Parallel Telephone With Secrecy And Call Prevention

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.

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 handset 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 don’t 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 confusion, a bridge rectifier can be used at the input of the circuit.

This circuit costs around Rs 80.

PROTECTION FOR YOUR ELECTRICAL APPLIANCES

Here is a very low-cost circuit to save your electrically operated appliances, such as TV, tape recorder, refrigerator, and other instruments during sudden tripping and resumption of mains supply. Appliances like refrigerators and air-conditioners are more prone to damage due to suchconditions.

The simple circuit given here switches off the mains supply to the load as soon as the power trips. The supply can be resumed only by manual intervention. Thus, the supply may be switched on only after it has stabilised.

The circuit comprises a step-down transformer followed by a full-wave rectifier and smoothing capacitor C1 which acts as a supply source for relay RL1. Initially, when the circuit is switched on, the power supply path to the stepdown transformer X1 as well as the load is incomplete, as the relay is in de-energised state. To energise the relay, press switch S1 for a short duration. This completes the path for the supply to transformer X1 as also the load via closed contacts of switch S1. Meanwhile, the supply to relay becomes available and it gets energised to provide a parallel path for the supply to the transformer as well as the load.

If there is any interruption in the power supply, the supply to the transformer is not available and the relay de-energises. Thus, once the supply is interrupted even for a brief period, the relay is de-energised and you have to press switch S1 momentarily (when the supply resumes) to make it available to the load.

Very-short-duration (say, 1 to 5 milliseconds) interruptions or fluctuations will  not affect the circuit because of presence of largevalue capacitor which has to discharge via therelay coil. Thus the circuit provides suitable safety against erratic power supply conditions.

OVER- / UNDER-VOLTAGE PROTECTION OF ELECTRICAL APPLIANCES

This circuit protects refrigerators as well as other appliances from over and under-voltage. Operational amplifier IC LM324 (IC2) is used here as a comparator. IC LM324 consists of four operational amplifiers, of which only two operational amplifiers (N1 and N2) are used in the circuit.

The unregulated power supply is connected to the series combination of resistors R1 and R2 and potmeter VR1. The same supply is also connected to a 6.8V zener diode (ZD1) through resistor R3.Preset VR1 is adjusted such that for the normal supply of 180V to 240V, the voltage at the non-inverting terminal (pin 3) of operational amplifier N1 is less than 6.8V. Hence the output of the operational amplifier is zero and transistor T1 remains off. The relay, which is connected to the collector of transistor T1, also remains de energised. As the AC supply to the electrical appliances is given through the normally closed (N/C) terminal of the relay, the supply is not disconnected during normal operation.

When the AC voltage increases beyond  240V, the voltage at the non-inverting terminal (pin 3) of operational amplifier N1 increases. The voltage at the inverting terminal is still 6.8V because of the zener diode. Thus now if the voltage at pin 3 of the operational amplifier is higher than 6.8V, the output of the operational amplifier goes high to drive transistor T1 and hence energise relay RL. Consequently, the AC supply is disconnected and electrical appliances turn off. Thus the appliances are protected against over-voltage. Thus the appliances are protected against over-voltage.

Now let’s consider the under-voltage condition. When the line voltage is below 180V, the voltage at the inverting terminal (pin 6) of operational amplifier N2 is less than the voltage at the non-inverting terminal (6V). Thus the output of operational amplifier N2 goes high and it energises the relay through transistor T1. The AC supply is disconnected and electrical appliances turn off. Thus the appliances are protected against under-voltage. IC1 is wired for a regulated 12V supply.
Thus the relay energises in two conditions: first, if the voltage at pin 3 of IC2 is above 6.8V, and second, if the voltage at pin 6 of IC2 is below 6V. Over-voltage and under-voltage levels can be adjusted using presets VR1 and VR2, respectively.

Earth Fault Indicator

In mains-powered equipment, exposed metal parts are connected to earth wire in order to prevent users from contact with high voltages if electrical insulation fails. Connections to ground through earth connection also limit the build-up of static electricity when handling electrostatic-sensitive devices. Earth in a mains electrical wiring system is a conductor that provides a low-impedance path to the earth to prevent hazardous voltages from appearing on equipment and hence the name.

Here is a simple tester to find whether the mains wiring is correct or not. This tester can be used to check sockets periodically before connecting appliances like heater and electric iron. It indicates the status of the mains wiring through two LEDs as shown in Fig. 1.

Fig. 1: Author’s prototype

Circuit and working
Fig. 2 shows the circuit of earth fault indicator. The circuit is built around a BC547 transistor (T1), two LEDs (LED1 and LED2), two 1N4007 diodes (D1 and D2) and five resistors (R1 through R5).

