32 December 2015
This article introduces programmable relays as a means of building a smart doorbell.
Programmable relays are microcontroller-based devices that can replace older hard-wired
electromechanical control systems. A single programmable relay can replace an array of
relays, timers, and counters.
THE SMART DOORBELL —
A Simple Programmable Relay Project
FIGURE 1. Ladder logic tool bar.
Programmable relays use a visual anguage based on relay logic. Virtual devices are selected from a toolbar, configured from a drop down menu,
and positioned on a ladder diagram between
the power rails to achieve the required
functionality (Figure 1).
The TECO SG2 Client Software Version
3. 37 illustrated in this article is downloadable
for free from http://teco.us.com.
The visual ladder logic language was
developed to ease the transition of people
familiar with older electro-mechanical systems
to newer microcontroller-based control
The software allows for the simultaneous
monitoring of multiple inputs, outputs, and
virtual devices. Control functions can be
complex, so the addition of comments into
the ladder logic program can help to explain
the workings of the system when in monitor
mode (Figure 2).
Monitor mode allows an online computer
to display the status changes of all inputs,
outputs, and virtual devices as they occur.
The system designed in this article is a
smart doorbell. It produces different ring
patterns depending on when the door bell is
pressed. The system utilizes a standard door
bell and buzzer (Figure 3).
The intelligence is added to the system
by the programmable relay. The
programmable relay used in this article is a
TECO SG2-12HT-D. It is available from
FAC TORYMATION for $94. Additionally
required is a SG2-Ulink cable for $33. Both
are available at www.factorymation.com.
The status of the input and output
terminals are indicated on the programmable
relay’s visual display.
The display in Figure 4 shows that both
input I01 and output Q01 are energized.
FIGURE 2. Program comments added to ladder logic provide