18. The 555 Timer

Moving on from the simple transistor, and still not involving any coding, some devices have a number of components incorporated into them, including transistors.  The 555 device is an integrated circuit (IC) containing about 23 transistors, 16 resistors and a couple of diodes.

There are three ways to use a 555 timer:
Monostable operation is when the output signal switches between the default off position and a temporary on position at regular intervals - most commonly used for timers. 
Astable operation is when the output signal rises and falls in a set pattern, making it an oscillator. The pattern can be varied, so this mode of operation has many uses.
Bistable operation is when the signal can be held in one of two positions, enabling the 555 timer to act as a basic unit of computer memory.

A more simplified version shows the circuit in terms of operational amplifiers and logic gates :

The three resistors are 5kΩ each, and this is where the name of the chip comes from.  The pin-out is shown below:

Below is my circuit -  when I press the pushbutton, the red and green LEDs in the video (D1 and D2 respectively) flash alternately at a frequency depending on the setting of the variable resistor R2 (the knob on the right in the video).  This is astable operation.

I designed the circuit using Circuit Lab (www.circuitlab.com) which is excellent for designing and examining waveforms in advance of actually putting the components together:

With the variable resistor set at 0.5 (halfway), the predicted signals to the LEDs (ie the voltages across the LEDs - [Vnode1-Vnode2] and [Vnode2-Vnode3)] were:

and with R2 set at 0.1 (a tenth), the predicted voltages across the LEDs were:

The upper graph represents the red LED, which can turn on at 2.1V but not at 1.8V.  The lower trace represents the green LED, which can illuminate at 1.9V but not at 200mV.  

Here is the real thing in action (using a 9V battery):

And here are some captured waveforms, corresponding to the voltage across the red and green LEDs respectively:

Note that the duty cycle is not what Circuit Lab predicted. (I must look into that!  If you have any explanation, I would be eternally grateful).

However, although the trigger positions are not identical, the red LED voltage (top) is much smaller than green's, and when one is on, the other is off - as predicted.