CALL PREVENTION  AND PARALLEL TELEPHONE WITH SECRECY  SYSTEM PROJECT

hello  friends  today I am gonna  telling  you  about  some  easy  and valuable  electronics projects for  school and  college  practical

This circuit provides secrecy when two or more telephones are connected in parallel to a telephone line. The circuit also prevents incoming calls to as well as outgoing calls from other telephones connected in parallel, except from the one lifted first

When someone picks up the handset of the telephone connected in parallel to the original (master) phone for making an outgoing call, no dial tone is heard and the phone appears to be dead But when a call comes, the ring signal switches the SCRs'  on 'and conversation can be carried out.  As soon as the handset is kept on the hook, the SCR goes off and the telephone can again only receive incoming calls. 

Fig1; circuit diagram

When a call comes , conversation can be made only from the telephone which is lifted up first . To carry out conversation from the other telephone, the hand set of the telephone that was lifted up first has to be placed on the hook and then the push - to - on switch of the associated circuit of the other telephone has to be pressed  after lifting up its handset.  Thus the circuit ensures privacy because both the telephones cannot be active at the same time

Those who are not need parallel telephones can rig up the associated circuit of a single telephone to work as an outgoing call preventer.  An outgoing call can be made only when one lifts up the handset and presses the push - to - on switch of its associated circuit. 

The polarity of the telephone line can be determined by a multimeter.  To avoid con fusion, a bridge rectifier can be used at the input of the circuit.  

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         7-SEGMENT DISPLAY DICE WITH

hello  friends  today I am gonna  telling  you  about  some  easy  and valuable  electronics projects for  school and  college  practical

A digital dice circuit can be easily realised using an astable oscillator circuit followed by a counter, display driver and a display.  

Here we have used a timer NE555 as an astable oscillator with a frequency of about 100 Hz.  Decade counter IC CD4026 or CD4033 (which- ever available) can be used as counter- cum - display driver.  When using CD4026, pin 14 (cascading output) is to be left unused (open), but in case of CD4033, pin 14 serves as lamp test pin and the same is to be grounded. 

The circuit uses only a handful of components.  Its power consumption is also quite low because of use of CMOS ICs, and hence it is well suited for battery operation.  In this circuit two tactile switches S1 and S2 have been provided.  While switch S2 is used for initial resetting of the display to '0' depression of Si simulates throwing of the dice by a player.  

When battery is connected to the circuit, the counter and display section around IC2 (CD4026 / 4033) is energized and the display would normally show '0', as no clock input is available.  Should the display show any other decimal digit, you may press reset switch S2 so that display shows 0% To simulate throwing of dice, the player has to press switch S1, briefly this extends the supply to the astable oscillator configured around IC1as well as capacitor C1 (through resistor R1), which charges to the  battery volt.  age.  

Thus even after switch S1 is released, the astable circuit around IC1 keeps producing the clock until capacitor C1 discharges sufficiently.  Thus for duration of depression of switch S1 and discharge of capacitor C1 thereafter, clock pulses are produced by IC1 and applied to clock pin 1 of counter IC2, whose count advances at a frequency of 100 Hz until CI discharges sufficiently to de activate ICI. 

Fig1; circuit diagram

When the oscillations from IC1 stop, the last (random) count in counter IC2 can be viewed on the 7 - segment display.  This count would normally lie between 0 and 6, since at the leading edge of every 7th clock pulse, the counter is reset to zero.  This is achieved as follows. 

Observe the behavior of 'b' segment output in the Table On reset, at count 0 until count 4, the segment 'b' output is high at count 5 it changes to low level and remains so during count 6. How ever, at start of count 7, the output goes from low to high state.  A differentiated sharp high pulse through C - R combination of C4 - R5 is applied to reset pin 15 of IC2 to reset the output to 'O' for a fraction of a pulse period (which is not visible on the 7 stops at seventh  count the display will segment display).  

Thus , if the clock segment display).  Thus, if the clock stops at seventh count, the display will read zero.  There is a probability of one chance in seven that display would show ' 0 ' In such a situation, the concerned player is given another chance until the display is non - zero 

Note

Although it is quite feasible to inhibit display of and advance the counter by '1' the same makes the circuit somewhat complex and there fore such a modification has not been attempted.

