Two-Line Intercom-Cum-Telephone Line Changeover

The circuit presented here can be used for connecting two telephones in parallel and also as a 2-line intercom. Usually a single telephone is connected to a telephone line. If another telephone is required at some distance, a parallel line is taken for connecting the other telephone. In this simple parallel line operation, the main problem is loss of privacy besides interference from the other phone. This problem is obviated in the circuit presented here. Under normal condition, two telephones (telephone 1 and 2) can be used as intercom while telephone 3 is connected to the lines from exchange. In changeover mode, exchange line is disconnected from telephone 3 and gets connected to telephone 2.

Circuit diagram:

2-Line Intercom-Cum-Telephone Line Changeover-Circuit-Diagram

2-Line Intercom-Cum-Telephone Line Changeover Circuit Diagram

For operation in intercom mode, one has to just lift the handset of phone 1 and then press switch S1. As a result, buzzer PZ2 sounds. Simultaneously, the side tone is heard in the speaker of handset of phone 1. The person at phone 2 could then lift the handset and start conversation. Similar procedure is to be followed for initiation of the conversation from phone 2 using switch S2. In this mode of operation, a 3-pole, 2-way slide-switch S3 is to be used as shown in the figure. In the changeover mode of operation, switch S3 is used to changeover the telephone line for use by telephone 2. The switch is normally in the intercom mode and telephone 3 is connected to the exchange line.

Before changing over the exchange line to telephone 2, the person at telephone 1 may inform the person at telephone 2 (in the intercom mode) that he is going to changeover the line for use by him (the person at telephone 2). As soon as changeover switch S3 is flipped to the other position, 12V supply is cut off and telephones 1 and 3 do not get any voltage or ring via the ring-tone-sensing unit. Once switch S3 is flipped over for use of exchange line by the person at telephone 2, and the same (switch S3) is not flipped back to normal position after a telephone call is over, the next telephone call via exchange lines will go to telephone 2 only and the ringtone-sensing circuit will still work. This enables the person at phone 3 to know that a call has gone through. If the handset of telephone 3 is lifted, it is found to be dead. To make telephone 3 again active, switch S3 should be changed over to its normal position.

Author : J. Srinivasan - Copyright: Efymag

Simple Electronic Quiz Switch

One of the common  rounds in the  quizzes is the buzzer round. We are describing here a simple electronic circuit that can be used in any test or quiz competition. In this circuit, only four persons can participate,  and  every  participant is assigned a certain number. Whenever a switch is pressed, the circuit locks the remaining three entries. At the same time, an alarm sounds and the designated switch number is displayed on the seven segment LED display.When a player presses his switch, the corresponding output of IC1 goes high. Let us suppose, when switch S1 is pressed, D1 input of IC1 goes low and its corresponding output Q1 goes high. As a result, current passes through D5 to piezo buzzer PZ1, which creates a beep. At the same time, current also passes through diodes D6-D7 to show the number on the LED display.

Circuit diagram:

Simple Electronic Quiz Switch Circuit Diagram

Simple Electronic Quiz Switch Circuit Diagram


Similarly, when any other switch (S2-S4) is pressed, the corresponding  number  gets  displayed  on  seven segment displaying DIS1 and buzzer sounds. Switch S5 is used to reset the display exclusively. Switch S5 is a push to on switch. The circuit is powered by 9V battery. Assemble the circuit on a general purpose PCB and enclose it in a suitable  case along with seven segment display and piezo buzzer. The assembled circuit can be kept near the host and the switches connected through the external can be assigned to the players.

