Strobe LED / Police Flasher Circuit

The strobe LED, also known as the police flasher circuit, is a signal lamp consisting of a pair of LED groups, one after the other that flashes. And the purpose of this article is to explain the principle of operation.

In this article, we will tell you how to make police strobes and how they work with 3 different circuit examples.

First Circuit (Only 555)

The police strobe made with a single 555 can only be used in small sizes for trial and test purposes, in short, to understand the operating principle of the 555 and interpret the signal.

This project is for flashing two LEDs at intervals of about 500ms. The circuit uses 555 timer IC along with several other electronic components and is built on a circuit board. In this circuit, 555 timer IC is used in unstable mode, which allows the output to continuously fluctuate between high and low until it turns off power.

You can also adjust the flashing rate of the LED by changing the capacitor or resistance values used.

Required Materials

Circuit Diagram

Single 555 Circuit Diagram

Second Circuit (Double 555)

In a previous tutorial: We learned how to configure the Adjustable Flashing LED Circuit using 555 Timer ICand 555 timer IC to work in unstable mode (astable). We also connected the 2 LEDs at reverse polarity at the output, so that they became ON and OFF at regular time intervals. You can refer to this tutorial to understand how the 555 timer IC operates in unstable mode and how capacitors and resistance values affect blink rate.

In this police strobe-style flashing LED circuit, we used two copies of similar unstable circuits configured at different frequencies. The first 555 timer IC uses a larger capacitor and therefore takes more time to change the output. The second 555 timer IC uses a smaller capacitor and therefore changes output very quickly.

Now as for the layout of leDs, the first LED group (Red LEDs) lights up when there is positive voltage in the anode and negative voltage in the cathode. This scenario occurs when the output of the first 555 timer IC at the same time is ON and the output of the second 555 timer IC is OFF.

Similarly, the second LED group (Blue LEDs) only outputs the first 555 timer IC off, and the second 555 timer IC is turned on if the output is ON.

Therefore, when the output of the first 555 timer IC is ON, there is only a chance that the first LED group is ON, and the second 555 timer flashes at the speed at which the IC changes output.

Similarly, when the first 555 timer IC is OFF, only the second LED group has a chance to be ON, and the second 555 times ic flashes at the speed at which it changes output.

Repeated this cycle creates this LED flasher effect, similar to the flashing lights used in police cars.

Required Materials

  • 2 x 555 Timer Integrated
  • Breadboard
  • 1 μF , 100nF Capacitor
  • 2 x 1MΩ, 2 X 68Ω Resistance
  • 2 x 1N4007 general purpose Diode
  • 5V-12V Power supply
  • Jumper cables
  • Blue and Red LED

Depending on the power supply used and the way the LEDs are connected (serial or parallel), you need to use a different resistance than the 68R used serially with LEDs. You can check out this calculation tool to calculate the resistance values to be used in basic scenarios.

Circuit Diagram

Double 555 Circuit Diagram

Third Circuit (555 and 4017)

Circuit 555 timer and decimal meter use IC CD4017. Here, the 555 timer runs in astable mode. The tener counts the incoming pulses of 4017 and activates their output, that is, Q0 is high for the first pulse and Q1 is high for the second pulse, and the Q0 status is high again for the 10th pulse.

Here the 555 timer produces continuous pulses over pin 3. The width of these pulses can be changed by changing resistance (R1,R2 ) or capacitance (C1). These pulses are given as input to the tenth counter. The output status of the tenner counter is increased for each incoming pulse.

  1. for impact – Q0 high – the glow of the blue LED
  2. for pulse – Q1 high (no connection) – all LEDs off
  3. for impact – Q2 high – the glow of the blue LED
  4. for impact – Q3 high – all LEDs off
  5. for impact – Q4 high – the glow of the blue LED
  6. For impact – Q5 high – red LED is on, blue LED off
    so blue LED flashes 3 times.
  7. for impact – Q6 high – all LEDs off
  8. for impact – Q7 high – the glow of the red LED
  9. for impact – Q8 high – all LEDs off
  10. for impact – Q9 high – the glow of the red LED
  11. for impact – Q0 high – blue LED on, red LED off

Therefore, red LEDs flash 3 times. This process is repeated continuously.

Where 555 timers run in free-to-work mode, i.e. InDeterminate Mode (astable). Produces pulses that can be changed in width. The 2nd and 6th pins are short-circuited to allow triggering after each cycle. 4. Pin is connected to Vcc to prevent sudden resets.

The 4017 Tenth Counter is a 10-bit counter. Counts incoming blows. The supply voltage range is -0.5 to +22V. The high pulse in the reset pin deletes the count to zero. The operating speed of this IC is up to 10 Mhz.

Output pins (Q0,Q2,Q4) blue LEDs are configured to flash 3 times and Q5, Q7 and Q9 pins are configured to flash red LEDs 3 times.

Depending on the outputs of the 4017 IC, two transistors (NPNs) turn leDs on and off.

R3, R4, R5, R6 resistors are used to protect LEDs from high voltage.

Required Materials

Circuit Diagram