circuitT here are many ways of battery charging but constant-current charging, in particular, is a popular method for lead-acid and Ni- Cd batteries. In this circuit, the battery is charged with a constant current that is generally one-tenth of the battery capacity in ampere-hours. So for a 4.5Ah battery, constant charging cur- rent would be 450 mA. This battery charger has the follow- ing features: 1. It can charge 6V, 9V and 12V bat- teries. Batteries rated at other voltages can be charged by changing the values of zener diodes ZD1 and ZD2. 2. Constant current can be set as per the battery capacity by using a potmeter and multimeter in series with the battery. 3. Once the battery is fully charged, it will attain certain voltage level (e.g. 13.5-14.2V in the case of a 12V battery), give indication and the charger will switch off automatically. You need not remove the battery from the circuit. 4. If the battery is discharged be- low a limit, it will give deep-discharge indication. 5. Quiescent current is less than 5 mA and mostly due to zeners. 6. DC source voltage (V CC ) ranges from 9V to 24V. 7. The charger is short-circuit pro- tected. D1 is a low-forward-drop schottky diode SB560 having peak reverse volt- age (PRV) of 60V at 5A or a 1N5822 diode having 40V PRV at 3A. Nor- mally, the minimum DC source volt- age should be ‘D1 drop+Full charged battery voltage+V DSS + R2 drop,’ which is approximately ‘Full charged battery voltage+5V.’ For example, if we take full-charge voltage as 14V for a 12V battery, the source voltage should be 14+5=19V. For the sake of simplicity, this con- stant-current battery charger circuit is divided into three sections: constant- current source, overcharge protection and deep-discharge protection sec- tions. The constant-current source is built around MOSFET T5, transistor T1, diodes D1 and D2, resistors R1, R2, R10 and R11, and potmeter VR1. Diode D2 is a low-temperature-coeffcient, highly stable reference diode LM236-5. LM336-5 can also be used with reduced operating temperature range of 0 to +70°C. Gate-source voltage (V GS ) of T5 is set by adjusting VR1 slightly above 4V. By setting V GS , charging current can be fxed depending on the battery capacity. First, decide the charging current (one-tenth of the battery’s Ah capacity) and then calculate the nearest standard value of R2 as follows: R2 = 0.7/Safe fault current Constant-Current Battery Charger 6oíteo uíth Io||x PDf 6d|tcr - |ree |or ooo-commercíol ose. Jo remove thís ootíce, vísít: uuu.íceoí.com/oolock.htm electroni cs for you • August 2009 • 115 w w w . e f y m A g . c o m R2 and T1 limit the charging cur- rent if something fails or battery termi- nals get short-circuited accidentally. To set a charging current, while a multimeter is connected in series with the battery and source supply is present, adjust potmeter VR1 slowly until the charging current reaches its required value. Overcharge and deep-discharge protection have been shown in dotted areas of the circuit diagram. All com- ponents in these areas are subjected to a maximum of the battery voltage and not the DC source voltage. This makes the circuit work under a wide range of source voltages and without any infu- ence from the charging current value. Set overcharge and deep-discharge voltage of the battery using potmeters VR1 and VR2 before charging the bat- tery. In overcharge protection, zener diode ZD1 starts conducting after its breakdown voltage is reached, i.e., it conducts when the battery voltage goes beyond a prefxed high level. Adjust VR2 when the battery is fully charged (say, 13.5V in case of a 12V battery) so that V GS of T5 is set to zero and hence charging current stops fowing to the battery. LED1 glows to indicate that the battery is fully charged. When LED1 glows, the internal LED of the optocoupler also glows and the internal transistor con- ducts. As a result, gate-source voltage (V GS ) of MOSFET T5 becomes zero and charging stops. Normally, zener diode ZD2 con- ducts to drive transistor T3 into con- duction and thus make transistor T4 cut-off. If the battery terminal voltage drops to, say, 11V in case of a 12V bat- tery, adjust potmeter VR3 such that transistor T3 is cut-off and T4 conducts. LED2 will glow to indicate that the bat- tery voltage is low. Values of zener diodes ZD1 and ZD2 will be the same for 6V, 9V and 12V batteries. For other voltages, you need to