Hex Schmitt 40106 Oscillator Help

[Clbraddock]

Long story short I am trying to replace the clock on a TR-505 with a new variable clock. I’m using a Hex Schmitt 40106 which I am getting to oscillate okay, but not in the frequency range I need. I’ve tried doing a RC calculation but I must be screwing it up.

What value resistance and capacitance would I need to be able to sweep from approx 20khz to 110khz? I realize I will prob need a second resistor in series with the potentiometer to keep a minimum resistance above 0. Every combination I try I either end up with a freq way too high or way too low.

 

[alphasnk]

Hi,

I did this mod to my 505 using this: Circuitbenders - reclocking a TR505 tutorial

You can set the maximum amount of speed with the trimmer, so the 505 doesn't crash, and i used it with TR-505 ROM Expansion Kit , it works very well together!

Good luck with the 40106!

 

[Clbraddock]

Nice work alphasnk! I too used the rom expansion, which is awesome. I'm wanting to have individual pitch mods which is why I'm trying to do it with the 40106, but who knows if I cant get it to work maybe Ill just do universal pitch mod from Circuitbenders. The rom expansion alone really turns the TR-505 from a throw away to something pretty fun to play with (Love me some LM-1 lol)

 

[Clbraddock]

I found this formula:
freq=(1.2x10^6)/(RxC)

with R being ohms and C being uF

http://soundmachines.ca/workshops/hex-schmitt-trigger-multi-voice-oscillator/
Wonder if that is accurate. I had just been calculating the RC frequency. I’m getting tired of order after order to mouser testing this out though lol

 

[Clbraddock]

Ok so using that formula I think a .33 uf cap for C and a 30 ohm resistor in series with a 250 ohm pot would work.

1,200,000/(30x.33) = 121,212.12 hz
1,200,000/(280x.33) = 12,987.01 hz

That is the approx range I want. Before I send mouser another $50ish can anyone confirm that this is correct??? Thanks!!

 

[Clbraddock]

Welp, that formula is trash. Plotting it out on a bread board it has no bearing on reality unfortunately. Back to square one

 

[radardoug]

The formula you want is the standard capacative reactance formula. Z(ohms) = 1/ (2 times pi times frequency(cycles per sec) times C(Farads))
Pi is 3.1416 to four places. Should be accurate enough.

 

[rogs]

One difficulty I have had calculating for a 40106 oscillator is that the Schmitt thresholds can vary for different manufacturers of the device. The data sheets show that there's even quite a wide variation of trigger limits between devices from the same manufacturer .
Remember the oscillator 'ramp' amplitude only switches between those Schmitt trigger levels.
In addition, remember those threshold level will depend on the supply voltage.

Probably easiest to breadboard with the actual device you are going to use to get the range of time constant components you need......

As an example -- For the 40106 oscillator I used on my project here: http://www.jp137.com/lts/LDCX2.notes.pdf
I find that with a 9V rail, 10K and 1nF for the oscillator component values gives me a frequency of c.125KHz, using a CD40106B

 

[alphasnk]

Clbraddock: yes the rom expansion is great indeed The LM-1 sounds super dusty, i understand why you like it! I was amazed by the sound of the modded 505 in a tube preamp, it really fits its 'dry transients' thing!

The 40106 was the first IC i messed with when i first got into electronic 6 years ago, you can even wire leds to it to see your bpm, you can mix the different channels together... It's only a matter of which capacitor and resistor you combine! You can even starve it! And i find hex inverters to be awesome distortion devices as well! Your idea is great, it would be super cool if it can work!

 

[Clbraddock]

Hmmm I wonder if there is another issue going on. I feel like I’m getting inconsistent results. I’m using Texas Instruments CD40106BE.

I can get it to oscillate, but the range seems random. I.e. changing resistor value results in a fairly unpredictable result. I have it hooked up to a scope since the freq range I’m trying for is above what’s audible.

I went ahead and ordered a LTC1799 board to see if that is anymore stable. Depending on how that works I guess I’ll decide how to move forward.

Appreciate all the suggestions and advice! I hope I’m at least half as knowledgeable as you guys someday.