Author : Siddeeq Basha  - Copyright : EfyMag

Pyroelectric Fire Sensor

Here is an ultra-sensitive fire sensor that exploits the direct piezoelectric  property  of  an ordinary piezo element to detect fire. The lead  zirconate  titanate  crystals  in the piezo element has a property to de-form and generate an electric potential plifier with gate protected p-channel.  MOSFETs in the inputs. It has high speed  of  performance  and  low  input current requirements. There are two inputs—the non inverting input (pin 3) connected to the piezo element through diode D7 (OA71) that carries the volt-age signal from the piezo element and the inverting input (pin 2) that gets a momentarily changes the voltage level at pin 3 of IC1 and its output swings high. Transistor T1 conducts taking the reset pin 12 of IC2 to ground. IC2 is now enabled and start oscillating. With the shown values of the oscillating components C3 (0.22) and R8 (1M), when heated, thus converting the piezo element into a heat sensor.

Pyroelectric Fire Sensor -Circuit-Diagram

Pyroelectric Fire Sensor -Circuit-Diagram

The circuit we  have  described  here  is  very  sensitive. It gives a warning alarm if the room temperature increases more than 10 degree Celsius. The entire circuit is divided  into  two  section—the  sensor and the power supply section. Sensor side circuit. Fig. 1 shows the fire sensor circuit. The front end of the circuit has a sensitive signal amplifier built around IC1 (CA3130). It gives a  high  output when the  temperature near the piezo element increases. IC CA 3130 is a CMOS operational am-preset  volt-age through VR1.By adjusting VR1, it is easy  to set the reference voltage level at pin 2. In normal condition, IC1 gives a low output and the remaining circuitry will be in a standby state. Capacitor C2(10P) keeps the non-inverting in-put of IC1 stable, so that even a slight change in voltage  level  in  the  inputs can change the output to high.

Normally, IC1 gives a low output, keeping transistor T1 non-conducting. Reseting pin 12 of IC2 (CD4060) connected to the collector of transistor T1 gets a high voltage through R6 and IC2 remains disabled. When the piezo element gets heat from fire, asymmetry in its crystals cause a potential change, enabling capacitor C2 to discharge. It the first output (Q3) turns high after 4 seconds and a red LED starts flashing. If the heat near the piezo persists, Q7 (pin 14) output of IC2 becomes high after  1  minute,  and  the  alarm  starts beeping. If heat continues, Q9 (pin15) turns high after four minutes and turns on the relay driver transistor T2. At the same time, diode D8 conducts and IC2 stops oscillating and toggles.The  solenoid  pump  connected  to the N/O (normally opened) contact of the relay starts spraying the fire-ceasing foam or water to the possible sites of fire.

Power supply with battery backup  circuit diagram Power Supply With Battery Backup  Circuit Diagram

Power  supply  circuit. Power sup-ply section (Fig. 2) comprises a 0-12V, 1 A step down transformer with a standard full wave rectifier formed by D1 through D4 and filter capacitor C1. A battery backup is provided if the mains supply cut-off due to short-circuit and fire. A 12V, 4.5 Ah rechargeable battery is used for backup to give sufficient current to the solenoid pump. When mains power is available, diode D5 forward biases. It provides power to the  circuit  and  also  charges  the  bat-tery 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  availability  of  mains power.Assemble the circuit on a common PCB and enclose it in a suitable case. Connect the piezo element to the circuit using thin plastic wire. Glue the flat side of the piezo element on a 30x30 cm 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 fixed in a suitable place. If only the alarm generator is needed, omit the relay driver section.

Author :D. Mohan Kumar - Copyright : EFYMag

Electric Window-Fence Charger

Here is the circuit of a simple electric window charger. With a couple of minor circuit variations, it can be used as an electric fence charger too. A standard 12V, 7Ah sealed maintenance-free (SMF) UPS battery is required for powering the entire unit. Any component layout and mounting plan can be used. However, try to keep the output terminals of transformer X1 away from the circuit board. Timer NE555 (IC1) is wired as a free-running oscillator with narrow negative pulse at the output pin 3. The pulse frequency is determined by resistors R2 and R3, preset VR1 and capacitor C3.

Circuit diagram :

Electric Window-Fence-Charger-Circuit-Daigram

Electric Window-Fence Charger Circuit Diagram

The amplitude of the output pulse can be varied to some extent by adjusting variable resistor VR1. You can vary the frequency from 100 Hz to 150 Hz. X1 is a small, iron-core, step-down transformer (230V AC primary to 12V, 1A secondary) that must be reverse connected, i.e., the secondary winding terminals of the transformer should be connected between the emitter and ground and the output taken across the primary winding.Switch S1 is used for power ‘on’/‘off’ and LED1 works as a power-‘on’ indicator. LED2 is used to indicate the pulse activity.

The output pulse from pin 3 of IC1 drives pnp transistor T1 into conduction for the duration of the time period. The collector of T1 is connected to the base of driver transistor T2 through resistor R5. When transistor T1 conducts, T2 also conducts. When T2 conducts, a high-current pulse flows through the secondary winding of transformer X1 to generate a very high-voltage pulse at the primary winding. This dangerously high voltage can be used to charge the window rails/fences. Ordinary silicon diode D1 (1N4001) protects T2 against high-voltage peaks generated by X1 inductance during the switching time. You can replace X1 with another transformer rating, and, if necessary, replace T2 with another higher-capacity transistor. The circuit can be used to charge a 1km fence with some minor modifications in the output section.

Caution:

  • Take all the relevant electrical safety precautions when assembling, testing and using this high-voltage generator.

Author : T.K. Hareendran  - Copyright : EFY Mag

Simple Mini Power Inverter

Even robot systems occasionally need a negative supply voltage for some purpose or other, and in this kind of application in particular there is a need for an effective circuit that does  not  make  greater demands  then  necessary in terms of current or space. If a low current 5 V supply is needed and only +5 V is available, a natural manufacturer to turn  to  is  Maxim,  and indeed in this case they do not let us down.The best known integrated  circuit made by this company is the MAX232, a level shifter for serial ports with an integrated charge pump that does not need an external inductor.

Project  image:

Mini Power Inverter Img

Simple Mini Power Inverter  

Along the same lines, although with a more stable output voltage and higher efficiency, is the MAX660. The device can ‘mirror’ any input voltage between 1.5 V and 5.5 V. With a 5 V input the output is typically –4.7 V with a load of 100 mA. Efficiency at 10 mA is around 96 % and at 100 mA is around 88 %. With an open-circuit output the IC draws a quiescent current of just 120 μA.There is little to say about the circuit itself.

Circuit diagram :

Simple Mini Power Inverter Circuit Diagram

Simple Mini Power Inverter Circuit Diagram

The 0 Ω resistor on pin 1 selects the operating frequency. With R1 fitted, the circuit operates at 80 kHz; without it, at 10 kHz. The combination of L1 and C5 slightly reduces ripple on the output voltage; the choice of inductor is not as critical as it would be if it formed part of the switching circuit.Gerber files for the printed circuit board (which uses some SMD components) are available for download from the Elektor website, ref. 070279-11.zip. R1, C1 and C4 are 0603 SMDs and C3 is an SMD tantalum electrolytic capacitor. Either the MAX-660CSA or the MAX660M can be used; both come in SO8 packages. L1 is a 10 μH SMD inductor rated at 300 mA.

Author : Alexander Wiedekind-Klein - Copyright : elektor

Fastest Finger First Indicator

Quiz-type game shows are increasingly becoming popular on tale vision these days. In such games, fastest finger first indicators (FFFIs) are used to test the player’s reaction time. The player’s designated number is dis played with an audio alarm when the player presses his entry button. The circuit presented here determines as to which of the four contestants first pressed the button and locks out the remaining three entries. Simultaneously, an audio alarm and the correct decimal number display of the corresponding contestant are activated.

Circuit Diagram:

Fastest Finger First Indicator Circuit-Diagram

Fastest Finger First Indicator Circuit Diagram

When a contestant presses his switch, the corresponding output of latch IC2 (7475) changes its logic state from 1 to 0. The combinational circuitry comprising dual 4-input NAND gates of IC3 (7420) locks out subsequent entries by producing the appropriate latch-disable signal. Priority encoder IC4 (74147) encodes the active-low input condition into the cor responding binary coded decimal (BCD) number output. The outputs of IC4 after inversion by inverter gates inside hex inverter 74LS04 (IC5) are coupled to BCD-to-7-segment decoder/display driver IC6 (7447). The output of IC6 drives common-anode 7-segment LED display (DIS.1, FND507 or LT543).

