555 Timer (thing)

Here's the circuit for an astable multivibrator using the ubiquitous IC 555

    ____________________________  +Vcc
   |            |     |
   |            |     |
   \        ____|_____|____
R1 /       |    4     8    |
   \       |               |
   /       |               |
   |       |               |
   |-------|7              |
   |       |               | Output
   \       |    IC 555     |       __   __ 
R2 /       |              3|---- _|  |_|  |_
   \       |               |
   /       |               |
   |_______|6              |
   |   |___|2   5     1    |
   |       |_______________|
 ----- C1       |     |
 -----      C2 ---    |
   |           ---    |
   |____________|_____|__________  Gnd

• 1. Ground.
• 2. Trigger.
• 3. Output.
• 4. Reset.
• 5. Control voltage.
• 6. Threshold.
• 7. Discharge.
• 8. +Vcc in.

• The threshold pin is initially at ground because C1 is charging through R1 + R2. The output pin is at +Vcc.
• As soon as the potential of the threshold pin reaches 2/3 of Vcc , the output pin switches to the off state and the discharge pin goes to ground potential.
• This causes the capacitor C1 to discharge through R2.
• When the trigger pin goes below 1/3 of Vcc , the output pin goes back to the on state and the discharge pin goes to a floating (disconnected) state.
• The entire cycle repeats indefinitely.