Introduction to Industrial Automation

2.3.1   Timers

A time delay relay or simply a “timer” is called an electronic or an electromechanical switch, 

where its SPDT contact output is activated with delay with respect either to the start or stop of the 

switching operation. In general, there are two basic types of timers from the time-function point 

of view: the ON-Delay and the OFF-Delay timer. Also, there are three basic kinds of timers from 

the operation principle point of view, the electronic, motorized, and pneumatic timers, which are 

presented next.

Electronic timers are pure electronic circuits with an operation that is based on a solid state, 

integrated circuit of timer type and a number of other electronic components that control the 

operation of a micro-relay, which is the timer’s output. A printed circuit board of such an elec-

tronic timer is shown in Figure 2.54. Some low-cost and low-accuracy timers base their operation 

on an RC circuit and use the initially linear behavior of capacitor charging as a time meter.

The more common time-function of an electronic timer is the ON-Delay response of its out-

put. When the input voltage V

in

 is applied to the terminals A

1

 and A

2

, a timing delay T begins 

while the output contacts remain in their initial state, as shown in Figure 2.55. When the preset 

time delay has expired, output contacts change state and remain there as long as the input volt-

age is applied. The output contacts return to their initial state when the input voltage is no longer 

applied. Of course, this output response or timer behavior is repeatable as many times as desired.

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Hardware Components for Automation and Process Control 

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An electronic timer is able to cover a large time range, and the time delay is adjustable within 

this range. There are various types of electronic timers with several time ranges (for example, 

1–10 s, 1–30 s, 1–6 min, etc.), offering a broad choice of timing ranges from a second or even 

less to several minutes, hours, or days. Also, there are several ways of timing adjustment from 

external knobs, dual in-line (DIP) switches, and thumbwheel switches to recessed potentiometers. 

Electronic timers can accept a variety of input voltages from 12 to 230 V AC or DC, while the 

switching capability of the output contacts is usually up to 12 A.

Pneumatic timers: The most common pneumatic timers are designed to operate in conjunction 

with a power relay or a contactor. They are mounted on top of a relay of the same manufacturer 

and base their operation on restricting the flow of air through an orifice to a rubber bellow, dia-

phragm, or small air cylinder. Figure 2.56a illustrates the principle of operation of a simple air 

cylinder timer versus the construction design. When the relay is activated, the piston is moved 

downwards, overcoming the spring force; the air enters into the cylinder, while the check valve 

and the output contacts remain in their initial state. At the end of this movement, the piston rod is 

released from the relay and the piston is allowed to return back, due to the spring force. The check 

valve is then closed, forcing the air to escape through the orifice, the size of which is adjustable 

via a suitably mounted knob. The rate at which the air is permitted to be exhausted (adjusted by 

knob) is proportional to the time interval (time delay) needed for the piston to return to its initial