The audio alarm generator comprises clock oscillator IC7 (555), whose output drives a loudspeaker. The oscillator frequency can be varied with the help of preset VR1. Logic 0 state at one of the outputs of IC2 produces logic 1 input condition at pin 4 of IC7, thereby enabling the audio oscillator.  IC7 needs +12V DC supply for sufficient alarm level. The remaining circuit operates on regulated +5V DC supply, which is obtained using IC1 (7805). Once the organiser identifies the contestant who pressed the switch first, he disables the audio alarm and at the same time forces the digital display to ‘0’ by pressing reset pushbutton S5. With a slight modification, this circuit can accommodate more than four contestants.

Author : P. Rajesh Bhat  – Copyright : EFY

Automatic Battery Charger

Normally, chargers available in the market do not have any sort of control except for a ro-tary switch that can select different tap-pings on a rheostat, to vary the charging current. This type of control is not adequate because of the irregular fluctuations in the mains supply, rendering the control ineffective.  A simple circuit intended for automatic charging of lead-acid batteries is presented here. It is flexible enough to be used for large capacity inverter batteries. Only the rating of transformer and power transistor needs to be increased.

Circuit diagram :

Automatic Battery Charger Circuit Diagram

Automatic Battery Charger Circuit Diagram

The circuit has been basically designed for a car battery (about 40 Ah rating), which could be used for lighting two 40W tube lights. The circuit includes Schmitt trigger relay driver,float charger,and battery voltage monitor sections.  The Schmitt trigger is incorporated to avoid relay chattering. It is designed for a window of about 1V. During charging, when the battery voltage increases be-yond 13.64V, the relay cuts off and the float charging section continues to work. When battery voltage goes below 11.66V, the relay is turned on and direct (fast) charging of the battery takes place at around 3A.  In the Schmitt trigger circuit, resistors R1 and R2 are used as a simple voltage divider (divide-by-2) to provide battery voltage sample to the inverting input terminal of IC1. The non-invert-ing input terminal of IC1 is used for reference input derived from the output of IC2 (7806), using the potentiometer arrangement of resistors R3 (18 kilo-ohm) and R4 (1 kilo-ohm).

LED1 is connected across relay to indicate fast charging mode. Diodes D3 and D6 in the common leads of IC2 and IC3 respectively provide added protecion to the regulators.  The float charging section, comprising regulator 7812, transistors T3 and T4, and few other discrete components, becomes active when the battery volt-age goes above 13.64V (such that the relay RL1 is deenergised). In the energised state of the relay, the emitter and collector of transistor T4 remain shorted, and hence the float charger is ineffective and direct charging of battery takes place.

The reference terminal of regulator (IC3) is kept at 3.9V using LED2, LED3, and diode D6 in the common lead of IC3 to obtain the required regulated output (15.9V), in excess of its rated output, which is needed for proper operation of the circuit. This output voltage is fed to the base of transistor T3 (BC548), which along with transistor T4 (2N3055) forms a Darlington pair. You get 14.5V output at the emitter of transistor T4, but because of a drop in diode D7 you effectively get 13.8V at the positive terminal of the battery. When Schmitt trigger switches ‘on’ relay RL1, charging is at high current rate (boost mode). The fast charging path, starting from transformer X2, comprises diode D5, N/O contacts of relay RL1, and diode D7.

The circuit built around IC4 and IC5 is the voltage monitoring section that provides visual display of battery voltage level in bar graph like fashion. Regulator 7805 is used for generating reference voltage. Preset VR1 (20 kilo-ohm) can be used to adjust voltage levels as indicated in the circuit. Here also a pot meter arrangement using resistors R7, R8, and R9 is used as ‘divide by 3’ circuit to sample the battery voltage. When voltage is below 10V, the buzzer sounds to indicate that the safe dis-charge limit has been exceeded.