suitably change the values of ZD1 and ZD2. Charging current pro- vided by this circuit is 1 mA to 1 A, and no heat-sink is required for T5. If the maximum charging current required is 5A, put another LM236-5 in series with diode D2, change the value of R11 to 1 kilo-ohm, replace D1 with two SB560 devices in parallel and provide a good heat-sink for MOSFET T1. TO-220 pack- age of IRF540 can handle up to 50W. Assemble the circuit on a gen- eral-purpose PCB and enclose in a box after setting the charging current, overcharge voltage and deep-discharge voltage. Mount potmeters VR1, VR2 and VR3 on the front panel of the box. 6oíteo uíth Io||x PDf 6d|tcr - |ree |or ooo-commercíol ose. Jo remove thís ootíce, vísít: uuu.íceoí.com/oolock.htm circuit ideas electroni cs for you • March 2009 • 81 w w w . e f y M a g . c o M Sandip Trivedi and p.d. LeLe TripLe power SuppLy in positive and negative regulated power supplies. LED1 glows to indi- cate that +5V is available, while LED2 indicates that –5V is available. Switch S1 is used for mains ‘on’/ ‘off’. Using switches S2 through S4, any of the three supplies can be independently turned off when not required in a particular experiment. This reduces unnecessary power dis- sipation and increases the life and reliability of the power supply. Since the circuit uses three terminal regula- tors, only capacitors are required at the input and output. The use of few components makes the circuit very simple. The three terminal regulators have heat-sink provision to directly deliver 1A output current. To ensure the maximum output, do not forget to T his low-cost, multipurpose power supply fulfils the re- quirements of almost all labora- tory experiments. Nonetheless, it can be easily fabricated by hobbyists. A single transformer is used to build this triple power supply. Regula- tor IC LM317 generates variable power supply of 1.25 to 20V, 1A. The dual ±12V, 1A power supply is generated by regulators 7812 and 7912. Similarly, dual ±5V, 1A power supply is gener- ated by regulators 7805 and 7905. ‘On’/‘off’ switches (S2 through S4) select the required power supply. Vari- able power supply is used to study the characteristics of devices. Fixed +5V power supply is used for all digital, microprocessor and microcontroller experiments. Dual ±12V power supply is used for op-amp-based analogue circuit experiments. Fig. 1 shows the circuit of the triple power supply, while Fig. 2 shows the pin configuration of the regulators used in the circuit. Transformer X1 steps down the mains power to deliver the secondary output of 18V-0-18V. The transformer output is rectifed by full-wave bridge rectifer BR1, fltered by capacitors C1, C2, C3, C7 and C8, and regulated by IC1 through IC5. Regulator IC1 (LM317) provides vari- able voltages (1.25 to 20V), while IC2 and IC4 provide regulated +12V and –12V, respectively. The output of IC2 is fed to regulator IC3 (7805), which pro- vides fxed +5V. Similarly, the output of IC4 is fed to regulator IC5 (7905), which provides fxed –5V. Capacitors C4 through C6, and C9 through C11, are used for further fltering of ripples Fig. 1: Tripple power supply OUT IN IC3 1 7805 2 3 GND F1 1.5A FUSE S1 ON/OFF SWITCH S2 230V AC 50Hz L N X1 BR1 W04 C1 1000µ 35V C5 10µ 16V C4 100µ 25V C9 100µ 25V C2 0.1µ C3 0.1µ C6 0.1µ C8 0.1µ C11 0.1µ C10 10µ 16V C7 1000µ 35V S3 S4 R2 330 R1 120 R3 330 LED1 LED2 GND GND +5V –5V OUT IN IC2 1 7812 2 3 GND OUT IN IC4 2 7912 1 3 GND OUT IN IC5 2 7905 1 3 GND OUT IN IC1 3 LM317 1 2 ADJ. VR1 2.2K +1.25 TO 20V +12V –12V X1 = 230V AC PRIMARY TO 18V-0-18V, 1.5A SECONDARY TRANSFORMER BR1-W04 1.5A, BRIDGE RECTIFIER GND BR1 W04 HEAT SINK HEAT SINK HEAT SINK HEAT SINK HEAT SINK S2 = FOR VARIABLE VOLTAGE S3 = FOR +12V AND +5V S4 = FOR –12V AND –5V S1-S4 = ON/OFF SWITCH POT 6oíteo uíth Io||x PDf 6d|tcr - |ree |or ooo-commercíol ose. Jo remove thís ootíce, vísít: uuu.íceoí.com/oolock.htm circuit ideas 82 • March 2009 • electroni cs for you www. e f y Ma g . c o M use heat-sinks for the regulators. The three-terminal regulators are almost non-destructible. These have inbuilt protection circuits including the thermal shutdown protection. Even if there is overload or shorting of the output, the inbuilt overload protection circuit will limit the current and slowly reduce the output voltage to zero. Similarly, if the temperature increases beyond a certain value due to excessive load and heat dissipation, the in-built thermal shutdown circuit will reduce the output current and the output volt- age (gradually) to zero. Thus complete protection is provided to the circuitry. Assemble the circuit on a general- purpose PCB and enclose in a box as shown in Fig. 3. The step-by-step procedure to build the triple power supply for the labora- tory follows: Fig. 2: Pin confgurations of regulators Fig. 3: Proposed cabinet for power supply 1. Collect all the components shown in the circuit diagram. 