The UJT below is configured to produce a sawtooth wave at a frequency of 104 Hz. The capacitor begins to charge through the 4.7 kOhm resistor when the circuit is initially powered up. The current suddenly flows from the UJT emitter (E) to the UJT Base (B1) when the voltage across the capacitor rises to a certain point. This will produce a voltage across the 100 ohm resistor which is the same as the output of the UJT relaxation oscillator on the Emitter.
The sawtooth waveform is applied to op amp pin 3 and the voltage divider is applied to op amp 2. The op amp produces a positive going output on [in 6 when the sawtooth is higher than the reference from the divider of 2.5 volts. The op amp acts as a comparator. [Circuit’s schematic diagram source: Chuck Bolin Electronic Project]