Fig. 2: Circuit of earth fault detector

The circuit takes advantage of the voltage that appears across the earth and neutral terminals. Neutral-to-earth voltage as measured at the load for a single-phase circuit is a function of the load current and the impedance of the neutral wire. Various standards limit this voltage drop in a branch circuit to 3 per cent (5 per cent total for feeder and branch circuit) for a reasonable efficiency of operation. Based on this, the neutral-to-earth voltage limit for a single-phase 120V AC circuit is 3.6V AC and for a single-phase 230V AC circuit 6.6V AC.

There is no additional power supply used to operate this circuit. The circuit is directly powered from the 230V AC mains supply. The combination of diode D1 and resistor R1 reduces the 230V AC mains to a low voltage for the circuit. A transistor switch is provided to light up the green LED (LED1) if earth is correctly connected. The base of T1 is connected to earth pin of the mains supply through a network of resistors R2 and R3 as shown in Fig. 2.
Working of the circuit is simple. The red LED (LED2) lights up if there is power in the socket and phase (L) and neutral (N) lines are connected correctly. Diode D2 protects the green LED (LED1) from damage when the polarity changes. Resistors R4 and R5 limit current through LED1 and LED2, respectively. 

When the mains wiring is proper, a potential difference develops between the neutral (N) and earth (E) lines and transistor T1 turns on to light up the green LED (LED1). This indicates that the earth connection is perfect. At the same time, the red LED also glows, indicating that the phase (L) and neutral (N) lines are connected properly. In brief, if the phase, neutral and earth connections are proper, both the red and green LEDs light up. When the earth connection is broken, the red LED2 glows but green LED1 does not.

Construction and testing
An actual-size, single-side PCB for the earth fault indicator is shown in Fig. 3 and its component layout in Fig. 4. After assembling the circuit on a PCB, enclose it in a suitable plastic case. The circuit can be housed in an adaptor box with three pins (see Fig. 1). 

Fig. 3: An actual-size, single-side PCB for the earth fault indicator

Fig. 4: Component layout for the PCB

http://www.electronicsforu.com/electronicsforu/circuitarchives/my_documents/my_files/142_EarthFaultIndicator.zip

To test the circuit for proper functioning, check input supply at TP1 with respect to TP0. Also check the voltage difference across neutral and earth pins as per the test point table.

Over-Voltage Protector

This circuit protects your television as well as other electrical appliances from over-voltage. It uses operational amplifier µA741 (IC1) as a comparator. The unregulated power supply is connected to resistor R3 and preset VR1 through resistor R2. Zener diode ZD1 provides reference voltage of 5.1V to the inverting input (pin 2) of IC1.
The non-inverting input (pin 3) of IC1 senses voltage fluctuation in the mains. Preset VR1 is adjusted such that for mains supply below 240V AC, the voltage at the non-inverting terminal of IC1 is less than 5.1V. Hence the output of IC1 is zero and transistor T1 is in non-conducting state. At the same time transistor T2 conducts to energise relay RL1 to connect the mains to the load.

When AC mains is beyond 240V, the voltage at pin 3 of IC1 goes above 5.1V. The high output of IC1 drives transistor T1 and transistor T2 stops conducting to de-energise the relay. Hence the appliance turns off.

Preset VR2 is used for proper biasing of transistor T1. The AC mains supply is stepped down by transformer X1 to deliver a secondary output of 7.5V-0-7.5V AC, 1A. The output of the transformer is rectified by a full-wave rectifier comprising diodes D1 through D4. Capacitors C1 and C2 act as filters to eliminate ripples. Regulator IC 7812 is used to provide regulated 12V supply.

Automatic Headlights Switcher

Automatic headlamps are the latest convenience in today's cars. These eliminate the need for the driver to manually switch on or switch off the headlamps in most driving situations. The automatic headlight system reacts like the human eye to outside light levels and independently turns the lights on and off when needed. Such a system offers both safety and convenience.

This circuit can be particularly helpful when driving on roads with many tunnels, at twilight or sunset, and even in foggy, icy, stormy and rainy conditions. For example, when the car enters a dark tunnel, the driver will not have to fumble for the headlights switch. The car’s headlights will automatically switch on after sensing the poorly lit tunnel. When the car comes out of the tunnel, the headlights will switch off.
The circuit is built around timer NE555 (IC1), light-dependent resistor LDR1 and some discrete components. Potmeter VR1 is used to set the light sensitivity of LDR1. On sensing the darkness, LDR1 turns the headlights ‘on’.

Basically, an LDR is a resistor whose resistance decreases with increase in the intensity of the incident light. Usually, an LDR exhibits very high resistance in darkness and low resistance in the presence of ambient light. Thus a varying voltage drop can be obtained across it with changing ambient light conditions.

The LDR1 is connected to the trigger input (pin 2) of IC1.The output of IC1 is connected to the base of relay-driver transistor T1. The 12V supply voltage is connected to the circuit through switch S1. LDR1 and the 100-kilo-ohm preset constitute a voltage divider arrangement at pin 2 of IC1.

