This is probably become the simplest radio frequency RF transmitter that you will find anywhere. It has a total of five parts and can be constructed into a very small space. It is great for science fair projects or other science related projects where short range transmission is useful.
It operates on 1.5 to 3 Volts, with small hearing aid batteries or lithium “coin” cells being ideal. A thermistor or photoresistor can be inserted in series with R1 to have a varying output frequency dependent on the input.
The frequency can also be changed by changing the value of C1. A 2N2222 transistor is recommended, but you can try other types also. Performance tends to vary from type to type as well as from transistor to transistor. L1 is 20 to 30 turns of thin magnet wire (24 to 32 ga.) close wound around a 1/8 to 1/4″ diameter non-conductive form. The coil is tapped 1/3 of the way from one end and the tap connected to the emitter of Q1. You may make experiment with all of the components value in this circuit. Nothing is critical in this circuit, but the performance can be varied considerably.
Simplest RF Transmitter Source: http://www.sentex.net/~mec1995/circ/rft1.htm