2. Connect switch S1, fuse, trans- former and mains cord to the assem- bled PCB as well as the box. 3. Keep the multimeter in DC volt- age range (more than 25V DC) and measure the DC voltage across ca- pacitors C1 and C7 (1000 µF, 35V). This voltage should be around 18V×1.41=25 to 26V DC. Check both positive and negative voltages with respect to ground. 4. It is advisable to use three-wire mains cable and plug. If you are using any metallic box, earthing wire/pin of the mains plug should be soldered to the body of the metallic box using an earthing tag. 5. If the 18V-0-18V transformer is replaced with 15V-0-15V trans- f ormer, t he out put voltage of the variable supply using LM317 will be correspond- ingly lower. 6. If proper voltages are available, go to step 7. Otherwise, check the connections. 7. Connect variable regulator LM317 to the circuit and check 1.25V to 20V output by varying the 2.2-kilo- ohm linear potentiometers. 8. Now connect ICs 7812, 7912, 7805 and 7905 to the circuit and check their output voltage. 9. Connect terminals, potmeter, switches and indicator LED on the front panel of the box and complete the connections. Close the box by us- ing screws. Precaution. At the primary side of the transformer, 230V AC could give lethal shocks. So be careful not to touch this part. EFY will not be responsible for any resulting loss or harm to the user. circuit ideas 88 • apri l 2009 • electroni cs for you www. e f y ma g . c o m D. Mohan KuMar reMote-operateD Master switch S.C. DW IVEDI tial divider comprising resistors R4 and R5 maintains half of 5.1V at pin 2 of IC1. In brief, the voltage at pin 2 of IC1 is higher than at pin 3 and its output remains low. LED2 remains ‘off’ and transistor T2 does not conduct. Relay RL1 remains de-energised and, as a re- sult, security lamps (both indoors and outdoors) remain switched off. When you press any key of the remote TV handset, IR rays fall on the receiver (IRX1) and its output goes low. LED1 fashes in sync with pulsation of the IR rays. At the same time, transis- tor T1 (BC558) conducts to take pin 3 of IC1 high. IC1 is used as a comparator with timer action. When transistor T1 conducts, pin 3 of IC1 gets a higher voltage than pin 2 making the output of IC1 high. Mean- while, capacitor C4 charges to full voltage and keeps pin 3 high for a few minutes even after T1 is non-conduct- ing. Resistor R3 provides discharge path for capacitor C4, which decides the time period for which the output of comparator IC1 should remain high. The high output of IC1 energises re- lay RL1 through relay-driver transistor T2. Thus the load, i.e., security lamps, turn on for three to four minutes. LED2 glows to indicate activation of the relay as well as switching ‘on’ of the security lights. Connect a single-pole, single-throw ‘on’/‘off’ switch (MS) to activate the security lamps manually when required. Zener diode ZD1 provides 5.1V DC for safe operation of the IR receiver and associated circuit. Power for the circuit is derived from a step-down transformer (X1) and a bridge recti- fer comprising diodes D1 through D4. Smoothing capacitor C1 removes rip- ples, if any, from the power supply. Assemble the circuit on a general- purpose PCB and enclose in a suitable cabinet. Drill holes on the front panel for mounting the IR sensor and LEDs. Connect the master switch between the normally-open (N/O) contact and pole of relay RL1 so that the master switch can be used when needed. The relay contacts rating should be more than 4A. Mount the unit near the master switch using minimal wiring. G enerally, a bedside master switch is used to switch on lamps both indoors and outdoors when there is a threat of intruder. This circuit can be used to activate the master switch from the bed without searching for the switch in darkness. It can be activated by the TV remote handset. The security lamps glow for three minutes and then turn