The Programmable Logic Controller is the ‘brains’ of the electrical control system but it still requires inputs from other sources, like low voltage keypads, to solve the complex switching requirements of today.

Low Volt Keypads

Eugene Kowch | P.I.D. Consultants Inc.

Low Voltage Keypads for the Programmable Logic Controller
By Eugene Kowch, P.I.D. Consultants Inc.

The Programmable Logic Controller is the 'brains' of the electrical control system but it still requires inputs from other sources, like low voltage keypads, to solve the complex switching requirements of today.


To utilize Programmable Logic Controllers (PLC) in an office or home environment it requires aesthetically pleasing keypads. Low voltage keypads, strategically placed throughout the office or home, provide the inputs for the PLC to determine which outputs to activate. These keypads should have multiple buttons in a single gang configuration, to remove the wall clutter that conventional switches create. These buttons, on the keypad, should also provide a momentary contact so that the latching feature may be preformed by the PLC, to keep lights 'On' or 'Off' for example.

There are numerous low voltage keypads available in the market place. One type is the simple Leviton single gang rocker, spring return to center, switch which provides two momentary contacts to the PLC discrete inputs. This switch looks the same as the conventional high voltage switches, but allows the electrical designer to turn 'On' and 'Off' two loads, one by pushing upwards and the other by pushing downwards. An application is at a stair landing, push down to turn 'On' or 'Off" the lights going down the stairs, push up to turn 'On' or 'Off" the lights on the stair landing. Three low voltage wires required to each switch.

National's modular buttons allows for one, two, three or more momentary contacts in a single or dual gang combination. Each button provides a very positive feedback with a clicking sound whenever the button is depressed. Comes with individual removable plastic covers so one can label each button as to function. An application is a school gym with numerous banks of lights that require switching from a number of different entrances and exits. One low voltage wire per button and a common wire required per keypad.

Xantech's eight button momentary contact in a single gang configuration. All eight buttons fit on a circuit board with a small terminal strip on the back of the keypad. Like the National's buttons, these buttons provide a very positive feedback with a clicking sound whenever the button is depressed. Comes with individual removable plastic covers so one can label each button as to function. These buttons are much smaller than National's and fit within the standard Decora face plates. Eight low voltage wires and a common wire required per keypad.

Lutron Homework's six or twelve contact closure keypads are also in a single gang configuration. These buttons are on a printed circuit board with special AMP connectors required to make wire terminations on the back of the keypad. Each keypad comes with LEDs to provide appropriate feedback.

These keypads do not fit the Decora standard and labeling is custom engraved. One low voltage wire per button, one wire per LED and four common wires required per keypad.

Lutron Grafik Eye systems have a six button, single gang, Decora style keypad. Four buttons provide status via LEDs, one button is the off button and the sixth button is a lower/raise rocker switch. These keypads communicate on a four wire proprietary bus with dip switches on the back to give a unique system address. When connected to a RS232 interface, these keypads communicate to the PLC via ASCII codes. Up to sixteen keypads can be connected over one system, thus giving the PLC over 100 virtual momentary contact switches.

Lovol's nine button, single gang, Decora style keypad was made exclusively for PLCs. Eight small buttons and one large button have back lit LEDs which stay half lit at night so as to view the buttons. These keypads are connected via CAT5 wire with RJ45 connectors. Four keypads may be daisy chained together with dip switches on the back to uniquely identify each keypad. Each keypad communicates on one 24VDC discrete input of the PLC via a series of pulses. This keypad maximizes the discrete input usage and frees up the RS232 port to communicate with other control systems.

The Programmable Logic Controller is the 'brains' of the electrical control system but it still requires inputs from other sources, like low voltage keypads, to solve the complex switching requirements of today.


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