Author : Yash Deep - Copyright : EFY Mag

Simple Switch Mode Power Supply

The SMPS described here is suit-able for high-wattage stereos and other similar equipment. The circuit employs two high-voltage power transistors (BU208D) which have built-in re-verse-connected di-odes across their collectors and emitters. It can supply about 250-watt out-put. The circuit uses a ferrite core transformer of 14mm width, 20mm height, and 42mm length of E-E cores. An air gap of 0.5 mm is required between E-E junction. Good insulation using plastic-insulating sheets (Mylar) is to be maintained between each layer of winding.

Circuit diagram:

Simple Switch Mode Power Supply Circuit Diagram

Simple Switch Mode Power Supply Circuit Diagram

The number of primary turns required is 90 with 26 SWG wire. The secondary winding employs 17 SWG wire (for 4A load current). Each turn of the secondary develops approximately 2 volts. The reader can decide about the output volt-age and the corresponding secondary turns, which would work out to be half the desired secondary voltage. The volt-age rating of capacitors C7 and C8 should be at least twice the secondary output of each secondary section. BY396 rectifier diodes shown on the secondary side can be used for a maximum load current of 3 amperes.

Two feedback windings (L1 and L2) using two turns each of 19 SWG wire are wound on the same core. These windings are connected to transistors T1 and T2 with a phase difference of 180o,  as shown by the polarity dots in the figure. First wind the primary winding (90 turns using 26 SWG wire) on the former. Then wind the two feedback windings over the secondary (output). Ensure that each winding is separated by an insulation layer.  Two separate heat sinks are to be pro-vided for the two transistors (BU208D).

The filter capacitor for mains should be of at least 47µF, 350V rating. It is better to use a 100µF, 350V capacitor. If the output is short-circuited by less than 8-ohm load, the SMPS would automatically turn off because of the absence of base current.  The hfe min (current amplification factor) of BU208D is 2.5. Thus, sufficien base current is required for fully satu rated operation, otherwise the transistors get over-heated. At times, due to use of very high value of capacitors C7 and C8 (say 2200mF or so) on the secondary side or due to low load, the oscillations may cease on the primary side. This can be rectified by increasing the value of capacitor C6 to 0.01mF.

Author :  Deepu P.A.- Copyright : Electronic for you

Radio Band Position Display

This circuit is an add-on unit for radio receivers that lack band-position display. The circuit presented here can show up to nine bands. It also incorporates a novel feature to make the display dance (blink) with the audio level from the receiver. The power-supply for the circuit can also be derived from the radio-set. The conversion of selected channel to BCD format is achieved using diodes D1 through D15 in con-junction with resistors R4 to R7. The voltages developed across these resistors (R4 through R7) serve as logic in-puts to BCD inputs of BCD to 7-segment de-coder IC1 (CD4511).

Circuit diagram :

Radio Band Position Display Circuit Diagram

Radio Band Position Display Circuit Diagram

When all switches are in  ‘off’ state, the volt-age across resistors R4 through R7 is logic zero, but when any of the switches S1 through S9 is slided to  ‘on’ position, the output across these resistors changes to output proper BCD code to represent the selected channel. This BCD code is converted to 7-segment display by IC1. By this arrangement of diodes, the need for another decimal-to-BCD converter IC and associated parts is obviated. Switches S1 through S9 are actually parts of existing band-switch of the radio.

Usually, one or two changeover contacts would be found extra in the modular pushbutton-type band-switches of the radios. IC1’s display blanking pin 4 is connected to a display-blinker-control circuit wired around transistors T1 and T2. A small part of the audio signal from the speaker terminals is applied to rectifier diode D16 and filter capacitor C1 to pro-duce a pulsating DC across preset VR1. The sliding contact of preset VR1 is connected to the base of emitter-follower stage comprising transistor T2. The out-put of transistor T2, as amplified by transistor T1, is connected to pin 4 of IC1.Thus turning  ‘on’/‘off’ of display is con-trolled by the pulsating voltage developed from audio output of radio.

