This circuit most likely the easiest and simplest radio transmitter which you will get anyplace. It uses total of 5 components and could be built into a really compact space. It's very good for science fair projects or other science related projects in which a small range transmission is useful.
It works on 1.5 to 3 V, with small hearing aid battery packs or lithium "coin" cells becoming excellent. A thermistor or photoresistor may be inserted in series with R1 to get a various output frequency depending on the input.
The frequency may also be adjusted by modifying the value of C1. A 2N2222 transistor is highly recommended, but you are able to try out other types as well. Performance is likely 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 about a 1/8 to 1/4" diameter non-conductive form. The coil is normally tapped 1/3 of the way from one end and also the tap connected to the emitter of Q1. Try out with all of the values in this particular circuit. Nothing is vital, but the result may be varied considerably.