Door bell timer using 555 IC?

[Nick]

In light of John's comment, it's definitely worth measuring the off-load voltage of the transformer - the 555 is only good for 18V DC.

The bipolar NE555 (the original) is good to a max Vcc of 18VDC but the CMOS versions (which are generally "better"), e.g. the TLC555, are only typically good to 15VDC.

Not all "555"s are the same

With 12VAC from the transformer you'll get a peak under load of about 17VDC (Vpeak=Vrms*sqroot(2) for a sine wave), subtract 2 x 0.7V for the two diodes in the bridge that are always in series and you're left with about 15.5VDC.

Note also that in the diagram above, the relay has 15.5V across it, not 12V , so with a 280ohm Rdc that's now 47mA through the coil - actually, a bit less due to the Vce of the transistor.

Using a 12V zener you'll be dropping 15.5V - 12V, i.e. 3.5V,over the 330R resistor which will dissipate about 40mW, so use an 1/8th or 1/4W for safety - the zener will need to carry this current which is about 11mA.

A zener is a good choice here as the difference between the unregulated output (15.5) and the regulated requirement (12) is small at 3.5V. If you use an ordinary small 12V linear regulator, e.g. a 78L12, it has a Vf (forward dop voltage) of 1.7V, i.e. it requires an absolute minimum of 13.7V in to get 12V out which only gives us 1.8V of headroom - not a lot. Its in situations like this that LDO (Low Drop Out) linear regulators are useful having a typical Vf of only 0.5V, though using an LDO here would be gross overkill