As described last month, basic applications requiring simple
on/off control can be driven directly from the PNS with only a few extra parts.
However, more complex applications often call for a dedicated controller,
perhaps based on a PICAXE chip.
Fortunately, a PICAXE chip in such a system can be programmed
to interact with the PNS by sharing information (such as sensor data) in a
common area of memory. These variables can then be displayed within web pages or
even manipulated by the PNS to alter system behaviour.
Environmental temperature control is one example of a process
that demands a dedicated control system. A temperature controller might be used
to regulate the temperature in a dwelling, greenhouse, fish tank or even
home-brew storage tank – to name a few examples!
An outline of a rudimentary controller that could be used to
maintain room temperature (in a cold-weather climate) within a specific range is
shown in Fig.1. It is based around a PICAXE-28X chip, which controls a heater
and fan via transistor switches and relays. A DS18B20 sensor provides
If the PNS were not connected to the temperature controller,
then the BASIC program in Listing 1 would be all that’s required. The program
waits until the temperature drops below a minimum of 16°C, at which point the
heater (on output 2) is switched on. When the temperature subsequently rises
above 22°C, the fan (on output 5) is switched on.