Update

October 22, 2009

Regrettably a few errors have crept into the circuit diagrams in Figures 1 and 2. In Figure 1, components K1 and K2 are drawn incorrectly and an inductor is missing between the analogue and digital ground lines.

In Figure 2, the relay contacts are drawn incorrectly in their non-actuated state, which suggests all inputs are interconnected. Also, two miniature chokes are missing in the +12 V and –12 V supply lines.

The electronic version (.pdf) of the article contains the corrected diagrams.