HOW DOES A TRANSISTOR WORK

Diagram "A" shows an NPN transistor with the legs covering the symbol showing the name for each lead.
The transistor is a "general purpose" type and and is the smallest and cheapest type you can get. The number
on the transistor will change according to the country where the circuit was designed but the types we refer to are all the SAME.
Diagram "B" shows two different "general purpose" transistors and the different pinouts. You need to refer to data sheets or test the transistor to find the correct pinout.
Diagram "C" shows the equivalent of a transistor as a water valve. As more current (water) enters the base, more water flows from the collector to the emitter.
Diagram "D" shows the transistor connected to the power rails. The collector connects to a resistor called a LOAD and the emitter connects to the 0v rail or earth or "ground."
Diagram "E" shows the transistor in SELF BIAS mode. This is called a COMMON EMITTER stage and the resistance of the BASE BIAS RESISTOR is selected so the voltage on the collector is half-rail voltage. In this case it is 2.5v.
To keep the theory simple, here's how you do it. Use 22k as the load resistance.
Select the base bias resistor until the measured voltage on the collector 2.5v. The base bias will be about 2M2.This is how the transistor reacts to the base bias resistor:
The base bias resistor feeds a small current into the base and this makes the transistor turn on and create a
current-flow though the collector-emitter leads.
This causes the same current to flow through the load resistor and a voltage-drop is created across this resistor. This lowers the voltage on the collector.
The lower voltage causes a lower current to flow into the base and the transistor stops turning on a slight
amount. The transistor very quickly settles down to allowing a certain current to flow through the collector-emitter and produce a voltage at the collector that is just sufficient to allow the right amount of current to enter the base.
Diagram "F" shows the transistor being turned on via a finger. Press hard on the two wires and the LED willilluminate brighter. As you press harder, the resistance of your finger decreases. This allows more current to flow into the base and the transistor turns on harder.
Diagram "G" shows a second transistor to "amplify the effect of your finger" and the LED illuminates about 100 times brighter.
Diagram "H" shows the effect of putting a capacitor on the base lead. The capacitor must be uncharged and when you apply pressure, the LED will flash brightly then go off. This is because the capacitor gets charged when you touch the wires. As soon as it is charged NO MORE CURRENT flows though it. The first transistor stops receiving current and the circuit does not keep the LED illuminated. To get the circuit to work again, the capacitor must be discharged. This is a simple concept of how a capacitor works. A large-value capacitor will keep the LED illuminated for a longer period of time.
Diagram "I" shows the effect of putting a capacitor on the output. It must be uncharged for this effect to work. We know from Diagram G that the circuit will stay on when the wires are touched but when a capacitor is placed in the output, it gets charged when the circuit turns ON and only allows the LED to flash.