The power-supply regulator stage is needed only when radio power-supply is greater than 6V DC.

Author : M.K. Chandra Mouleeswaran Copyright : Electronic for you 2000

Inverter Overload Protector With Delayed Auto Rest

An overload condition in an inverter may  permanently  damage  the  power transistor array or burn off the transformer. Some of the domestic inverters sold in the market do not feature an overload shutdown facility, while those incorporating this feature come with a price tag.the circuit presented here is an overload detector which shuts down the inverter  in  an  overload  condition. 

Circuit diagram:

Inverter Overload Protector With Delayed Auto Rest -Circuit -Diagram

Inverter Overload Protector With Delayed Auto Rest Circuit Diagram

It  hasthe following desirable features:

  • It shuts down the inverter and also provides  audio-visual  indication  of  the overload condition.
  • after  shutdown,  it  automatically restarts  the  inverter  with  a  delay  of  6 seconds. thus, it saves the user from the inconvenience  caused  due  to  manually resetting the system or running around in darkness to reset the system at night.
  • It  permanently  shuts  down  the inverter  and  continues  to  give  audio warning,  in  case  there  are  more  than three  successive  overloads.  Under  this condition, the system has to be manually reset.(Successive overload condition indicates that the inverter  output  is  short-circuited or a heavy current is being drawn by the connected load.)

Inverter Overload Protector

Inverter Overload Protector With Delayed Auto Rest

The circuit uses an ammeter  (0-30a)  as  a  transducer  to  detect  overload condition.  Such  an  am-meter  is  generally  present in  almost  all  inverters.  this  ammeter  is connected between the negative supply of the battery and the inverter, as shown in Fig. 2. the voltage developed across this ammeter, due to the flow of current, is very small. It is amplified by IC2, which is wired as a differential amplifier having a gain  of 100. IC3 (NE555) is connected as a Schmitt ‘trigger’, whose output goes low when the voltage at its pin 2 exceeds 3.3V. IC4 (again an NE555 timer) is configured as  a  monostable  multivibrator  with  a pulsewidth of 6 seconds. IC5 (CD4017) is a CMOS counter which counts the three overload  conditions,  after  which  the  sys-tem has to be reset manually, by pressing push-to-on switch S1. the  circuit  can  be  powered  from  the inverter battery. In standby condition, it consumes 8-10 ma of current and around 70 mA with relay (RL1), buzzer (PZ1), and LED1 energised.

Please note the following points carefully:

  • Points A and B at the input of IC2 should be connected to the corresponding points (A and B respectively) across the ammeter.
  • Points C and D on the relay terminals  have  to  be  connected  in  series  with the  already  existing  ‘on’/‘off’  switch  leads of inverter as shown in Fig. 1. this means that one of the two leads terminated on the existing  switch  has  to  be  cut  and  the  cut ends have to be connected to the pole and N/O contacts respectively of relay RL1.
  • The  ammeter  should  be  connected in series with the negative terminal of the battery and inverter, as shown in Fig. 2.Move the wiper of preset VR1 to the extreme position which is grounded. Switch ‘on’ the inverter. For a 300W inverter, connect about 250-260W of load. Now adjust VR1 slowly, until the inverter just trips or shuts down.  repeat the step if necessary. Use good-quality preset with dust cover (e.g. multiturn trimpot) for reliable operation.the circuit can be easily and success-fully installed with minimum modifications to the existing inverter. all the components used are cheap and readily avail-able. the whole circuit can be assembled on a general-purpose PCB. The cost of the whole circuit including relay, buzzer, and PCB does not exceed Rs 100.

Author : Siddharth Singh - Copyright : EFY Mag

Power Supply Failure Alarm

Most of the power supply failure indicator circuits need a separate power-supply for them-selves. But the alarm circuit presented here needs no additional supply source. It employs an electrolytic capacitor to store adequate charge, to feed power to the alarm circuit which sounds an alarm for a reasonable duration when the mains supply fails. During the presence of mains power supply, the rectified mains voltage is stepped down to a required low level.

