Home heating monitor

Archiver

I want to build a monitor to watch the 6 zones of my home heating system so
I can tell which zone is on and for how long each day. Should be easy to
program on a stamp.

The interface point is a relay box where the thermostat wires come to the
boiler. They are simple 24v AC thermostats, probably a mercury switch. I
need to hook across this line, but cannot draw enough current that my
interface trips the furnace relay. Their is also some current sensing in the
thermostat that controls its hystresis that I dont want to confuse by
drawing much current.

Any good ideas for a 24v AC to 5v logic interface?

ron ginger

Archiver

Ron,

You might take a look at Optoisolators that have a high current
transfer ratio such as the NTE3093, available from Mouser. The LED
only requires 1.6ma and the output is TTL compatible. However the LED
reverse voltage is only 5V so I would suggest using a 1N4002 in series
with it to block the negative swing of the 24VAC.

The output will be a 60HZ squarewave that you can filter or simply
detect with the Stamp software.

Dave


--- In basicstamps@yahoogroups.com, "Ron Ginger" <ginger@m...> wrote:
> I want to build a monitor to watch the 6 zones of my home heating
system so
> I can tell which zone is on and for how long each day. Should be
easy to
> program on a stamp.
>
> The interface point is a relay box where the thermostat wires come
to the
> boiler. They are simple 24v AC thermostats, probably a mercury switch. I
> need to hook across this line, but cannot draw enough current that my
> interface trips the furnace relay. Their is also some current
sensing in the
> thermostat that controls its hystresis that I dont want to confuse by
> drawing much current.
>
> Any good ideas for a 24v AC to 5v logic interface?
>
> ron ginger