Hello everyone !)
I am the newcomer in the sphere of electronics and have just started to learn it using the book "Make:electronics"
There in one of the projects I have constructed the circuit, shows below

Generally speaking, I understand it's joking principles, but still have some more questions:
1. What is the role of each resistor hear ?
2. Why is the PUT closed from the beginning ?
3. What makes the PUT open (why does the voltage in this part changes) ?

Will be happy to hear all the answers)

 

hi #1,
Look through this Uni-junction PDF, it explains the operation.
Page # 11- 4

E

 

I know hoe PUTs generally work, I have only some questions concerning this particular circuit

 

Found this short article that explains the circuit: http://www.electroschematics.com/6904/programmable-unijunction-transistor-put-flasher-circuit/

Looks like:
1.
1.a. 15k and 27k form voltage divider that is used to determine the gate voltage. The gate voltage is used to "program" the transistor. When voltage across 2.2uF exceeds gate voltage, the transistor is shorted, current flows to the led and led turn on.
1.b. 470k is used to control the current flow to the capacitor. Together they from RC network needed to calculate RC time constant, tau=R*C. You then plug tau into: frequency=1/(2*pi*tau)=1/(2*pi*R*C), this gives you the frequency at which the Anode will be charged.

2. Assuming the initial condition that capacitor is discharged. Then transistor Anode will have voltage lower than transistor Gate voltage, in this condition the transistor is not conducting, it is closed to current.