Police flashing strobe light circuit using 555
Strobe lights are commonly used as emergency vehicle lights for visual warnings. Their light effects are similar to the flashing light in police vehicles, ambulance, beacon light bars, etc.
The given police light circuit consists of two 555 astable multivibrators. One with a lower oscillation frequency (IC1 multivibrator section) and other with a higher frequency (IC2 multivibrator section).
The IC2 consists of a simple LED flasher circuit, which continuously flashes the LED. And using the signal from the low frequency multivibrator, the signals from the IC2 are alternately switched to the RED and BLUE LEDs. Here the LED set consists of a 3×3 Red and 3X3 blue LEDs.
The square wave output of IC2 is connected to the RED and BLUE LEDs through an NPN transistor Q1-BC547 and a PNP transistor Q2-BC557.
The base of both transistors is biased from the output of low frequency multivibrator. So it switches the transistor Q1 and Q2 alternatively with the high and low state at the output.
On high output state of IC1, the Q2 switches to forward active mode and drives the flashing signals to BLUE LEDs. Similarly, in the low output state of IC2, the Q1 drives the signals to the RED LEDs.
The rate of flashing of LED sets and switching time between the LED sets can be adjusted by varying the time period of the two multivibrator circuits.
Integrated circuit – IC1,IC2 – NE555
Transistor – Q1 – BC557, Q2 – BC547
Resistor – R1, R3, R10, R11 – 1K, R2 – 470K, R4-R9- 220ohms, R12- 100K
Capacitor – C1,C4 – 2.2uf, C2,C3 – 0.01uf, C5 – 220uf
Diode – D1 – D9 – Red LED, D10 – D18 – Green LED