PUT symmetrical square wave generator

It is possible to generate symmetrical square waves using the PUT configuration. To obtain a symmetrical square wave output at node 2, diode D1 and R5 have been added. By adding D1, the junction between C1 and D1 is not fixed to the ground potential anymore, but can go negative. During charging of C1, D1 is forward biased and the junction between C1 and D1 will be at +0.7V. When the voltage at the junction of C1 and R4 reaches the trigger voltage of the PUT, Q2 will start conducting and will pull the junction between R4 and C1 down to the ground potential. When this happens, the voltage at the junction of C1 and D1 will go negative, because C1 is still charged and D1 became reverse biased, so it will not conduct anymore. C1 will then be discharged via R5, which is about twice the value of R4, so the discharge rate of C1 via R5 is equal to the charge rate via R4, resulting in a symmetrical waveform at node 2.

Discharge of C1 happens via R5 and discharges C1 from a negative voltage via R5 to the positive supply. Charging of C1 happens via R4 and charges C1 from 0V via R4 to the positive supply. To have an equal discharge and charge current, R5 should be twice the value of R4, so the discharge and charge currents are equal.P.S. : R3 is not necessary in the circuit below and can be omitted.