Added a diode bridge, to avoid any chance of reversing the connection and destroying the
78L09 regulator. Now the full-wave rectifier will always deliver the correct polarity to the
regulator. The added bridge makes this look like a classic AC to DC rectifier circuit, but
the bridge is only to allow the 12VDC to work regardless of the connection's polarity.
The pnp transistor Q1 was changed to a TIP42. This transistor allows more maximum current
to the coil.
The assembly has been simplified by using two PC boards. The result is a lower parts cost
and the savings have been passed on to customers ($49 versus the old price of $75 US).