PUT sawtooth generator

The following circuit uses a bootstrapping constant current source to charge C1 with a constant current, so we achieve a linear sawtooth waveform at node 1. The bootstrap is formed by Q3, that will conduct more current away from R5 and into the ground when the base emitter voltage exceeds 0.7V. So when the current through R5 increases such that the voltage over R5 exceeds 0.7V, the transistor will eat current away from R5 by letting more current flow into the ground. So it Q3 acts as a kind of overflow valve that draws abundant water into a well at the moment that the water flow is too high.
Suppose the current through R5 would increase. Then the base-emitter voltage of Q3 increases, because the voltage over R5 increases. The result is that Q3 will conduct more, pulling its emitter "lower" (closer to ground) by adding extra current through R4.
When the voltage over R4 increases, this means the voltage over R5 must decrease, so the current through R5 decreases. This way, Q3 keeps the current through R5 constant and C1 will be charged with a constant current, resulting in a linear rising voltage. As soon as the voltage reaches the threshold voltage at node 2, that is set by R1 and R2, the PUT will latch into conduction and fast discharges C1 via R3. The voltage at node 2 goes down to 0.7V volts. When C1 is fully discharged, the PUT does not receive enough current to stay latched, and unlatches.
When the PUT unlatches, the voltage at node 2 jumps up to the threshold voltage again (which is close to the supply voltage) and C1 is charged again by the constant current source.
The cycle repeats over and over again …