Introduction to Industrial Automation

2.1.8   Pulse Bistable Relays

Pulse bistable relays are small electromechanical devices, which have the ability to open and close 

their contacts in a preset sequence. Although they are called “relays”, their operation principle, con-

struction, external form, and dimensions are quite different from that of the standard relays described 

in Section 2.1.1. Their use in automation systems is limited, but in the cases where this is required, 

it facilitates immensely the implementation of an application. A pulse bistable relay consists of a cam 

mechanism, an electromagnet, and the electric contacts as shown in Figure 2.12. The cam is a double 

ON button

OFF button

Set coil

Reset

coil

Reset

coil

(a)

(b)

Pivot

R

N

R

N

Power

contact

Power

contact

ON button

OFF button

Set coil

Pivot

Latch

state

Release

state

Figure 2.11  Schematic diagram of latching relay: (a) relay tripped and power circuit open, and