I was reminded that a simple 555 timer can create an astable oscillator that can be used with a DVM to measure a transformer windings when at a rally or when there are no markings.
The values for R1, R2 and C1 will give you the frequency and the duty (charge/discharge).
t1 = 0.693 (R1 + R2) x C
t2 = 0.693 x R2 x C
You might then follow that with a simple op amp integrator to turn the square wave into a sine wave.
Or another option without the op amp so a similar small component count is a saw toothed oscillator.
The frequency of the circuit is: