This police siren simulated this project uses two 555 timers IC to generate a sound similar to the police siren. A single 556 timer IC which consists of two 555 timers can also be used. In this circuit, both of the timers are configured as astable circuit. The first timer is configured as a square wave close to 1 Hz astable oscillator. The output of this timer is used to feed the control voltage of the second timer where it is subjected to frequency modulation. This frequency modulation will generate a tone similar to the siren used by the police. The frequency of this tone generator can be varied by changing the value of potentiometer VR1. When set to its maximum value of 220k ohm, it will have a tone frequency of approximately 320 Hz.

Schematic Diagram

The schematic of the project is as shown below.

When S1 is switched ON, the circuit will be powered ON and U1 will start to oscillate at a frequency given by the formula:

f = 1.44/[(R1 + 2R2)(E1)]

= 1.44/[(10 + 2*82)(10)] Hz

= 0.8 Hz

This output frequency from pin 3 of U1 is fed into pin 5 of U2 where it is subjected to frequency modulation through resistor 10K. The tone generated can be varied by changing the values of potentiometer VR1. Experiment with the sound and settle with the best sound of your choice. The output of U2 is used to drive a power transistor which in turn drives an 8 ohm speaker. Diode D2 is used to prevent the damage of transistor Q1 due to the back emf generated by the speaker during the ON/OFF driving of the speaker.

Parts List

The parts list of the project is as shown below.

Analog Timing Light Project

This analog timing light project uses RC circuit as a delay OFF timer to control the duration an incandescent light turns ON. When the accuracy of a timer is not critical, the use of RC circuit is a good choice as it is more cost effective and simple. Once the normally open switch SW is pressed, the light will turn ON for a duration of 10 - 20 seconds before it turns OFF. The duration of the turn ON time can be varied by varying the values of R1, R2 and E1.

When SW is pressed, the base of the transistor Q1 is forward bias and it turns ON. This turns ON the 12V relay that is connected to the transistor. The contact of the relay RLY must be able to withstand the current of the load. At the same time, the electrolytic capacitor E1 is being charged to a voltage of approximately 0.7V.

Once SW is released, E1 will discharged through resistor R2 and the base of the transistor. After some time, When the voltage across E1 drops to approximately 0.5V, the transistor will turn OFF. This in turn will cause the relay to turn OFF and the incandescent light will turn OFF. The timing of the turn OFF can be changed by changing the values of E1, R1 and R2.

Parts List

The parts list of the project is as shown below.

Countdown Timer Project

Oscillator

The 555 is configured in the standard astable oscillator circuit designed to give a square wave cycle at a period of around 1 cycle/sec. A potentiometer is included in the design so the period can be set to exactly 1 second by timing the LED flashes. A jumper connection is provided so the LED can be turned off. As soon as power is applied to the circuit counting begins. The output pulse from pin 3 of the 555 is fed to a the clock input pin 10 of the 14-stage binary ripple counter, the 4020 (or 14020.)

Ripple Counter

The counter output wanted is set by a jumper. Ten counter outputs are available: 8/16/32/64/128/256/512/1024/4096 and 8192 counts. If the 555 is set to oscillate at exactly 1.0Hz by the on-board trimpot then the maximum timer interval which can be set is 8192 seconds (just over 2 hours.) At the end of the counting of the countdown timer period a pulse is output on the pin with the jumper on it. The 14020 ripple counter advances its count on each negative transistion of the clock pulse from the 555. So for each output cycle of low-high-low-high the count is advanced by two. It can be set to an zero state (all outputs low) by a logic high applied to pin 11.

In this circuit C3, R4 and D1 are arranged as a power-on reset. When power is applied to the circuit C3 is in a discharged state so pin 11 will be pulled high. C3 will quickly charge via R4 and the level at pin 11 falls thus enabling the counter. The 14020 then counts clock pulses until the selected counter output goes high. D1 provides a discharge path for C3 when the power is disconnected.

You can change the components values of R1 and C1 to set the 555 count frequency to more than 1.0 Hz. If you change the count to 10 seconds then a maximum timer delay of 81920 seconds, or 22.7 hours, can be obtained.

Transistors

The output from the 4020 goes to a transistor switch arrangement. Two BC547 are connected so that either switching option for the relay is available. A jumper sets the option. The relay can turn ON when power and counting start then turn OFF after the count period, or it can do the opposite. The relay will turn ON after the end of the count period and stay on so long as power is supplied to the circuit. Note that the reset pin of the 555 is connected to the collector of Q1. This enables the 555 during the counting as the collector of Q1 is pulled low.