The circuit shown in this web page is based on the information presented in this article:

https://www.instructables.com/Oscillator-LED-Dimmer-1/

I modified the circuit to include Zener diode maximum LED current protection. I implemented the "Zener" diode with 7 typical silicon diodes because I already had those components:

VcapacitorMax = 7 * Vd = 7 * 0.7 V = 4.9 V

You can see the photo of my circuit here:

I added an additional LED (that is not needed):

Step 1: Design the Circuit

I drawn the circuit in PSpice student edition software:

The optional circuit diode should be a Schottky diode (preferably).

Calculate the maximum LED current:

Imax = (Vs - 2*Vled) / Rd2

= (5 V - 2 * 2 V) / 100 ohms

= 10 mA

Step 2: Simulations

Time domain:

Frequency domain:

Step 3: Testing

I tested the diode protection circuit (without Schottky - diode shorted):

I tested the diode protection circuit (with Schottky):

Step 4: Playing

I continues to play around with my circuit: