Hello everyone, I've come up together with a few circuits that will produce an oscillating signal with variable frequency. Unfortunately, I couldn't remember some formulas, and I don't have the time right now to look for them online. You are more then welcome to correct me, and add anything. :)

Here we go.

Manual starting Wien-bridge sine oscillator:

C1 = C2 = C
R1 = R2 = R
V1 = V2 = Vout (peak)
F = 1 / (2*pi*R*C)

To begin oscilations, set (Rf/Ri) > 3, then to keep the stable oscillations, set (Rf/Ri) = 3

Self starting Wien-bridge sine oscillator using zener-diodes:

A little modification allows for a self-starting oscillator. 
C1 = C2 = C
R1 = R2 = R
V1 = V2 = Vout (peak)
F = 1 / (2*pi*R*C)

Zeners' Vref should be smaller then V1
This is easy to build, but will NOT result in a clean sine-wave

Self starting Wien-bridge sine oscillator using JFET:

C1 = C2 = C
R1 = R2 = R
V1 = V2 = Vout (peak)
C3 is electrolytic (!)
F = 1 / (2*pi*R*C)
Rf= 2 * (R3 + r'ds)  !!! r'ds is a drain-source resistance of JFET (look in JFET's datasheet for this value)

Triangular-wave oscillator:

The first op-amp works as a comparator, second - as an integrator.
f = (R2 / R3) * ( 1 / (4*R1*c))

Square-wave oscillator:

Unfortunately no formulas.

Nice Stan, thank you.

The op-amp is a 741 or equivalent correct?

V2 would be a negative power supply correct?

Now some circuits driven by the mcu would be interesting.

Pin change high/low and PWM signals.

Thanks to the formulas I should be able to work out the parameters so that I can learn how to read/understand my oscilloscope.

I have my Forrest Mims Electronics Tablets with similar circuits but I like having them here.

Ralph

Yes, op-amp is 741 (those guys are really useful).

Yes, V2 is negative, sorry I didn't mark them, but generally on a DC power supply symbol the side that ends with a longer line is positive, and the side that ends with a shorter - negative.

If you want something cool, driven by the MCU, try this:

Integrator:

C1 = C
R1 = R
Vc = ( Ic / C ) * time
Iin = ( Vin / R )
Ic = Iin

( Change in Vout ) / ( change in time) = - ( Vin / ( Ri*C ))

Differentiator:

R1 = R
C1 = C
Vc = Vin
Ic = ( Vc / t )*C
Vout = Ic * R
Vout = -( Vc / t )*RC

Digital to Analog converter:

This can be a really useful circuit. Not too expensive either - for an N-bit converter you need 1 op-amp and N+1 resistors. It just works as weighted summing amplifier. Resistor 2R is twice as resistive as R, 4R is 4 times, and so on. You HAVE to keep this pattern.

LSB Vout = -Vin*(Rf / R), since TTL levels are 0 and +5v, we can rewrite that
Vout = -5*( Rf / R ) when we input 1 on LSB

The whole expression is (D0 - voltage at LSB; D8 - voltage at MSB):

Vout = -( D0*(Rf/R) + D1*(Rf/2R) + D2*(Rf/4R) + D3*(Rf/8R) + 
           D4*(Rf/16R) + D5*(Rf/32R) + D6*(Rf/64R) + D1*(Rf/128R))

Note the "-" at the beginning. If you want positive output, just pass it through an inverting amplifier. The output of this will be composed of a series of "steps" of voltages, instead of smooth analog signal, this is known as alaising. To smooth out the signal, (operation known as anti-alaising), just connect the output of the DAC to an integrator mentioned above.

Oh! it's off-topic, but I'm getting the same scope XD

The main problem with that type of DAC is that you need very accurate resistors. In order for R and 128R to be in the same ratio, you need better than 1% tolerance, or else test a lot of resistors to find ones that match the best.

If you actually want to build this circuit for use with the MCU, I'd stick to 6 or fewer bits (stop at 32R).

(or use a resistor ladder)

bretm, Yea you are right, if you really need a precise DAC. If someone just wanted to test their oscilloscope with it, or just to play around, that'd work with any 10% tolerance resistors :)

But if you don't need a precise DAC, you can just use 5 bits instead of 8 in which case 10% is appropriate anyway.