On Tue, 25 Feb 2014 00:19:15 +0000, Bill
wrote:

In message , "Dave Plowman (News)"
writes
In article ,
Fredxxx wrote:
Most small transformers consume a few watts, say 5?

Where did that figure come from?

Given there are
8,700 hours a year that equates to 43.5kWHrs, which at 10p per unit
would be roughly £4.35

That pays for quite a few alkaline batteries at Poundland. Plus
batteries don't "buzz" in the bedroom above.

Batteries go flat. Usually at the most inconvenient time.

As this is a DiY group, how about having 2 batteries, main and standby,
with an auto change over circuit that gives an indication when it has
operated so that you have a couple of years warning that you need to
change the flat battery?


Many years ago, after we moved into the current dwelling, I fitted a
GPO trembler bell ('perk of the job' item) and used a couple of 8 x AA
cell holders to make up a 24v battery pack hooked onto a screw in the
door frame immediately below where the bell had been fitted using a
spare oblong door bell push just the other side.

This worked just fine for about a decade or so before the alkaline
cells started to leak, ruining the battery holder. Rather than waste
time trying to clean up the battery holder and spend a small fortune
on another 16 AA cells, I simply bought a pack of 3 Carbon Zinc PP3s
from a local Pound Shop and soldered straps to turn the pack of PP3s
into a 27v battery (nothing was wasted!) which I hung on the screw
head used by the previous bulky "battery". The result looked a lot
neater than the previous arrangement.

A few years ago, prior to the PP3 battery pack job, I'd bought myself
one of those wireless door bells so that I could still be alerted when
I had my upstairs office door closed and noisy equipment running/
music playing.

I mounted the wirless sender to the inside of the door frame and
connected it to the existing wired door bell using a blocking diode
and a zenner 'dropper' so that its internal 12v battery only got used
whenever it was pressed to test the 'Office Bell'[1] (callers
operating the wired bell push caused the 24v (now 27v) to power the
sender as well as the bell).

I did the 27v PP3 battery replacement about a year or so back and
it'll probably be good for another 2 or 3 years before I need to spend
a pound on another 3x PP3 pack. I spend more on the 3 AA cells used by
the wireless bell unit which seems to need a fresh set every 12 months
or so (again, a set of four alkalines can be had in most pound shops).

The door bell case is the classic example of disposable battery power
being an effective alternative to mains power (no PSU failure to worry
about and the cost of battery replacement is on a par with or better
than the cost of the parasitic load of a bell PSU).

Being primary cells, I get ample warning of imminent battery failure
without the need for any fancy voltage monitoring circuitry.

[1] My office door bell enjoys (endures?) frequent testing by SWMBI
whenever she comes back with a load of shopping.
--
Regards, J B Good