Simplest oscillators for DC

[Belfior]

I am trying to create my own test setup and I would really go as simple as I can. Now I am trying to use a transistor in reverse avalanche mode to create an oscillator, but that works very randomly.

Could you guys help me find different kind of oscillators?

There can be a maximum of one transistor or mosfet. Using just inductors, resistors and caps would be optimal.

My setup is battery feeding a cap and then when the voltage reaches the avalance voltage, it goes through the transistor and L1

[Hoppy]

https://www.electronics-tutorials.ws/power/unijunction-transistor.html

[F6FLT]

.

[gyulasun]

Just to widen your choices,      see the Lambda diode oscillator consisting only of an n and p channel JFET
(if you cannot obtain a p channel JFET, it can be replaced with a pnp transistor plus 2 resistors):
http://www.zen22142.zen.co.uk/Theory/neg_resistance/negres.htm 

also see this collection here, mainly towards the bottom part:  http://zpostbox.ru/g1_e.htm 

Gyula

[Belfior]

Flyback could also work. Simple one here:

https://www.youtube.com/watch?v=BoUivNtYFIA

Just trying to get it as simple as it could be. Wondering if a 12MHz crystal would work with a mosfet somehow