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LCD SCREENS CCFL TESTER PROJECT

 

      LCD SCREENS CCFL TESTER

hello  friends  today I am gonna  telling  you  about  some  easy  and valuable  electronics projects for  school and  college  practical

Liquid crystal displays (LCDs) are used in a wide range of products including flat screen computer monitors, laptop computers, tablet PCs, PDAs, digital cameras, and portable devices. Compact cold cathode fluorescent lamp (CCFL) based backlight (illumination source for LCD screen)The arrangement in these applications enables a well-observable display in both dim and bright ambient light conditions. The LCD backlight mainly consists of a light source, diaphragm, light-leading board and plastic frame. It has high brightness, long lifetime and good uniformity characteristics.

The CCFL often uses a fluorescent lamp, which has a phosphor coated glass cylinder with a cathode at each end. The CCFL tube is powered by a small electronic inverter (CCFL inverter) circuit that flashes the screen electronically. The inverter circuit accepts a low-level DC input voltage and provides a high-level AC output to drive the backlight CCFL tube (s). Figure 1 shows the CCFL for LCD. The need often arises to check for a faulty LCD backlight circuit to find dead components..

Fig1; circuit diagram

Absence of back light means either the CCFL inverter and / or the CCFL tube is in dead state.  Here is a simple circuit to test the CCFL tube in a LCD backlight unit, which might help the hardware technician to speed up the repair work.  This basic circuit (go / no go test) is portable, 6V battery operated and can be used to test almost all types of LCD backlight CCFL tubes.  

The working of the CCFL circuit is shown in Fig. 2 A timer IC NE555 (IC1) is wired as astable multivibrator (AMV) to drive a standard MOSFET T1 (IRF 512 / IRF 830).  Components R2, R3 and C3 determine the operating frequency of the AMV.  MOSFET T1 switches the inverter output transformer (X1) to produce high-voltage AC supply at its output terminals.  Here an ordinary step - down transformer is used as the inverter transformer by reversing its primary and secondary windings.  

Assemble the circuit on a general - purpose PCB and en close in a suitable cabinet.  Fix the 2 - pin connector on front side of the cabinet in such a way that the CCFL under test can easily be connected here.  Fix the test switch on top of the frame.  Keep the 6V battery in side the cabinet.  To test a CCFL, connect its ter minals to the primary leads of transformer X1 and press test switch S1 momentarily.  If the CCFL is good, you will notice a dim / bright glow in it (depends on the wattage rating of the CCFL under test).  Always try to keep the test time as short as possible.  

Warning

Even for experienced technicians, repairing the LCD back light unit is not an easy task. If you do something wrong you can permanently damage the LCD screen and have to buy a new one!

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HOME APPLIANCE CONTROL USING TV REMOTE 

hello  friends  today I am gonna  telling  you  about  some  easy  and valuable  electronics projects for  school and  college  practical

This circuit is designed to turn on / off any household or industrial appliance using a TV / DVD remote controller. Depending on the remote used, the circuit can be operated up to a distance of 5–10 m.

The circuit consists of a step-down transformer X1 (6V - 0-6V, 250mA secondary), 5V regulator 7805 (IC1), two 5V, 1 change-over (C / O) relay, one timer NE555 IC (IC2). An IR receiver module (IRX1 TSOP1738) and some discrete components. The circuit operates at regulated 5V, which is received from X1 and controlled by IC1. The home appliance is controlled by pressing any key on the remote or manually pressing the switch SI in the 'on' position

The TV / DVD remote controller produces 38kHz frequency.  The IR receiver module operates at this frequency.  It is used to control relay RL2.  The relay triggers IC2, which is wired in a bistable mode to control the home appliance connected at the contacts of relay RLI.  

Timer IC2 toggles relay RL1 when switch Si is pressed momentarily.  Threshold and trigger input pins 6 and 2 of IC2 are held at one - half of the power supply voltage (5V) by resistors R2 and R3.  When output pin 3 of IC2 is high, capacitor C4 charges through resistor R4, and discharges when the output pin 3 is low.  When switch S1 is Pressed, capacitor C4 voltage is applied to pins 2 and 6 of IC2, which causes the output of IC2 to change from low to high, or high to low.  When switch Si is released capacitor C4 charges or discharges to the original level at the output pin 3 of IC2. 

