC555
Texas Instruments   TLC555

Edit 2: (CMOS as mentioned by Steve above).

[armdnrdy]

I'll echo what has already been stated.

Try a CMOS LMC555CM or a ICM7555 timer.

Or...there are many other ICs that can be used for clocks/LFOs that play nice in an audio setting.

[duck_arse]

330R CLR? that looks way low to me. can you increase that value and still get the circuit to work? maybe switch to an ultrabrite led, use even less current.

you need a low-value series resistor before the "?" cap to make that filter work right, and it surely won't hurt to include a series resistor/cap to ground in the audio half of the circuit as well.

and the 7555 draws less current to start with, so +1 there.

[jim68000]

OK, cool: I've ordered a 7555 + Ultrabrite LEDs from Banzai. I'll also try putting a low pass in front of the LED to let it brighten slower - that should sound better too, although I think LDRs don't react instantly to changes in brightness, so should smooth out the waveform there.

Thanks all.