🚀 go-pugleaf

In the spirit of the poster's first article, the following
discrete circuit will get the job done:

                               +5.0V      +5.0V    +5.0V
                                 |          |       |
                                 |          |       |  minus
            C1                 C |          |      D1
           10uF                 /         relay   1N4148
logic     + | | -         B  | /          coil      |
 in  >------| |-----+--------|  Q1          |       |  plus
            | |     |        | \            +--------
                    |         E \           |
                    R1          |         C /
                    50K         |      B | /
                    |           +--------|  Q2
 Q1 and Q2          |           |        | \
 are NPN            |           R2        E \
 general purpose    |          3.3K         |
 (like 2N2222)      |           |           |
                    |           |           |
                   GND         GND         GND

When the logic signal makes a low-to-high transition,
the base of Q1 rises.  This turns on Q1.  The emitter
of Q1 rises, and that turns on Q2 (energizing the relay).

After a delay equal to (R1 * C1) = 0.5 seconds, the base
of Q1 falls low again, turning off Q1.  This turns off
Q2 and the relay.

Protection diode D1 clamps the inductive back-EMF
induced when the current in the relay coil cuts off
to zero.

TTL compatibility is achieved by setting the turnon
point to 2*VBE (same as TTL).