On 10 Apr 2007 16:39:22 -0700, "
wrote:

I need a unit that can detect a downward change in temperature which
can be adjustable in the area of 31 degrees F. The trip point must be
accurate to one degree or better. The sensing element needs to be a
probe which can be remoted from the control unit by means of a cable.
The control unit will be mounted in a weather proof box ouside. The
unit must provide a contact closure at the preset temperature and hold
it until the temperature rises. It has to be battery powered,
preferably from 12.0VDC. The units that I've found thus far do not
have the accuracy required. Does any one know of any such equipment
available? Thanks, Lenny Stein, Barlen Electronics.

Probably can be done as a one-off design for under $150.

PICAXE microprocessor (under $10)
DS18B20 temperature sensing chip ($5)
one 2n2222 transistor (under $1)
78L05 regulator (under $3)
one relay (depends on the load)
about 60 lines of code (priced by the hour or by the project)

The PICAXE would read the DS18B20 chip every minute (or whatever) and
compare the temperature to the set point. If the temp is low, it
would operate the relay to power the transmitter for 10 seconds (or
whatever). At this point, the PICAXE can set a flag that the alert
has been sent, and continue to check the temp every minute. If the
temp is still low after 15 minutes (30 minutes, 1 hour, whatever), it
can turn the transmitter on again (same 10 seconds or longer).

If you want spares, you could duplicate the circuit as many times as
you wish.

John

Email for details: picaxe at jecarter dot com