PUT relaxation oscillator

The following circuits show 2 implementations of a relaxation oscillator using the PUT transistor combination.
The oscillators can be used with a wide range of power supply voltages.
The first circuit generates a non-linear sawtooth waveform with a rising ramp.
At nodes 1 and 2, different waveforms are produced. At node 1, a ramping waveform is available and at node 2, a pulsating waveform is available. The slope, and thus frequency, of the ramping waveform is controlled by R4 and C1 and depends on the power supply voltage.
Increasing R4 or C1 will decrease the frequency, same as when increasing the power supply
The pulse width of the pulses, that are generated when the ramp waveform resets, is controlled by R3.
How the relaxation oscillator works: Initially, C1 is discharged, so node 1 is at 0V. Q2's base-emitter is reversed biased (the base of Q2 is at Vdd/2, while its emitter is at 0V), so Q2 will not conduct. Because Q2 is open, Q1 doesn't get any base current and will not conduct either. The voltage over C1 will increase while the capacitor is being charged by the current flowing from the power supply via R4. At node 1, the voltage increases exponentially while C1 is charged. When the voltage over C1 reaches the point where the emitter-base junction of Q2 becomes forward biased (at about Vdd/2 + 0.7V), Q2 will start conducting. As a result, Q1 will get base current via Q2 and will also start conducting, thereby pulling the base of Q2 low. The PUT combination is now latched in conduction and will discharge the capacitor C1 via R3. Because R3 has a low value, the capacitor is discharged fast via the PUT combination. The voltage a node 2 will not drop to 0V. When the voltage over C1 has dropped below 0.7V, Q2 will stop conducting and the PUT combination "unlatches" again, so C1 can charge via R4. The voltage at node 2 is now Vdd/2.
This process repeats itself over and over.
When R1 is changed to f.e. 1K, the waveform amplitudes at node 1 and 2 will be bigger, because the voltage at which the PUT combination will latch will be closer to Vdd.
R3 is added so the pulse width of the pulses at node 2 can be adjusted. When R3 is increased, the pulse width of the pulses will increase.

The following circuit works in the same way as the first circuit, but generates a non-linear sawtooth waveform with a falling ramp.
The waveforms of this circuit are now related to Vdd instead of ground, because the circuit is "upside-down" compared with the previous circuit.
When R2 is changed to f.e. 1K, the waveform amplitudes at node 1 and 2 will be bigger, because the voltage at which the PUT combination will latch will be closer to ground.
R3 is added so the pulse width of the pulses at node 2 can be adjusted. When R3 is increased, the pulse width of the pulses will increase.

Important notes for both circuits :

It is possible that the oscillator does not start when you are tweaking C1 and R4 to change the frequency. When that happens, then probably R4 has become too low. When R4 is too low, the PUT will not unlatch itself, because even though it has discharged C1, it still receives enough current via R4 to stay latched. In that case, increase R4 again and use a lower capacitor to get the desired frequency. Also, R1 and R2 play a role in this game. When R1 and R2 are too high, the PUT does not receive enough gate current to firmly latch and C1 will not be discharged deep enough to unlatch the PUT again. So it is better to have a low value of R1 and R2 than a high value.
R3 can not be increased without a limit.
At a certain point, R3 will be too high, so the PUT does not receive enough current to firmly latch into conduction and can not discharge C1 far enough to unlatch again. So the oscillator does not start when R3 is too high.
For the same reason, R4 has a high value, so the PUT can discharge the capacitor far enough to unlatch again. When lowering R4, R1 and R2 should be lowered to compensate, so the PUT can draw more current out of C1 to discharge C1 deep enough to unlatch again.
When f.e. changing R4 to 100K, R1 and R2 should be lowered to about 470E.