Astable Multivibrator

http://www.electronics-tutorials.ws/waveforms/astable.html

Good oscillator circuit. It looks like R1 and R4 don't really matter, they generally seem pretty low (~500Ω).

 T = t1 + t2
 tx = 0.69*Cn*Rn
 

http://www.sentex.ca/~mec1995/tutorial/xtor/xtor7/xtor7.html

http://www.nutsvolts.com/magazine/article/bipolar_transistor_cookbook_part_6