# Variable Sawtooth Core Oscillator

#### romons

Joined Feb 12, 2017
8
That should be fine. Remember that if you can't get down to 20hz, use a higher value pot, or a larger timing cap.

#### drmanmachine

Joined Mar 12, 2016
68
Thanks again Romons. You've been more than helpful. I will post my results and let you know.

#### drmanmachine

Joined Mar 12, 2016
68
I'm using Multisim for my simulation and I'm having a challenging time being able to acquire the frequency range of 20 Hz. to 20 kHz.

Is there a proper formula for this circuit to acquire the frequency?

#### romons

Joined Feb 12, 2017
8
I'm using Multisim for my simulation and I'm having a challenging time being able to acquire the frequency range of 20 Hz. to 20 kHz.

Is there a proper formula for this circuit to acquire the frequency?
The current is equal to the voltage drop across the pot, which is vcc - 0.6. So input current is

I = (vcc-0.6)/Rpot

The rate of change of voltage is thus

dv/dt = I/C

The voltage must climb from 1/3 to 2/3 of vcc. So,

Vcc*C/I/3 = t, so f = (3*I)/(vcc*C)

If vcc = 5, and C = .68uF, then 20 hz corresponds to 22.6uA. 20kHz corresponds to 1000 times that, or 22.6mA.

That works out to 194 ohms for 20k, and 194k ohms for 20 hz.

I actually built the circuit, and it only works at the desired range using a CMOS 555. I used a 0.68uF timing cap, and a 100k ohm pot. There is clearly some leakage, probably through the 555

#### romons

Joined Feb 12, 2017
8
The current is equal to the voltage drop across the pot, which is vcc - 0.6. So input current is

I = (vcc-0.6)/Rpot

The rate of change of voltage is thus

dv/dt = I/C

The voltage must climb from 1/3 to 2/3 of vcc. So,

Vcc*C/I/3 = t, so f = (3*I)/(vcc*C)

If vcc = 5, and C = .68uF, then 20 hz corresponds to 22.6uA. 20kHz corresponds to 1000 times that, or 22.6mA.

That works out to 194 ohms for 20k, and 194k ohms for 20 hz.

I actually built the circuit, and it only works at the desired range using a CMOS 555. I used a 0.68uF timing cap, and a 100k ohm pot. There is clearly some leakage, probably through the 555
Correction. Using a 180 ohm resistor gets 20khz, and using a 220k resistor gets 20 hz. You are going to need a very good pot to get both. My 20 turn cermet resistor didn't go high enough. You can use a 220k ohm 20 turn resistor, which should work properly.