LED Flasher using a Thyristor (SCR)
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Just one active component in this LED flasher circuit - a thyristor or silicon controlled rectifier (SCR). With a couple of resistors and an electrolytic capacitor, the LED can be made to flash at a few Hertz.
https://en.wikipedia.org/wiki/Thyristor
https://en.wikipedia.org/wiki/Silicon_controlled_rectifier
https://www.schematicsforfree.com/archive/file/Oscillators%20and%20Generators/Misc/SCR%20Relaxation%20Oscillator.pdf