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.

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Touch Based Blinking Lights

This circuit demonstrates the principle and operation of application based on touch sensor. The circuit is divided into three parts: Input, 555 timer and output. A touch plate is used for the input and output can be seen across an LEDor a buzzer. Some application of the circuit include touch based blinking lights, touch buzzer, touch switch etc.
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The touch plates is connected to the trigger pin of the 555 timer IC. When we touch the touch plates the output of the touch plates becomes zero thereby providing an active low trigger to the IC 555. The IC is configured in the monostable mode .The output of the IC is a pulse whose frequency is set by the resistor (R1) and  capacitor (C1) according to the formula [F=1/(1.1*R*C)]. In this case it produces a pulse with frequency of about 0.9 hertz and time period of 1.1 seconds. In monostable mode pin7 (discharge pin) and pin6 (threshold pin) are shorted while R1 is connected between pin7 and Vcc and C1 is connected between pin 6 and ground. Reset pin (pin4) is connected to Vcc while control pin is connected to ground through a capacitor. The output can be observed on the LED, which glows for a small duration. This circuit can be used in the following applications:

To detect stray voltages produced by mains or to detect electrostatics build up in a room.

To make touch buzzers.

To implement touch switches like for a bell.

circuit 2, shows an improved version of this circuit. This circuit has a higher sensitivity as compared to circuit in figure 1. The output of the touch plate is connected to the base of npn transistor T1 (BC 547). The collector is connected to the Vcc through a resistor R1. The trigger pin of the IC is connected to the collector of the transistor. When no input is there on the base of the T1, T1 is in cut off state and hence the trigger pin is at logic high and therefore 555 do not produce any output. When we touch the plates T1 behaves like a closed switch trigger pin gets connected to ground, thereby producing the output at pin 3 of the IC.