How do you create a siren like led effect? Is there a component to do that?

Is there a hardware piece that behaves like a siren light say on a fire truck/snow plow/etc without the sound?
Or does have to be done in software?


Not sure where you live, in California US, I think there are some kind of mirror that rotates. Is that what you want?

I think there are some kind of mirror that rotates

I think that's the ooooooold way... pretty sure they're all solid state nowadays

Do you mean flashing?
Blink without delay type sketch powering a transistor to turn on or off your backlight would work.
Slowly rising and falling could be done with a for loop, same hardware, PWM pin.
I usually have to find the problems in my sketches to stop the actual letters ect from flashing, clearing the screen in the wrong place is usually the problem there, I have used it to flash warnings.

The Multiblink sketch at my web site (see below in the signature block) implements that sort of flashing as an example. It came up a while ago when I was helping someone else. The pattern was something like two flashes on one side of the vehicle and two on the other. Anyway, if the pattern is not there it is easy enough to modify the program data to get what you need.

To answer your question, yes. Two chips can readily be configured to act as an led "siren" flasher

LM3909 specifically designed for this, but no "police" pattern, duty cycle and speed are set with two resistors & 1 capacitor
LM555 timer IC - identical to the above chip, no "police" pattern

If your looking to duplicate the patterns seen on official vehicles (cop cars, fire/tow trucks or ambulances), then this would be much easier to do in software. It's actually multiple patterns repeating. A for loop with a pin bitmask of however many outputs you wanted to have, something like this:

led_pins = 0,1,2,3 = outputs
variables = pattern_a, pattern_b, i
disable_pin = 4 //input pin to disable the outputs
              // pins = 0123
#define pattern_a %1100 
#define pattern_b %0011
#define pattern_z %0000
i = 1

loop while i  //run this main loop until the input pin for i is high
  loop from 1 to 2
    led_pins = pattern_a  //turn on leds connected to pins 0 & 1 for 1/8 second
    wait 1/8 sec
    led_pins = pattern_z //turn off leds
   wait 1/4 sec            //wait 250ms then repeat loop one more time
  loop from 1 to 2
    led_pins = pattern_b  //turn on leds connected to pins 2 & 3 for 1/8 second
    wait 1/8 sec
    led_pins = pattern_z //turn off leds
   wait 1/4 sec

  if i = 0
      exit loop //if disable pin is brought to ground then exit the flash loop

That mini program wont compile, doesn't actually do anything. It's so you can get the idea of how to do what you want in software. this particular code will gennerate the alternating double flash often seen on police cars. Of course, if your driving many leds on each output, you will need a transistor interfaced with your pins, Mosfets are a very good choice because the inputs dont really draw any current from your pins and they can switch heavy currents without getting very hot.

If what you're trying to do is just flash a few leds on two outputs with a disable, you can connect 3 D-type flip flop gates, a clock source like an LM555, two AND gates, and two output transistors to actually flash the leds. A few dollars in parts.