At normal condition, when IR rays are not incident on TSOP1738, its out put at pin 3 remains high.  When any TV remote key is pressed, IR rays fall on the TSOP1738 and its output goes low.  At the same time relay RL2 energises for a few seconds through pnp transistor T2 (BC558). 

The working of the circuit is simple.  Initially, when there are no IR rays falling on the IR receiver module, its output remains high.  Transistor T2 is in cut-off condition.  Relay RL2 does not energise and hence IC2 does not toggle.  As a result home appliance connected at the contacts of relay RLI remains switched off.  

When you press any remote key for the first time, the IR receiver module's output goes low and the collector of the transistor T2 goes high.  Relay RL2 energises and triggers IC2.  Output of IC2 goes high and relay RL1 energises to switch on the appliance.  Once relay RLI is energized it remains in that state.  So the appliance which is connected at the contacts of relay RLI remains switched on 

Now when you press any remote key the second time, relay RL2 energises and re - triggers IC2.  Output of IC2 goes low and relay RL1 de energises to switch off the appliance.  Once relay RL1 de energises it remains in that state.  So the home appliance remains off.  This cycle repeats when any key of the TV remote is pressed to switch on / of the home appliance 

Assemble the circuit on a general purpose PCB and enclose in a suitable cabinet fix TSOP1738 and switch S1 on front side of the cabinet.  Place trans former inside the cabinet and mains power cord at the back of the cabinet

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                  PULSE RATE METER

hello  friends  today I am gonna  telling  you  about  some  easy  and valuable  electronics projects for  school and  college  practical 

Here is a meter to measure the human heart rate. The pulses are picked up around the wrist (just as it is examined by the doctor) using a condenser microphone fastened with a suitable belt.

The circuit is built around a low- power audio amplifier LM358 (IC1), NAND gates CD4011 (IC2), dual up- counters CD4518 (IC3 and IC4), timer IC NE555 (IC5), clock pulses, four NPN transistors BC548 (T1-T4) along with some discrete components. The human pulse rate is indicated in binary-coded display format on LED2 through LED9.

To know the heart rate one has to fasten the condenser microphone properly to the wrist and press the RESET / START switch S1 momentarily.  The counters built around IC3 and IC4 are inhibited for some time.  The inhibit LED10 glows for a brief period.  As soon as the inhibit LED10 goes down the counting starts.  The display (LED2 through LED9) remains off during counting of pulse rate to save power during battery operation.  

The divider counter IC4 counts disabled by the logic of the circuit.  for one minute after which it is The one-minute timing is derived from common quartz clock pulser,

Fig1; circuit diagram

which can be easily borrowed from any old quartz clock.  The clock pulser provides two outputs - Q1 and Q2 - which drive the coil of the quartz clock.  One of the outputs (Q1) of the clock pulser is used to obtain the required timing of one minute (60 seconds, which corresponds to 30 counts).  Each output gives a pulse at every two seconds.  Q1 is fed to pin 2 of IC4 through transistor T3.  At the end of one minute, the divider counter built around IC4 and the rate counter built around IC3 are inhibited ed.  The pulse rate count is displayed on the LEDs powered by transistor T2, which becomes conductive at the end of one minute through NAND gates N2 and N3

The preset VRI associated with the audio amplifier circuit built around ICI has to be adjusted to get clear heart beat signals.  The condenser microphone should be placed on the pulsating vein near the wrist.  After the condenser microphone is fastened with a suitable belt or rubber band, you can take a fresh reading.  For this you have to press the RESET START switch S1 momentarily 

Assemble the circuit on a general purpose PCB and enclose in a suit able cabinet.  Fix the 2 - pin connector on front side of the cabinet in such a way that the condenser microphone can be placed firmly on the skin of the lower wrist to get a good signal, preferably where the pulse can be easily sensed.  Keep the 6V battery inside the cabinet.  Fix the display LEDs in BCD format to read the pulse rate easily.  •

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DIGITAL VOLT METER PROJECT

  DIGITAL VOLT METER PROJECTS

hello  friends  today I am gonna  telling  you  about  some  easy  and valuable  electronics projects for  school and  college  practical 

A voltmeter finds its importance wherever voltage is to be measured.  Here we present an easy  to build and accurate digital voltage meter that has been designed as a panel meter and can be used in DC power supplies panels or where it is necessary to have an accurate indication of the voltage.

circuit description

The circuit uses  an analogue to digital converter ICL7107 made by INTERSIL.  Fig.  1 shows the pin detail of IC L7107.  This IC incorporates in a 40 pin case all the circuitry necessary to convert an analogue signal into digital and can drive a series of four seven segment LED displays directly.  The circuit  built into the IC are an analogue to digital converter (ADC) a comprator a clock a decoder and a seven segment LED display driver. 