Circuit diagram :

Power-Supply-Failure-Alarm Circuit Diagram

Power Supply Failure Alarm Circuit Diagram


A zener is used to limit the filtered voltage to 15-volt level. Mains presence is indicated by an LED. The low-level DC is used for charging capacitor C3 and reverse biasing switching transistor T1. Thus, transistor T1 remains cut-off as long as the mains supply is present. As soon as the mains power fails, the charge stored in the capacitor acts as a power-supply source for transistor T1. Since, in the absence of mains supply, the base of transistor is pulled ‘low’ via resistor R8, it conducts and sounds the buzzer (alarm) to give a warning of the power-failure.

With the value of C3 as shown, a good-quality buzzer would sound for about a minute. By increasing or decreasing the value of capacitor C3, this time can be altered to serve one’s need. Assembly is quite easy. The values of the components are not critical. If the alarm circuit is powered from any external DC power-supply source, the mains supply section up to points ‘P’ and ‘M’can be omitted from the circuit.

Following points may be noted:

1. At a higher  DC voltage level, transistor T1 (BC558) may pass some collector-to-emitter leakage current, causing a continuous murmuring sound from the buzzer. In that case, replace it with some low-gain transistor.

2. Piezo buzzer must be a continuous tone version, with built-in oscillator. To save space, one may use five small-sized 1000µF capacitors (in parallel) in place of bulky high-value capacitor C3.

Author : M.K. Chandra Mouleeswaran Copyright: EFY Mag

Emergency Light

The circuit of emergency light presented here is unique in the sense that it is automatic, com-pact, reliable, low-cost, and easy to assemble for anyone. The circuit consists of four sections, namely, battery charging section, inverter section, changeover section, and low battery voltage indication section. In the battery charging section, 230V AC mains is converted to 9V AC using step-down transformer X1. The diodes D1 and D2 from a full-wave rectifier, and capacitor C1 filters the rectified voltage.

Emergency-Light-Circuit-Daigram

The output of filter is about 12V DC, which is connected to the collector of transistor T1 provides a fixed bias of 8.2V. Thus, transistor T1 works as a regulator and provides a constant voltage for charging the lead-acid battery. LED1 indicates the charging of battery. The inverter section comprises transformer X2, transistor T2, capacitor C2 and resistor R3. Transformer X2 is ferrite core type. Its winding details are shown in Fig. 2.

Emergency-Light-Circuit-Daigram 2

While core details are shown in Fig. 3. Resistor R3 pro-vides DC bias to the base of transistor T2, while capacitor C2 couples the positive AC feed-back from winding L1 to the base of transistor T2 to sustain the oscillations. The AC power developed across primary winding L2 is transferred to secondary winding L3, which ultimately lights up the fluorescent tubes.

Emergency-Light-Circuit-Daigram 3

The changeover section uses diodes D3 and D4 as an automatic switch. In the presence of AC mains supply, diode D3 keeps transistor T2 in its cut-off state, while diode D4 provides DC path for charging of the battery. But, in the absence of AC mains supply, diode D4 is reverse biased and acts as an ‘off’ switch, inhibiting the conduction of diode D3, which allows normal functioning of transistor T2. The inverter can be switched ‘off’,when not required, by using ‘on/off’ switch S1.

Low battery voltage indicator circuit comprises transistor T3, senser diode D6, LED 2, variable resistor VR1, and resistors R4 through R6. The low battery indication can be adjusted from 4.7V to 5V by using variable resistor VR1. When the battery voltage is above 4.7V, zener diode D6 comes out of conduction, keeping transistor T3 at cut-off level. At the same time,LED2 gives the indication of low battery voltage.

The whole circuit can be assembled in a cabinet of emergency light suitably.

Author : Rajesh Kamboj - Copyright: EFY