555

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Universal timer.

When CV is not connected it is held at 2/3.VCC through a 5k/10k divider (larger values on the 7555).

     +---+--+---+            +------+--------+------*---+-----+
 GND |1  +--+  8| VCC        | /RST |  /TR   | Thr  | Q | Dis |
 /TR |2        7| Dis        +======+========+======*===+=====+
   Q |3   555  6| Thr        |  0   |   X    |  X   | 0 |  0  |
/RST |4        5| CV         |  1   | < CV/2 |  X   | 1 |  Z  |
     +----------+            |  1   | > CV/2 | < CV | - |  -  |
                             |  1   | > CV/2 | > CV | 0 |  0  |
                             +------+--------+------*---+-----+