555 Timer – Make a Siren LED Blinker

image of 555 circuit on breadboard

Basically, you can use a 555 IC to generate a square pulse. Depending on how you wire it takes a DC input, like a 9v battery, outputs a blinking signal. Hooking an LED up to the output will cause it to blink on and off at a frequency dependent of the value of the size of resistors you use. There’s a couple of ways you can wire it up to do different things, but in this case, we’re going to operate the 555 timer in astable mode, meaning it takes a constant input, and outputs a pulse. From there, will take that pulse output and use a capacitor and resistor  to make a simple differentiator, which transforms the square wave into a much cooler spikey wave.

Here’s what you’ll need:

  • 555 Timer
  • 1 100uF Capacitor
  • 1 0.47uF Capacitor
  • 2 1MΩ Resistors
  • 1 330Ω Resistor
  • 1 680Ω Resistor
  • Breadboard
  • Various wires
  • 9v Battery or 4 AA 6v Battery Pack

Dont’ worry if you don’t have the exact resistors above. Swapping out different resistor and capacitors is half the fun of 555 timers. Provided you do it right, you’ll see differences in frequency and in the way the LEDs blink.

Wiring up a 555 can be a little messy sometimes. If you’re used to programmable circuit layouts, the 555 wiring is much messier than that. You’ll have to watch where you plug things in.

Basically the 555 operates in its astable mode, and generates a square wave. Then the differential section creates a spikey wave which you wire to the LEDs. Here’s what the progression of the signal from beginning to end.

graphs of voltage to time

Build your circuit based on the schematic below, referring to my image for the  wiring setup. Watch the polarity of the LEDs and caps though. The interesting thing about this circuit is that one of the LEDs is wired in reverse, meaning the red LED in the image below is wired backward compared to the green one. On the above graph, you can see the differentiator output voltage alternates between negative and positive. It’s like the voltage goes backward then forward. When you have two LEDs hooked to the circuit opposite each other polarity-wise, then they’ll blink back and forth. If you put them both in the same polarity-wise, then they’ll blink at the same time, either both on the negative spike, or both on the positive spike.

If you’re finding it’s not blinking or working consistently, the problem most likely lays with the values of you caps and resistors. The two resistors in the astable mode 555 control its output frequency. The higher the resistor value, the slower the frequency. It’s best to stay with high value resistors when dealing with LEDs so the blinking will be visible to your eyes. For the differentiator, stay with lower resistor values so the LEDs don’t get too dim. Keep the differentiator cap higher in uF than the 555 cap. There are mathematical equations that relate resistor and capacitor values to frequency and the differentiation factor, but that’s a little to complicated for here.

fritzing 555 diagram


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s