The circuit can display any DC voltage in the range of -1999 volts to 1999 volts. It operates off 5  Volts. In order to understand the operating principle of the circuit, it is necessary to explain how the ADC IC works an analogue - to - digital converter, better known as a dual slope converter or integrating converter, is generally preferred over other  types as it offers accuracy, simplicity in design and a relative indifference to noise, which makes it very reliable. 

The operation of the circuit is de scribed in two stages for better under-standing. During the first stage, for a given period the input  voltage is integrated, and the output of the integrator at the end of this period is voltage which is directly proportional to the input voltage.  At the end of this period is a voltage Which is directly proportional to the input voltage  

Fig1; circuit diagram

At the end of the preset period, the integrator is fed with an internal reference voltage and the output of the circuit is gradually reduced until it reaches the level of the zero reference volt age.  This second phase is known as the negative slope period and its duration depends on the out put of the integrator in the first period.  As the duration of the first operation is fixed and the length of the second is variable, it is possible to compare the two and this way the input voltage is in fact compared to the internal reference voltage and the result is coded and sent to the  display

All this sounds quite simple and easy but in fact it comprises a series of complex operations, which are all made by the ADC IC with the help of a few external components that are used to configure the circuit for accurate measurement.  Fig.  2 shows the circuit of the digital voltmeter.  The voltage to be measured is applied across points IN - L 'and' IN - H 'of pins 30 and 31 of ICI, respectively, through circuit resistors R5, R6, R7 or R8 and preset VRI.  Resistor R2 together with C5 forms the circuit used to set the frequency of the oscillator (clock), which is set at 48 kHz approximately at this clock rate, there are about three different readings per second. 

Capacitor C4, which is connected between pins 33  and 34 of IC1, compensates for the error caused by the internal reference voltage and also keeps the display steady.  Capacitor C2 and resistor R1 together forms the circuit that does integration of the input Voltage and at the same time prevents any division of the input voltage making the circuit faster and more reliable Capacitor  C1 forces the instrument to display zero when there is no voltage at its input

Resistor R3 together with reference control preset VR2 is used to adjust the instrument during  setup so that it displays '0' when the input is zero.  Resistor R4 controls the current that is allowed to flow through the displays so that there is sufficient brightness without damaging them. 

The IC, as mentioned earlier, is capable of driving four common - anode, seven - segment displays Displays DIS1 through DIS3 are connected such that they can display all the numbers from "O 'to' 9 'while display DIS4 can display only digital  (and - sign when the voltage is negative). 

When the TEST switch is pressed, the test pin is pulled high (to V +) and all segments turn on to show' 1888 'on the display. The  TEST pin will sink about 15 mA under this condition. In the lamp test mode, the segments have a constant DC power supply. Pin 36 (REF HI) is made high through VR2 to adjust the reference voltage and pin 35 (REF LO) is connected to the negative terminal of the input Preset VR1 is used to adjust the range of voltage divisions. 

Resistors R5 R6 R7 and R8 connected to pin 31 are used for range selection.  Switch S2 is two - pole, four position (each ganged) rotary switch Decimal point is connected to resistor R4 via rotary switch S2 for different range selection 

The entire circuit operates off a dual 5V DC power supply which is applied to IC1 as follows: pin  1 to + 5V.  pin 21 to OV and pin 26 to -5V.  Dual power supply is derived from two 9V  batteries.  Regulators IC2 and IC3 are used to provide a regulated + 5V and 5V, respectively to the circuit capacitors C6 and bypass any ripple from the regulated supply

Assembling

an actual size, single - side PCB layout for the digital voltmeter is shown in Fig.  3 and its component layout in Fig.  4. Connection points for decimal point and for range selection resistor are provided on the PCB for connecting rotary switch S2 IC base is used for ICI.  Rotary switch is fitted outside the PCB.  Connections from the PCB to the rotary switches are made using suitable lengths of shielded wire

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       ANALOG CAPACITANCE  METER 

hello  friends  today I am gonna  telling  you  about  some  easy  and valuable  electronics projects for  school and  college  practical 

The principle of operation of this meter is based on pulse- width modulation.  Larger the capacitance, higher is the pulse width and vice - versa.  The meter given here has one oscillator and two one - shot generators (OSG) built around RC components.  An OSG is triggered by a free - running oscillator built around IC NE555 (C1) The pulse - width of the OSG is determined by the capacitor under measurement (Cx).  By passing the pulses from the OSG through a linear ammeter it is possible to measure its duty cycle.  In other words, the deflection of the ammeter can be made equal to the duty cycle, which here becomes proportional to the value of the capacitor Cx due to linear property of the OSG time period.

Fig.  1 shows the circuit diagram of analogue capacitance meter.  The OSG comprises timing resistors (R4) through Ro) and capacitor Cx, whose voltage is sampled by CMOS gate NI.  Cx is discharged through transistor 13 connected in parallel.  Cx is connected to pin 2 of gate N1 The voltage at pin 2 of gate N1 is used as the reference voltage to which Cx has to charge.  Once this voltage is reached the sampling gate output makes the transistion from high to low state, indicating the completion of the charging time.  Charging time (on - time) of capacitor Cx is given by:

Fig1; circuit diagram

Here, V. is supply voltage and V is the threshold input voltage of gate N1.  R = R4 through Ro and C = Cx.  Due to this linearity of the RC circuit, a capacitor with twice the value would take twice the time.  Thus the OSG pulse - width and hence the duty cycle of the output waveform, becomes pro portional to the value of the capacitor due to stray capacitance of connecting wires the OSG does not produce a zero pulse without capacitor to eliminate this, a similar auxiliary OSG, comprising presets VR1 through VR3 and a capacitor C3 is used.  The presets VR1 through VR3 are adjusted such that for each range the meter gives zero indication when capacitor cx is not connected with these adjustments the meter is ready for use 

The oscillator built around IC1 should be configured such that is on time period (To) slightly  exceeds the OSG on - time period () with the full range capacitor value and off - time period (Tom) occurs just before complete discharge of capacitor.  This becomes very important for capacitors having large capacitances

The calibration of the ammeter is easy it suitable reference capacitor is available.  It is recommended to permanently include a reference capacitor say, 100nF With a reference capacitor connected, you can easily set the calibration resistance using preset VR3.  to obtain a full - scale deflection This would finish the calibration for other ranges as well.  The capacitance of cx is given by the following expression

 Here, (Vg2)  and ( Vg1) are the output gate voltages of N4 and N2, respectively . Nd is the total number of divisions on the ammeter scale, R., and R = VR3 are the meter and series calibration resistances, and is the full-scale deflection current of the meter the range of the instrument can be further extended by including lower  valued range resistances (1k gives 0-100nF and 100Ω gives 0 - luf) We have used a basic instrument to keep the diagram as simple as possible 

Since NE555 cannot supply the required current providing a con The stand voltage, additional driver stage of transistors T1 (2N3904) and T2 (N3906) have been included.  The pulsed drive automatically cuts - off the supply during off - cycle, enabling a proper discharge of Cx, which is important to get the right value of the capacitor otherwise partially charged capacitor will be subjected to the next charge cycle and indicating a lower value An indicative timing diagram (not to scale) is given in Fig 2

Fig2; waveforms diagram

The circuit takes some time to stabilize thermally and is  More suitable as a lab instrument It is recommended to include a lower valued variable resistance in series  with the preset controlling the current through the meter This will allow easy frequent calibration of the instrument 

Assemble the circuit on a general - purpose PCB in a suitable  enclosure Approximately 63.5cm (25 - inch) long test leads with crocodile clips are used for compensating the capacitance of this capacitance can be changed according to the length of the test leads.  Fix the ammeter in the front panel.

