40106 schmitt trigger for Tim Escobedo's PWM

[Stompin Tom]

I've been thinking about trying to put together Tim's PWM... and I have a quick beginner's question.  When I do a search for 40106 on mouser I get several (schmitt trigger-type) chip options... does it matter which one I use?

Also, has anybody had any problems with this circuit?  Anything a relative novice like myself should know about?  Thanks!

[gez]

Tim's site seems to be down and I don't have the schematic, but if the 40106 (any chip should do) is used for an LFO and only one gate is used, then it would be easier to wire up a 7555 timer as a Schmidt trigger and use it instead (less space taken up).

Example in this thread (circuit not my idea), though LED is not required.

http://www.diystompboxes.com/smfforum/index.php?topic=24256.msg156734#msg156734

[gez]

PS  Scroll up a few posts (result of the search function)...

[Tim Escobedo]

Searching for "40106" on mouser's site yields several sources that are in stock. Commonly named CD40106, hey are 14 pin Hex Schmitt Trigger chips. I usually recommend buying a DIP-14 version, unless you specifically want surface mount parts.

The circuit could probably be modified using 7555 chips. However, I have not done it, and you'd have to roll your own.

[niftydog]

The 40106 is more than just the LFO, it's crucial to the effect, just to clarify.

I have successfully breadboarded a version of this circuit using a 74C14 CMOS compatible schmitt trigger inverter, and these can sometimes be much easier to find than the 40106.

Whatever you get, make sure it's a 14 pin DIP package and not surface mount. My advice would be to go with the cheapest 14 pin DIP package type - more expensive varietys have unecessarily high temperature stability and tolerance figures. (Mil spec is just mojo for our purposes, IMO)

You might wish to experiement a bit with the 5nF capacitor. Changing this changes the RC time constant and effectively results in a shift in the PWM duty cycle. To be more specific; the duty cycle of the PWM wave depends on the frequency of the input signal and on the RC time constant created by the 5nF cap and the 500k pot. So, by changing the capacitor value you can change the way the duty cycle responds to the input frequency and hence you affect the overall sound of the effect. I have had to do this in my experiments because I'm making mine for bass guitar, so I wanted to search around for the best sounding results.

I'm hoping to try some more mods on this design, eventually coming up with a frequency independent duty cycle. More soon.

[Dolly Parton]

I'm hoping to try some more mods on this design, eventually coming up with a frequency independent duty cycle. More soon.

You can do this using a 'stepped' approach - divide up a complete cycle using a 4046 with something or other in the loop (number of options) then switch in each segment using logic. 

Low parts count in terms of caps and resistors, but more chips than the sequins on my dress!  Slightly different effect too, more like a '70s computer' sound.