Police Siren Project

This police siren simulated electronic 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.

ref:  electronics-project-design.com/PoliceSiren.html

Timer

Introduction
Timer circuit has been used in many projects and there are basically 2 types that are used these days. One of them is the use of analog RC circuit where charging of the capacitor circuit determined the T(time) of the circuitry. This type of circuitry has larger tolerance and is used in applications where the T is not so critical as the T is affected by the tolerance of the RC components used.
The other is the use of crystal or ceramic resonators together with microprocessor, microcontroller or application specific integrated circuit that need higher precision T in the tolerance of up to 5 ppm (parts per million).



555 IC
One commonly used circuit is the 555 IC which is a highly stable controller capable of producing timing pulses. With a monostable operation, the T(time) delay is controlled by one external resistor and one capacitor. With an astable operation, the frequency and duty cycle are accurately controlled by two external resistors and one capacitor. The application of this integrated circuit is in the areas of PRECISION TIMING, PULSE GENERATION, TIMING DELAY GENERATION and SEQUENTIAL TIMING.
A typical 555 IC block diagram is as shown below.




Monostable Operation
Figure below shows the monostable operation of a 555 IC.





In this mode, the device generates a fixed pulse whenever the trigger voltage falls below Vcc/3. When the trigger pulse voltage applied to pin 2 falls below Vcc/3 while the its output is low, its internal flip-flop turns the discharging transistor Tr off and causes the output to become high by charging the external capacitor C1 and setting the flip-flop output at the same instant. The voltage across the external capacitor C1, VC1 increases exponentially with the time constant T=RA*C1 and reaches 2Vcc/3 at td=1.1RA*C1. Hence, capacitor C1 is charged through resistor RA. The greater the time constant RA*C1, the longer it takes for the VC1 to reach 2Vcc/3. In other words, the time constant RA*C1 controls the output pulse width. When the applied voltage to the capacitor C1 reaches 2Vcc/3, the comparator on the trigger terminal resets the flip-flop, turning the discharging transistor Tr on. At this time, C1 begins to discharge and its output goes to low.
Astable Operation




An astable operation is achieved by configuring the circuit as shown above. In the astable operation, the trigger terminal and the threshold terminal are connected so that a self-trigger is formed, operating as a multivibrator. When its output is high, its internal discharging transistor Tr turns off and the VC1 increases by exponential function with the time constant (RA+RB)*C. When the VC1, or the threshold voltage, reaches 2Vcc/3, the comparator output on the trigger terminal becomes high, resetting the F/F and causing its output to become low. This in turn turns on the discharging transistor Tr and the C1 discharges through the discharging channel formed by RB and the discharging transistor Tr. When the VC1 falls below Vcc/3, the comparator output on the trigger terminal becomes high and the timer output becomes high again. The discharging transistor Tr turns off and the VC1 rises again. The frequency of oscillation is given as below.



ref: electronics-project-design.com/Timer.html

Phone In Use Project

Phone In Use Project

This project is a simple phone in use indicator that one can design and construct that displays the status of the phone line. If the phone line is in use, the yellow LED will turn ON. If it is not in use, the green LED will turn ON. By having this indicator, the user of the phone will not be interrupted by another user who would want to use the same line. There are altogether 13 electronic parts that are used. They are 4 diodes, 2 LEDs, 5 resistors and 2 NPN transistors.

Circuit Description

When the phone is in on-hook condition, the voltage across the tip and ring is in the range of 48V DC to 50V DC. When it is in off-hook condition (the receiver is taken off its hook), the voltage drops to the range of 6V DC to 15V DC.

As shown in the circuit, diodes D1, D2, D3 and D4 are used to ensure that in the event that the tip and ring of the telephone line is reversed, the circuit can still be used. When the telephones connected to this line are on-hook, there is enough voltage to turn on transistor Q2 through voltage divider R4 and R5. When Q2 is ON, the green LED L2 will slightly turn ON which indicates that the phone line is not in use.

When the telephone goes to off-hook condition, Q2 will turn OFF. This allows current to flow through transistor Q1 causing yellow LED L1 to turn ON.

When the phone is ringing, both the LEDs will flash.

Parts List

ref: electronics-project-design.com/phoneinuse.html