off. The circuit is sensitive and can be activated from a distance of up to 25 metres. IR receiver module TSOP 1738 (IRX1) is used to sense the pulsed 38kHz IR rays from the TV remote handset. The IR receiver module has a PIN photodiode and a preamplifer enclosed in an IR flter epoxy case. Its open-collector output is 5 volts at 5mA current in the standby mode. In the standby mode, no IR rays from the remote handset fall on the IR receiver, so its output pin 3 remains high and LED1 doesn’t glow. Through resistor R2, the base of transistor T1 remains high and it does not conduct. As a result, the voltage at pin 3 of IC CA3130 (IC1) remains low. The poten- circuit ideas 94 • December 2009 • electroni cs for you www. e f y ma g . c o m T oday telephone has become an integral part of our lives. It is the most widely used communication device in the world. Owing to its immense popularity and widespread use, there arises a need for call recording devices, which fnd ap- plication in call centres, stock broking frms, police, offces, homes, etc. Here we are describing a call re- corder that uses very few components. But in order to understand its working, one must frst have the basic knowl- edge of standard telephone wiring and a stereo plug. In India, landline telephones pri- marily use RJ11 wiring, which has two wires—tip and ring. While tip is the positive wire, ring is the negative one. And together they complete the telephone circuit. In a telephone line, voltage between tip and ring is around 48V DC when handset is on the cradle (idle line). In order to ring the phone for an incoming call, a 20Hz AC cur- rent of around 90V is superimposed over the DC voltage already present in the idle line. The negative wire from the phone line goes to IN1, while the posi- tive wire goes to IN2. Further, the negative wire from OUT1 and the posi- tive wire from OUT2 are connected to the phone. All the resis- tors used are 0.25W carbon flm resistors and all the capaci- tors used are rated for 250V or more. The negative terminal of ‘To AUX IN’ is connected to pin 1 of the stereo jack while the positive terminal is con- nected to pins 2 and 3 of the stereo jack. This stereo jack, in turn, is con- nected to the AUX IN of any recording device, such as computer, audio cas- sette player, CD player, DVD player, etc. Here we shall be connecting it to a computer. When a call comes in, around 90V AC current at 20Hz is superimposed over the DC voltage already present in the idle line. This current is converted into DC by the diodes and fed to resis- tor R1, which reduces its magnitude and feeds it to LED1. The current is further reduced in magnitude by the resistor R2 and fed to the right and left channels of the stereo jack, which are connected to the AUX IN port of a computer. Any audio recording software, such as AVS audio recorder (available at: http://www.avs4you.com/AVS- Audio-Recorder.aspx), Audacity audio recorder (http://audacity.sourceforge. net/), or audio recorder (http://www. audio-tool.net/audio_recorder_for _free.html), can be used to record the call. When a call comes in, one needs to launch the audio recording software and start recording. For phone recording, simply con- nect the stereo jack to the AUX IN port of the PC. Install the Audacity audio recorder (different versions are available for free for different op- erating systems at http://audacity. sourceforge.net/) on your PC. Run the executable Audacity fle. In the main window, you will fnd a drop- down box in the top right corner. From this box, select the AUX option. Now you are ready to record any call. As soon as a call comes in, press the record button found in the Audacity main window and then pick up the telephone receiver and answer the call. Press the stop button once the call ends. Now go to the fle menu and select the ‘Export as WAV’ option and save the fle in a desired location. You may change the value of resis- AlizishAAn KhAtri telephone cAll recorder S.C. DW IVEDI Fig. 1: Call recorder circuit Fig. 2: Pin confguration of stereo jack Fig. 3: RJ connector circuit ideas electroni cs for you • December 2009 • 95 www. e f y ma g . c o m tor R2 if you want to change the output volume. You can use a variable resistor in series with R2 to vary the volume of the output. The recorded audio clip can be edited using different options in the Audacity software. You can assemble the circuit on a general-purpose PCB and enclose it in a small cabinet. Use an RJ11 connec- tor and stereo jack for connecting the telephone set and computer (for call recording). Telephone cords can be used to connect to the phone line and the circuit. Use of a shielded cable is recommended to reduce disturbances in the recording. These can also be reduced by increasing the value of R2 to about 15 kilo-ohms. EFY note. Audacity recording software is included in this month’s EFY-CD under ‘Utilities’ section. circuit ideas electroni cs for you • December 2009 • 93 www. e f y ma g . c o m power is available, diode D5 forward biases. It provides power to the circuit and also charges the battery through resistor R2, and it limits the charging current to 120 mA. When power fails, diode D5 reverse biases and diode D6 forward biases, giving instant backup to the circuit. LED1 indicates the avail- ability of mains power. Assemble the circuit on a general- purpose PCB and enclose it in a suit- able case. Connect the piezo element to the circuit using a thin insulated wire. Glue the fat side of the piezo el- ement on a 30×30cm aluminium sheet to increase its sensitivity. Fix the sheet with the piezo sensor to the site where protection is needed. The remaining circuit can be fxed at a suitable place. If only the alarm generator is needed, omit the relay driver section. overcharge voltage and deep-discharge voltage. gate-source voltage (VGS) of MOSFET T5 becomes zero and charging stops. w w w. you need to suitably change the values of ZD1 and ZD2. and no heat-sink is required for T5. e f y m Ag . say. adjust potmeter VR1 slowly until the charging current reaches its required value. Overcharge and deep-discharge protection have been shown in dotted areas of the circuit diagram. Assemble the circuit on a general-purpose PCB and enclose in a box after setting the charging current. it conducts when the battery voltage goes beyond a prefixed high level.R2 and T1 limit the charging current if something fails or battery terminals get short-circuited accidentally.e. i. TO-220 package of IRF540 can handle up to 50W. As a result. Adjust VR2 when the battery is fully charged (say. Mount potmeters VR1. zener diode ZD1 starts conducting after its breakdown voltage is reached. To set a charging current.. replace D1 with two SB560 devices in parallel and provide a good heat-sink for MOSFET T1. put another LM236-5 in series with diode D2. If the maximum charging current required is 5A. This makes the circuit work under a wide range of source voltages and without any influence from the charging current value. Values of zener diodes ZD1 and ZD2 will be the same for 6V. Set overcharge and deep-discharge voltage of the battery using potmeters VR1 and VR2 before charging the battery. LED1 glows to indicate that the battery is fully charged. zener diode ZD2 conducts to drive transistor T3 into conduction and thus make transistor T4 cut-off. 11V in case of a 12V battery. adjust potmeter VR3 such that transistor T3 is cut-off and T4 conducts. If the battery terminal voltage drops to. VR2 and VR3 on the front panel of the box. Normally. while a multimeter is connected in series with the battery and source supply is present. 13. change the value of R11 to 1 kilo-ohm. For other voltages.5V in case of a 12V battery) so that VGS of T5 is set to zero and hence charging current stops flowing to the battery. the internal LED of the optocoupler also glows and the internal transistor conducts. LED2 will glow to indicate that the battery voltage is low. Charging current provided by this circuit is 1 mA to 1 A. co m e l e c t ro n i c s f o r yo u • Au g u s t 2 0 0 9 • 1 1 5 . In overcharge protection. All components in these areas are subjected to a maximum of the battery voltage and not the DC source voltage. 9V and 12V batteries. When LED1 glows. BRIDGE RECTIFIER S4 IN 2 C9 100µ 25V GND 1 3 OUT IN 2 LED2 3 OUT C10 10µ 16V R3 330 –5V C11 0.5A FUSE L 230V AC 50Hz N X1 BR1 W04 BR1 W04 C1 1000µ 35V 1 S3 IC2 7812 2 3 1 IC3 7805 2 3 OUT GND C4 100µ 25V GND C5 10µ 16V R2 330 C6 0. C2. which provides fixed +5V.d. 1. Since the circuit uses three terminal regulators. 1A power supply is generated by regulators 7812 and 7912. respectively. The output of IC2 is fed to regulator IC3 (7805). Variable power supply is used to study the characteristics of devices. C7 and C8. LED1 glows to indicate that +5V is available. 1A power supply is generated by regulators 7805 and 7905. 1: Tripple power supply w w w. are used for further filtering of ripples HEAT SINK IN S2 in positive and negative regulated power supplies. Regulator IC LM317 generates variable power supply of 1. filtered by capacitors C1. any of the three supplies can be independently turned off when not required in a particular experiment.1µ GND 1 BR1-W04 1.1µ 3 IC1 LM317 2 1 ADJ.5A SECONDARY TRANSFORMER C7 1000µ 35V C8 0. The dual ±12V. Transformer X1 steps down the mains power to deliver the secondary output of 18V-0-18V. To ensure the maximum output. do not forget to S2 = FOR VARIABLE VOLTAGE S3 = FOR +12V AND +5V S4 = FOR –12V AND –5V S1-S4 = ON/OFF SWITCH C3 0. The transformer output is rectified by full-wave bridge rectifier BR1. Similarly. T his low-cost. Nonetheless.1µ +5V LED1 C2 0.25 to 20V). the output of IC4 is fed to regulator IC5 (7905). A single transformer is used to build this triple power supply. multipurpose power supply fulfils the requirements of almost all laboratory experiments.5A. Capacitors C4 through C6. while LED2 indicates that –5V is available. Dual ±12V power supply is used for op-amp-based analogue circuit experiments. Regulator IC1 (LM317) provides variable voltages (1. e f y M ag .1µ GND X1 = 230V AC PRIMARY TO 18V-0-18V. This reduces unnecessary power dissipation and increases the life and reliability of the power supply.2K POT HEAT SINK IN HEAT SINK OUT IN GND +12V S1 ON/OFF SWITCH F1 1. C3. Fixed +5V power supply is used for all digital. dual ±5V. 1 shows the circuit of the triple power supply. while IC2 and IC4 provide regulated +12V and –12V. OUT R1 120 +1.circuit ideas TripLe power SuppLy Sandip Trivedi and p LeLe .1µ IC4 7912 IC5 7905 HEAT SINK HEAT SINK –12V GND Fig. ‘On’/‘off’ switches (S2 through S4) select the required power supply. co M e l e c t ro n i c s f o r yo u • M a r c h 2 0 0 9 • 8 1 . and C9 through C11.25 TO 20V VR1 2. it can be easily fabricated by hobbyists. Fig. microprocessor and microcontroller experiments. Similarly. while Fig. 1A.25 to 20V. only capacitors are required at the input and output. which provides fixed –5V. Using switches S2 through S4. and regulated by IC1 through IC5. The use of few components makes the circuit very simple. 2 shows the pin configuration of the regulators used in the circuit. Switch S1 is used for mains ‘on’/ ‘off’. The three terminal regulators have heat-sink provision to directly deliver 1A output current. Assemble the circuit on a generalpurpose PCB and enclose in a box as shown in Fig. These have inbuilt protection circuits including the thermal shutdown protection. transformer and mains cord to the assembled PCB as well as the box. The step-by-step procedure to build the triple power supply for the laboratory follows: Fig. switches and indicator LED on the front panel of the box and complete the connections. fuse. Keep the multimeter in DC voltage range (more than 25V DC) and measure the DC voltage across capacitors C1 and C7 (1000 µF. 8 2 • M a r c h 2 0 0 9 • e l e c t ro n i c s f o r yo u w w w. 7805 and 7905 to the circuit and check their output voltage.2-kiloohm linear potentiometers. This voltage should be around 18V×1. Close the box by using screws. If the 18V-0-18V transformer is replaced with 15V-0-15V transformer. 8. Precaution. the output voltage of the variable supply using LM317 will be correspondingly lower. e f y M ag . co M . 3. go to step 7.25V to 20V output by varying the 2.circuit ideas Fig.41=25 to 26V DC. Connect terminals. 2. the in-built thermal shutdown circuit will reduce the output current and the output voltage (gradually) to zero. 7912. Check both positive and negative voltages with respect to ground. 35V). If proper voltages are available. EFY will not be responsible for any resulting loss or harm to the user. Otherwise. Similarly. At the primary side of the transformer. 3. 4. if the temperature increases beyond a certain value due to excessive load and heat dissipation. If you are using any metallic box. Collect all the components shown in the circuit diagram. Thus complete protection is provided to the circuitry. check the connections. Connect variable regulator LM317 to the circuit and check 1. 2: Pin configurations of regulators use heat-sinks for the regulators. 6. earthing wire/pin of the mains plug should be soldered to the body of the metallic box using an earthing tag. potmeter. 