A tutorial on how to make a Police Light Blink LED Circuit using a 555 Timer IC and some other electronic components. These circuits alternately blink between red and blue LEDs while individually blinking, much like a police strobe. An explanation of how the circuit works is also included in the video tutorial.
- 2 x 555 Timer ICs
- LED’s: 3 x Red, 3 x Blue
- Resistors: 2 x 1M, 2 x 68R (Resistors for LED’s)
- Capacitors: 1uF, 100nF
- Few Breadboard Connectors
- (5-12)V Power Supply
Note: Depending on the power supply used and the way LEDs are connected (series or parallel) you need to use a different resistor than 68R used in series with the LED’s. Refer to the table shown in the video tutorial above for resistors to be used for basic scenarios. You may also visit http://ledcalc.com/ to calculate the resistor based on LED arrangements.
In the previous Tutorial: Timer Circuit Using 555 IC (Adjustable Auto On Off Delay), we learned how to configure a 555 Timer IC to run in steady mode. We also connected 2 LEDs of opposite polarity at the output so that they turn on and off at regular intervals. You can refer to this tutorial to understand how the 555 timer IC works in stable mode and how the value of the capacitor and resistor affect the blink rate.
In this police strobe-style blinking LED circuit, we use two copies of a similar astable circuit configured with different frequencies. The first 555 timer ICs used larger capacitors and thus required more time to toggle the output. The second 555 timer IC uses a smaller capacitor so it can toggle the output very quickly.
Now let’s look at the arrangement of the LEDs. The first set of LEDs (red LEDs) light up when there is a positive voltage on the anode and a negative voltage on the cathode. This occurs when the output of the first 555 timer IC is ON and the output of the second 555 timer IC is OFF at the same time.
Likewise, the second set of LEDs (the blue LEDs) will only light up when the output of the first 555 timer IC is OFF and the output of the second 555 timer IC is ON.
So when the output of the first 555 timer IC is on, only the first set of LEDs will be on, and they will blink at the rate the second 555 timer IC is switching the output.
Likewise, when the first 555 timer IC is off, only the second set of LEDs have a chance to turn on, and they blink at the rate the second 555 timer IC toggles the output.
Repeating this cycle over and over creates this cool LED turn signal effect, similar to the flashing lights used on police cars.