A single transistor makes a simple phase shift oscillator. The output is a sine wave with distortion of about 104. The sine wave purity can be increased by putting a variable resistor (25 ohms) in the emitter lead of Q1 (x). The resistor is adjusted so the circuit is only just oscillating, then the sine wave is relatively pure. Operating frequency may be varied by putting a 10 K variable resistor in series with R3, or by changing CI, C2, and C3
Making CI, 2,-3 equal to 100 nF will halve the operating frequency Operating frequency canalsobe voltage controlled by a FET in series with R3, or optically controlled by an LDR in series with R3.