7. 230V AC could give lethal shocks. 3: Proposed cabinet for power supply 1. the inbuilt overload protection circuit will limit the current and slowly reduce the output voltage to zero. Even if there is overload or shorting of the output. Now connect ICs 7812. 5. Connect switch S1. It is advisable to use three-wire mains cable and plug. The three-terminal regulators are almost non-destructible. So be careful not to touch this part. 9. Relay RL1 remains de-energised and. It can be activated by the TV remote handset. dwiv edi glows to indicate activation of the relay as well as switching ‘on’ of the security lights. Resistor R3 provides discharge path for capacitor C4. Its open-collector output is 5 volts at 5mA current in the standby mode. LED1 flashes in sync with pulsation of the IR rays. When transistor T1 conducts. Power for the circuit is derived from a step-down transformer (X1) and a bridge rectifier comprising diodes D1 through D4. Meanwhile.e. IR receiver module TSOP 1738 (IRX1) is used to sense the pulsed 38kHz IR rays from the TV remote handset. i.1V at pin 2 of IC1. IR rays fall on the s. Zener diode ZD1 provides 5. At the same time. co m . which decides the time period for which the output of comparator IC1 should remain high.. This circuit can be used to activate the master switch from the bed without searching for the switch in darkness. the base of transistor T1 remains high and it does not conduct. Connect a single-pole. capacitor C4 charges to full voltage and keeps pin 3 high for a few minutes even after T1 is non-conducting. The poten- receiver (IRX1) and its output goes low. the voltage at pin 2 of IC1 is higher than at pin 3 and its output remains low. as a result. Thus the load. LED2 remains ‘off’ and transistor T2 does not conduct. When you press any key of the remote TV handset. security lamps (both indoors and outdoors) remain switched off. a bedside master switch is used to switch on lamps both indoors and outdoors when there is a threat of intruder. so its output pin 3 remains high and LED1 doesn’t glow. Drill holes on the front panel for mounting the IR sensor and LEDs. no IR rays from the remote handset fall on the IR receiver. Assemble the circuit on a generalpurpose PCB and enclose in a suitable cabinet. The relay contacts rating should be more than 4A. LED2 when required. transistor T1 (BC558) conducts to take pin 3 of IC1 high. The high output of IC1 energises relay RL1 through relay-driver transistor T2. Connect the master switch between the normally-open (N/O) contact and pole of relay RL1 so that the master switch can be used when needed. Mohan KuMar G enerally. 8 8 • a p r i l 2 0 0 9 • e l e c t ro n i c s f o r yo u w w w.1V DC for safe operation of the IR receiver and associated circuit. Through resistor R2. e f y m ag . In the standby mode. The security lamps tial divider comprising resistors R4 and R5 maintains half of 5. IC1 is used as a comparator with timer action. pin 3 of IC1 gets a higher voltage than pin 2 making the output of IC1 high. Mount the unit near the master switch using minimal wiring. As a result. turn on for three to four minutes. from the power supply. security lamps.c. single-throw ‘on’/‘off’ switch (MS) to activate the security lamps manually glow for three minutes and then turn off. Smoothing capacitor C1 removes ripples. The circuit is sensitive and can be activated from a distance of up to 25 metres. The IR receiver module has a PIN photodiode and a preamplifier enclosed in an IR filter epoxy case. the voltage at pin 3 of IC CA3130 (IC1) remains low. if any. In brief.circuit ideas reMote-operateD Master switch D. Now go to the file menu and select the ‘Export as WAV’ option and save the file in a desired location. stock broking firms. police. Now you are ready to record any call. This stereo jack. Owing to its immense popularity and edge of standard telephone wiring and a stereo plug. In India. a 20Hz AC current of around 90V is superimposed over the DC voltage already present in the idle line. in turn. audio cas- sette player. As soon as a call comes in. 1: Call recorder circuit Fig. simply connect the stereo jack to the AUX IN port of the PC. co m 9 4 • D e c e m b e r 2 0 0 9 • e l e c t ro n i c s f o r yo u .aspx). etc. The negative terminal of ‘To AUX IN’ is connected to pin 1 of the stereo jack while the positive terminal is connected