Introduction to Industrial Automation

Figure 2.54  Printed circuit board of an electronic switch timer.

Timer connection diagram

Time ON-delay (T) operation

T

Power

supply

t

t

ON

Open

Output signal

(time delay NO

contact 11–13)

Closed

OFF

A

1

A

2

12

13

11

SPDT

output

V

in

V

in

Figure 2.55  Connection diagram and ON-delay response of an electronic timer.

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  Introduction to Industrial Automation

position. At the end of the piston’s back movement, the projected part of the rod piston causes 

the output contacts to change state. The output contacts remain in this state until the relay is de-

energized, when another mechanism (not shown in the figure) returns the contacts to their normal 

state. The described behavior of the pneumatic timer corresponds to the ON-Delay time function 

of the electronic timers. In a similar mode of operation, pneumatic timers can offer an OFF-Delay 

time function of output contacts. With an OFF-Delay pneumatic timer, timing begins when the 

relay is de-energized, as shown in Figure 2.56b. The output contacts change state immediately 

after the relay activation and return to their normal state after the time delay T has expired. 

Some pneumatic timers are designed to offer both time functions (ON-Delay and OFF-Delay) 

and permit the timer to be changed from one to the other by a simple mechanical micro-switch. 

Electronic timers with an OFF-Delay time function are also available, but they need one more 

input signal for triggering the off-event time instant and an uninterrupted power supply.

Pneumatic timers are popular throughout the industry because they have some basic advan-

tages. For example, they are unaffected by variations in ambient temperature or atmospheric pres-

sure, they are adjustable over multiple time ranges, they have a single turn time-adjustment knob, 

and they are simply convertible from ON- to OFF-Delay and vice versa. But the most significant 

advantage, in the authors’ opinion, is that the pneumatic timer is the only device that can produce 

a kind of electric signal (electric contact change) with a time delay after the general failure of the 

central power supply system. In such case, the electric contact can be combined with a battery 

bank to activate with time delay, or a siren or a flashing light for indicating the existence of a fault, 

a functionality that cannot be achieved with other types of timers.

Motorized or electromechanical timers are motor driven timers with single or multiple time 

ranges, accurate and quick recurring operations, and instantaneous and/or delayed contacts. They 

use either a clock quartz motor or a synchronous motor to rotate a set of gears through which a 

mechanical activation of the electric contacts is achieved. If an electromechanical timer has mul-

tiple time ranges, the selection is performed by changing the gear ratio through a small recessed 

switch and a friction-clutch mechanism. Figure 2.57 shows two electromechanical timers from 

different manufacturers, on the front of which the time adjusting knob is visible. In timers with 

motors, the adjusting knob is back-rotated during timing, thus offering a moving pointer function.

(b)

Time OFF-Delay (T) operation

Relay

SPDT

output

Actuator assembly

mounted on movable

core of the relay  

Controlled

air exhaust

Air exhaust

adjustment knob

Check

valve

Spring

Cylinder

Piston

Timer

housing

One direction

gearing mechanism

T

Relay

power

supply

t

t

ON

Open

Output signal

NO contact 

Closed

OFF

(a)

Pneumatic (air) timer block