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     BATTERY DEEP DISCHARGE  SAVER 

hello  friends  today I am gonna  telling  you  about  some  easy  and valuable  electronics projects for  school and  college  practical 

This circuit can be used to ensure that a 12V sealed lead acid battery does not go into the condition of deep discharge.  The circuit disconnects the battery from load when its voltage goes below a preset voltage say (10.5v) level.  The load remains disconnected until the battery is recharged 

Initially, when the circuit is switched through switch S1. capacitor C1 momentarily acts as short circuit It connects the coil of relay RL1 to the battery. Relay RL1 energies and  connects the load to the battery.  At the same time battery terminal voltage is divided by resistor R2 and the combination of preset VR1 and R3 that provide reference voltage . The battery voltage does not go below the reference voltage preventing it from going into deep discharge 

Fig1;circuit diagram

The heart of the circuit is IC TL431 ( IC1) , which is a well - known voltage reference IC . The battery voltage is compared with the internal reference voltage of regulator IC1 . As long as the voltage set by the preset VR1 is above the internal reference voltage , IC1 allows relay RL1 to energise connecting the load to the battery As the battery begins to discharge its terminal voltage decreases When the battery voltage reaches the set voltage , say 10.5V , the cathode of IC1 goes high , which de - energies relay RL1 and disconnects the load from the battery  

Assemble the circuit on a general purpose PCB and set the low cut - off voltage (deep discharge voltage) of the battery.  For setting the low cut off voltage, remove the battery from the circuit and connect the circuit to a variable 12V DC power supply.  Also connect a digital multimeter across the variable power supply.  Increase the voltage from 0 to 12V through the variable power supply and read the voltage in multimeter Ads preset VR1 in such a way that relay RL1 gets energed and load is connected to the battery. Now decrease the supply to 10.5V (low battery volt age setting recommended by manufacturers to avoid deep discharge).  Move the preset VR1 slowly until the relay de - energises.  You may try this adjustment more than once to get IC to switch at exactly 10.5V.  After the adjustment is done remove the variable power supply and multimeter and reconnect the battery and switch on SL 

NOTE  After calibration, endose the assembled unit in a suitable shock - proof cabinet fix on / off switch S1 on the front panel and a two -  pin connector for load on the back panel.  Keep the battery inside the cabinet •

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 VARIVALE TIMER WITH MUSIC PROJECT 

hello  friends  today I am gonna  telling  you  about  some  easy  and valuable  electronics projects for  school and  college  practical

This simple timer can be set to run for different time settings (30 seconds to 4.5 minutes). Also, a musical note is played at the end of the set timing.

Timer IC NE555 (IC1) is wired as an astable multivibrator (AVM). The AVM is set to 0.033HZ frequency using preset VR1. It means that the time period is around 30 seconds. The output of IC1 goes high for around 20 seconds and low for around 10 seconds. As it changes from low to high, decade counter IC CD4017 (IC2) is advanced by one count. IC2 has ten outputs Q0 through Q9.

Switch S2 resets the counter which takes Q0 to logic high. Counting starts when switch S2 is released. Output Q1 goes high on the next positive-going pulse from IC1. IC2 is then incremented on each clock. Outputs Q1 through Q9 of IC2 go high one-by-one at interval of 30 seconds. 

The rotary switch S3 selects one of these outputs and provides 3.1V power supply to melody generator IC UM66 (IC3). UM66 produces a musical signal on its output pin 1. The musical signal is amplified by transistor T1 and drives the speaker to produce the musical note.

Fig1; circuit diagram

When rotary switch S3 is connected to Q1 output of IC2 and reset switch S2 is pressed, a musical note is heard after a few seconds for 30 seconds. For positions 2 to 9, the silent period increases by 30 seconds at each step. At position 10 of rotary switch S3, the delay will be of four and a half minutes, followed by 30 seconds of music. 

The best way to think about this is that positions 1 to 9 add 30-second delay each. If you let the timer continue to run, it repeats the tune every 5 minutes, irrespective of the setting of S3. 

The circuit runs on 6V, provided by four AA cells in a battery holder. A PP3-type clip is used to connect this to the circuit. S1 is a simple single-pole, single-throw (SPST) switch.

Assemble the circuit on a general- purpose PCB and enclose it in a suitable box. If you have a multimeter with a frequency-measuring facility, it would be ideal for setting the frequency of IC1 at pin 3 at around 0.033HZ. 

Check that IC2 outputs run through the correct sequence (30 SECOND ). Wire the loud- speaker to its terminal pins using external wires. Fix rotary switch S3 in the front panel for selecting the time. The positions of rotary switch S3 can be labeled with stick-on tape labels. A simpler technique is to punch small discs from insulating tapes of various colours and stick these around the knob.

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