to pins 2 and 3 of the stereo jack.html). or audio recorder (http://www. one needs to launch the audio recording software and start recording. CD player. All the resistors used are 0.c. 3: RJ connector widespread use. you will find a dropdown box in the top right corner. Further. dwiv edi AlizishAAn KhAtri T oday telephone has become an integral part of our lives. And together they complete the Fig. landline telephones primarily use RJ11 wiring. In order to ring the phone for an incoming call. press the record button found in the Audacity main window and then pick up the telephone receiver and answer the call. there arises a need for call recording devices. around 90V AC current at 20Hz is superimposed over the DC voltage already present in the idle line. such as AVS audio recorder (available at: http://www. Here we shall be connecting it to a computer. etc. while the positive wire goes to IN2. 2: Pin configuration of stereo jack Fig. can be used to record the call. offices. For phone recording. Any audio recording software.sourceforge. sourceforge. But in order to understand its working. In the main window. audio-tool. ring is the negative one. When a call comes in. You may change the value of resisw w w. This current is converted into DC by the diodes and fed to resistor R1. is connected to the AUX IN of any recording device. which find application in call centres. DVD player.com/AVSAudio-Recorder.net/) on your PC. voltage between tip and ring is around 48V DC when handset is on the cradle (idle line).avs4you.circuit ideas telephone cAll recorder s. While tip is the positive wire. In a telephone line. The current is further reduced in magnitude by the resistor R2 and fed to the right and left channels of the stereo jack. It is the most widely used communication device in the world. select the AUX option. Run the executable Audacity file. one must first have the basic knowl- telephone circuit. Install the Audacity audio recorder (different versions are available for free for different operating systems at http://audacity. net/). The negative wire from the phone line goes to IN1. such as computer. which are connected to the AUX IN port of a computer.net/audio_recorder_for _free. which has two wires—tip and ring. Press the stop button once the call ends. homes. the negative wire from OUT1 and the positive wire from OUT2 are connected to the phone.25W carbon film resistors and all the capacitors used are rated for 250V or more. When a call comes in. which reduces its magnitude and feeds it to LED1. Here we are describing a call recorder that uses very few components. From this box. e f y m ag . Audacity audio recorder (http://audacity. co m e l e c t ro n i c s f o r yo u • D e c e m b e r 2 0 0 9 • 9 5 .circuit ideas tor R2 if you want to change the output volume. These can also be reduced by increasing the value of R2 to about 15 kilo-ohms. Use of a shielded cable is recommended to reduce disturbances in the recording. The recorded audio clip can be edited using different options in the Audacity software. w w w. Use an RJ11 connector and stereo jack for connecting the telephone set and computer (for call recording). Audacity recording software is included in this month’s EFY-CD under ‘Utilities’ section. Telephone cords can be used to connect to the phone line and the circuit. You can use a variable resistor in series with R2 to vary the volume of the output. EFY note. You can assemble the circuit on a general-purpose PCB and enclose it in a small cabinet. e f y m ag . Connect the piezo element to the circuit using a thin insulated wire. giving instant backup to the circuit. It provides power to the circuit and also charges the battery through resistor R2. Glue the flat side of the piezo el- ement on a 30×30cm aluminium sheet to increase its sensitivity. diode D5 forward biases. co m e l e c t ro n i c s f o r yo u • D e c e m b e r 2 0 0 9 • 9 3 . Assemble the circuit on a generalpurpose PCB and enclose it in a suitable case. omit the relay driver section. The remaining circuit can be fixed at a suitable place.circuit ideas power is available. When power fails. Fix the sheet with the piezo sensor to the site where protection is needed. diode D5 reverse biases and diode D6 forward biases. and it limits the charging current to 120 mA. w w w. LED1 indicates the availability of mains power. If only the alarm generator is needed. e f y m ag .