Engineering · PLC · Project 9 of 15 · Signals and alarms · Level 1

Flasher (blinking lamp)

Two timers feed each other to blink a warning lamp. Used in nearly every alarm program.

Learning model only. This page teaches the program logic. A real machine needs a proper control panel with an isolator, fuses, motor protection and a hard-wired emergency stop. Mains and motor wiring is for qualified electricians. A PLC output never drives a motor directly.

You will learn

T1 counts the OFF time. When it finishes the lamp turns on and T2 starts. When T2 finishes it breaks the T1 rung, which resets both timers, and the pattern repeats. Many PLCs also have a built-in clock bit that does this for you.

How it works

Flip Flasher ON. The beacon blinks about once a second.
Flip it off. The timers reset and the lamp stays off.

In real life

Alarm beacons, fault indicators, battery-low lamps.

Inputs, outputs and addresses

The same signals on seven PLC brands. Inputs and outputs are numbered in order; check your own hardware and software, because addresses depend on the model and module position.

Name	What it is	Type	Siemens S7-1200	Allen-Bradley CompactLogix	Mitsubishi iQ-F FX5U	Delta DVP / AS	Omron CP2E	Schneider M241	Inovance H5U
ENABLE	Flasher enable switch	Input	`%I0.0`	`Local:1:I.Data.0`	`X0`	`X0`	`0.00`	`%IX0.0`	`X0`
BEACON	Warning beacon	Output	`%Q0.0`	`Local:2:O.Data.0`	`Y0`	`Y0`	`100.00`	`%QX0.0`	`Y0`

Ladder diagram (shown for Siemens, live state)

Green wires carry power in the moment captured below. Switch the brand and the language in the simulator.

Structured text, Siemens SCL

(* Siemens S7-1200, CPU 1214C DC/DC/DC - TIA Portal - SCL *)
(* Rung 1: OFF time (0.5 s) *)
"T1"(IN := "ENABLE" AND NOT "T2".Q, PT := T#500ms);
(* Rung 2: ON time (0.5 s) *)
"T2"(IN := "T1".Q, PT := T#500ms);
(* Rung 3: Beacon follows the first timer *)
"BEACON" := "T1".Q;

Structured text, Allen-Bradley

(* Allen-Bradley CompactLogix 5069 (L306ER) - Studio 5000 Logix Designer - Structured Text *)
(* Rung 1: OFF time (0.5 s) *)
Timer1.PRE := 500;   (* ms *)
Timer1.TimerEnable := ENABLE AND NOT Timer2.DN;
TONR(Timer1);
(* Rung 2: ON time (0.5 s) *)
Timer2.PRE := 500;   (* ms *)
Timer2.TimerEnable := Timer1.DN;
TONR(Timer2);
(* Rung 3: Beacon follows the first timer *)
BEACON := Timer1.DN;

Try this

Make it blink faster by changing both presets to 200 ms.
Make the ON time shorter than the OFF time.

Parts to buy

The links open a search on each store, so you see what is in stock today. Check the model, voltage and price before you order.

#	Item	Buy / compare
1	PLC	Amazon.in · IndiaMART · Robu.in
2	Beacon or lamp	Amazon.in · IndiaMART · Robu.in

← 8. Traffic light All projects 10. Part counter with batch stop →

This is a learning tool. It teaches the logic, not the exact keystrokes of each vendor’s software. Check the manual of your own PLC.