We looked at the 555 timer device used as an interval timer and as an oscillator; a typical example of a class of circuits know as 'Relaxation Oscillators'. This gave us an opportunity to revise op-amps, as there are 2 of them in the 555 chip, acting as voltage comparators.
Unfortunately, one of the demonstration circuits refused to work properly. This was subsequently found to be due to an electrolytic capacitor been connected upside-down, which destroyed the dielectric layer. A few seconds after re-connecting it with the correct polarity the dielectric reformed and the circuit worked perfectly, and will be demonstrated again at the next session. On most electrolytics the arrow points to the negative terminal, but this one was unusual in that it pointed to the positive terminal - And the moral is - always check the polarity!
We had a look at Charlie's non-functioning rain-gauge, but were frustrated by not having a screwdriver to remove the (totally unnecessary) security screws on the cover.
We finished with Mike's question about interfacing the 12V output of a central-heating valve to the 3V input of a microprocessor and considered the merits of various methods of limiting the voltage, including Zener/resistor networks and voltage control chips.