Components

1 Smowboard Think Mini
1 Speaker
1 BC547 transistor
1 100k ohm resistor
2 100 ohm resistors
1 10uF capacitor
2 red LEDs
5 male-to-female jumper wires
1 male-to-male jumper wires

In this circuit, we connect a transistor on the breadboard. We connect the 100k ohm resistor across the base-collector junction of the transistor. We connect the negative terminal of the capacitor to the base of the transistor and the positive terminal of the capacitor to the pin number 21 of Smowboard. We connect one terminal of the speaker to Vin pin of the Smowboard while its other terminal is connected to the collector of transistor. The emiiter of the transisitor is grounded by connecting it to the ground of Smowboard. We connnect both the LEDs on the breadboard. The LEDs are grounded by connecting their cathode to the ground of Smowboard. A 100 ohm resistor is connected in series to the cathode side of each LED to limit the current. The anode of one LED is connected to the pin number 2 of Smowboard while the anode of other LED is connected to the pin number 4 of Smowboard. Both the LEDs are controlled by the Smowboard.

The speaker is connected to the Smowboard using capacitor and transistor. The Smowboard controls the frequency of the sound. Thus the speaker produces the sound of a siren by producing sound of frequencies used in police sirens. Also the Smowboard controls the voltage across the LEDs. A current flows across the LEDs due to this voltage and the LEDs glow alternatively giving the effect of an actual police siren.

Police Siren