Silicon Chip – June 2019

96 Silicon chip Australia’s electronics magazine siliconchip.com.au

Our lab has an air conditioner which
is needed to keep the computer equip-
ment at a reasonable temperature dur-
ing summer.
But if we simply use the thermostat,
it will run continuously on a summer’s
day and that is not required; we want
a lower duty cycle than that. So I de-
cided to build an Arduino-based tim-
er to switch it on and off based on the
time of day. But others in the lab want
to be able to see the time and date dis-
play from this timer and also get an
idea of when the air conditioner is to
be switched on and off.

I found that difficult to incorporate

into the timer software without inter-

fering with the operation of the tim-

er. So I used a second Arduino chip

to drive the display and it reads the

time and date out of the same real-

time clock module.

So this project demonstrates how

a single RTC module can be shared

between multiple microcontrollers

which are doing different jobs. I didn’t

want to use two separate RTC modules

since there’s no guarantee that they

will not drift apart.

As the old adage goes, “a man with

Two micros control an aircon with a single real-time clock module

one watch always knows what time

it is. A man with two watches is nev-

er sure.”

The circuit is based on a DS3231

real-time clock module, two Arduino

ATmega328 chips and two small 5V

DC coil relays which drive the 5kW

contactor that controls the air con-

ditioner.

You can share the RTC module this

way because the ATmega328’s hard-

ware I^2 C implementation supports

the “multi-master” bus mode, which

can handle the case when both devic-

es want to use the bus at once – one