Simple Siren using op-amp

Intoduction :-

Device such as burglar alarms and sirens, whose basic purpose is to monitor certain conditions. The above figure shows a simple siren/alarm circuit using a dual-amp MC1458, audio amplifier LM380, and a 1-W speaker. The dual op-amp is used  as a signal generator that produces square, pulse and triangular or sawtooth wave froms.

Operation :-

The A1 and A2 op-amps make up a waveform generator in which the output of A1 is a square wave or pulse waveform and that of A2 is either a triangular or sawtooth waveform. The potentiometer R2 controls the frequency as well as the type of output waveform of op-amps A1 and A2. The switch SW1 connect the output of A1 or A2 to the audio power amplifier LM380, this in turn drives the speaker. Although not used in the circuit of figure 1, a potentiometer may be connected between (+) and (-) inputs of the power amplifier to control its voltage gain, which in turn controls the sound volume. The sound level produced depends on the position of switch SW1, the wiper setting of potentiometer R2, and the value of capacitor C4. Therefore, sound of varying intensities can be obtained by adjusting SW1, R2, and C4.

For higher output power (sound intensities), audio power amplifier may be used in the bridge from. This configuration will also require a higher wattage speaker.

PARTS LISTS :-

R1, R­4 = 10 KΩ
R2 = 20 KΩ potentiometer
R3 = 39 KΩ
C1, C3 =0.1 µF
C2 = 100 µF
C­4 = 0.05 µF
IC1 = MC1458 (dual op-amp)
IC2 = LM380    (audio power amplifier)
8Ω 1-W speaker
SW1 = Three-position switch

Zero Degree Celsius Alarm

Description.

This simple circuit will produce an alarm whenever the temperature falls below zero degree. A thermistor is used here to sense temperature. The op-amp LM7215 is used to compare the reference voltage and voltage from the thermistor network. Reference voltage is given to the non inverting input (pin3) of the IC and voltage from thermistor network is given to the inverting input (pin4).When temperature becomes less than zero degree the voltage at the non inverting input becomes larger than the voltage at the inverting input and the output of the op-amp becomes high. This makes the transistor Q1 ON and drives the piezo buzzer to make the alarm. In the power supply section, IC 7805 is used to derive 5V from the 9V battery.

Notes.

• Assemble the circuit on a good quality PCB.
• The thermistor used here is a glass bead thermistor, type No: KEYSTONE RL0503-5536K-122-MS (361K @ 0 degree Celsius and 100K @ 25 degree Celsius).
• The IC1 must be mounted on a holder.
• The battery B1 can be a 9V PP3 battery.