Fire alarm

At times we forget to switch off our heating devices and eventually they get damaged. Therefore there is always a need of a device which can alert by sounding an alarm if the device temperature goes beyond a particular value. Same concept can be used in fire alarm. If a building catches fire then it will raise an alarm and people could evacuate the building.

This fire alarm circuit is based around LM35 which is a temperature sensor and could be used to switch an alarm when temperature goes beyond a preset value. In this circuit the output of LM35 is fed to the negative pin of comparator of LM339. The positive input is connected to a preset VR1 of value 10K. This preset is used to set the reference temperature. If output of LM 35 is less than V reference than the output of comparator is low. The IC LM339 has four inbuilt comparators.

The output of comparator is fed to the base of transistor T1(BC547). As the output is low, T1 is in the cut off state and behaves like an open switch. As a result point A is at high potential. This in turn biases the transistor T2, which behaves like a closed switch. The potential of point B is low, which in turn gives a low signal to the reset pin 4 of the IC 555. When the reset pin is low, there is no output at the output pin 3 and hence the buzzer does not sound.

When the temperature exceeds the reference temperature, the output of the comparator becomes high and T1 gets biased. The point A comes at a low potential thereby making the point B at higher potential. Since the point B is connected to the reset pin, it also becomes high and the 555 timer which is configured in the astable mode, starts producing pulses and the buzzer makes a sound to alert the rise in temperature.

Variable power supply

The electronic devices are very sensitive to the fluctuations in the power fed to them. This problem can be solved by using regulated power supply for them. This power supply circuit is equipped with an adjustable voltage regulator to adjust the output in accordance with the requirement. Adjustable voltage regulators have both line and load regulation which is better than standard fixed regulators.

The stepped down AC from the secondary winding of the transformer is fed to the bridge rectifier for rectification. The output has pulsating DC which is filtered by a capacitor C1 (1000uF). This output needs to be stabilized against the input variations by using a voltage regulator IC (LM317) in series with the bridge rectifier. This regulator not only guarantees stable output but can also be adjusted to obtain a desired level of voltage output. The output from the regulator can be varied from 1.2V to 25V. This is achieved by adjusting the value of variable resistor (preset) VR1 (5k). A fixed resistor R1 (220) along with VR1 forms a potential divider. A capacitor C2 (0.1uF) is used to suppress the oscillations, if any, produced in the voltage regulator.

Light sensor using photodiode

The light sensor circuit shows the application of photodiode to detect the presence of light. This light sensor can be used as an intermediate circuit in various applications to detect the presence or absence of light. The sensitivity of the sensor can be adjusted using the preset.

A photodiode, used as a photo detector, generates current in the circuit when light incidents on it. This circuit uses the photodiode in reverse bias mode with resistor R1(10k). This resistor does not allow too much current to flow through the photodiode in case a large amount of light falls on the detector. Initially when no light falls on the photodiode, it results in high potential at the inverting input of a comparator (pin6) of LM339. When light falls on the photodiode, it allows current to flow through the diode, and thus drops the voltage across it. The non-inverting input (pin7) is connected to a variable resistor VR2 (preset) which is used to set the reference voltage of the comparator. A comparator works on the principle that its output remains high as long as the non-inverting input is at higher level than that at its inverting input. Here, the output (pin1) is connected to an LED. The reference voltage is set to correspond to a threshold illumination through a preset VR1 (10k). 

The LED at the output will glow when light falls on the diode. In such a condition, the inverting input drops to a lower value than the reference set at the non-inverting input and so the output goes providing the necessary forwards bias to the LED.  

Music transfer by IR by amplification

The circuit can be broadly divided into two parts: the transmitter and the receiver. The transmitter consists of a condenser mic, which is used as sound sensor to convert the sound signals into electrical signals. The electrical signals are amplified by the amplifying unit. For details about the amplifying unit refer to Sound Amplifier. Since the strength of the amplified output from the transistor is not enough for IR transmitter to produce high energy IR signals, so the amplified signals are further amplified using an Op-amp as amplifier with negative feedback configuration. The amplification factor is decided by the resistors R6 (1K) and R7 (10K). The output voltage is determined according to the formula V0=-(R7/R6)*Vin. This amplified signal is transmitted through the IR LED.

The receiver receives the IR signals through an IR receiver and converts them back to sound signals through a speaker.  For details of receiver section refer to Sound/Voice transmission through IR. This circuit has a better range since the strength of the transmitted signals is high. This circuit can operate at a range of around 10-15 cm.

DTMF Decoder

DTMF (Dual tone multiple frequency) is the most popular and nowadays ubiquitously used telecommunication signalling method. A DTMF decoder detects the DTMF tones and generates the binary sequence corresponding to key pressed in a DTMF keypad. The circuit presented here is a DTMF decoder. DTMF keypads are used in almost all landline and mobile handsets. The DTMF decoders, therefore, are used at the telephone switching centres to detect the number dialled by the caller. The DTMF version used in pushbutton telephones is called touch tone and is a registered trademark of AT&T.

The circuit uses a DTMF decoder IC (HT9170). The DTMF tones are generated by the keypad of a cell phone or by the computer software available on www.polar-electric.com.

 

The tones generated from the speaker (audio signals) of keypad are given to microphone which converts audio tones into electrical signals. The signals from the microphone are processed by the DTMF decoder IC which generates the equivalent binary sequence as a parallel output.

 

 

The electrical signals from microphone (mic) are fed to inverting output (pin2; VN) of Op-amp, present in IC, through a series of resistance and capacitance of value 100 k and 0.1 µF respectively. The non-inverting input pin (pin1; VP) of Op-amp is connected to pin4 (V_REF). The voltage at V_REF pin is V_DD/2. A feedback signal is provided, by connecting the output of Op-amp (pin3; GS) to inverting input pin (pin2; VN) through a resistor R2 (100 k).

 

The output of Op-amp is passed through a pre filter. The output from this pre filter is then supplied to low group and high group filters. These filters consist of switched capacitors and divide DTMF tones into low and high group signals. High group filters pass the high frequencies while low group filter pass low frequencies. These frequencies are then passed through frequency detector and code detector circuits. Finally the four-digit code is latched on the output pins of HT9170. The output on these pins has been used to drive a set of four LEDs.

 

The whole process, from frequency detection to latching of the data, is controlled by steering control circuit. So it is very important part of the whole process and is mainly controlled by RT/GT and DV pins of DTMF decoder IC. RT/GT pin is connected to V_DD through a capacitor of 0.1 µF. The EST pin is connected to RT/GT pin through a resistor of 300k.

 

Pin6, PWDN pin (active high) inhibits the working of oscillator thereby stopping the working of circuit; and Pin5, INH pin (active high) inhibits detection of the tones of character A,B,C,D. The pin 10; OE (output enable; active high) enables the latching of the data on the data pins. Pin15; DV (Data valid pin) becomes high on detection of valid DTMF tone otherwise it remains low. An oscillator of frequency 3.579545 MHz is connected between pins 7 and 8.

•