Working of the circuit is simple. Enable the circuit using switch S1. When there is sufficient ambient light, the resistance of LDR1 remains low (a few hundred ohms). The voltage at pin 2 is greater than two-third of 12V. The output at pin 3 of IC1 remains low—stable state for monostable mode of operation—and the headlights of the vehicle connected to the normally-open (N/O) contacts of relay RL1 remain off.

When the ambient light decreases, the resistance of LDR1 shoots up to a few mega-ohms and the voltage at the trigger input (pin 2) of IC1 decreases to less than one-third of 12V. The output at pin 3 of IC1 goes high to energise relay RL1 and turn the headlights ‘on’. Switch S2 can be used to manually operate the headlights.

Assemble the circuit on a general-purpose PCB and enclose in a small suitable cabinet such that the LDR sensor receives ambient light. Connect power supply switch S1 on the rear side of the cabinet to connect/disconnect the 12V car battery. Connect potmeter VR1 at the front side of the cabinet for varying the sensitivity to light as desired. Now your headlight circuit is ready for use.

Battery-Low Indicator

Rechargeable batteries should not be discharged below a certain voltage level. This lower voltage limit depends upon the type of the battery. This simple circuit can be used for 12V batteries to give an indication of the battery voltage falling below the preset value. The indication is in the form of a flickering LED.

At the heart of the circuit is voltage comparator IC LM319 (IC1). It is a dual comparator with a TTL-compatible output. We have used only one comparator here. A reference voltage of 1.2 volts generated by band-gap reference diode D1 (LM385) is applied to the non-inverting input (pin 4) of the comparator. The inverting input (pin 5) of the comparator is fed a voltage generated from the potential divider arrangement built around resistors R2 and R3 and preset VR1. That means, if you are using a 12V battery and want an indication as soon as the battery voltage goes below 10.5V, adjust the voltage at the inverting input using reset VR1 so as to get a voltage of 1.2 volts (with battery voltage at 10.5V).

Initially, when the battery is fully charged, the voltage at the inverting input of IC1 is higher than the non-inverting input and output pin 12 of IC1 remains low. The reset pin (pin 4) of IC2 connected to pin 12 of IC1 also remains low and the astable multivibrator built around IC2 does not oscillate. As a result, LED1 does not flicker.

When the battery voltage falls below 10.5V, the voltage at the inverting input of IC1 becomes lower than the non-inverting input and the output of IC1 goes high. The reset pin of IC2 connected to pin 12 of IC1 also goes high and the astable multivibrator built around IC2 starts oscillating. LED1 flickers to indicate that the battery voltage is low and the battery needs to be charged before further use. Both IC1 and IC2 operate off regulated +5V DC generated by voltage regulator IC 7805 (IC3).

Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. Mount LED1 and switch S1 on the front side of the case. Connect a 12V battery to check its voltage level. 

Night Lamp From Scrap CFL

Compact fluorescent lamps (CFLs) are available in different shapes and power ratings. These consist of an electronic ballast circuit and a programmable-logic-controlled tubelight. Most CFL manufacturers offer a guarantee of half a year on their product. 

In an unserviceable CFL, the filament has reached the end of its life but there is every possibility that the electronic ballast circuit inside the bottom of the CFL is in working condition. The night lamp circuit described here uses the serviceable electronic circuit fitted in the base of an 11-watt CFL.

Fig. 1: An efficient night lamp circuit

For constructing this night lamp, remove the CFL glass tube and replace it with four white LEDs as described below. You should be careful not to break the tube as it contains hazardous materials such as mercury. Carefully open the base of CFL holder using an appropriate tool. You can see the electronic ballast circuit on a circular PCB.

Fig. 2: LEDs arrangement in waste CFL

Use the components from the ballast circuit and a series combination of four bright white LEDs as shown in Fig. 1. Remove all other components from the original ballast circuit. As per the requirement of light intensity in your room, you can increase the number of white LEDs up to eight.

As shown in Fig. 1, the full-wave bridge rectifier comprising diodes D1 through D4 converts AC voltage into DC voltage. Snubber capacitor C1 at the input reduces the line input voltage of 230V to a very low-level AC voltage. Series current-limiting resistor R2 and series inductor coil L1 avoid voltage spikes. 

Emergency-Cum-Ultraviolet Light

This circuit serves both as an emergency light (using a 6W fluorescent tube) and an ultraviolet light (using a 6W ultraviolet tube).

Step-down transformer X1, diodes D1 through D4, capacitor C1 and 6V regulator IC1 form the regulated power supply. The series combination of diodes D5 through D7 connected to common pin of IC1 increases the output voltage to 7.8 volts. This regulated voltage is applied through resistor R1 and diode D8 to charge the battery.

When mains power is available, the battery starts charging and LED2 glows. However, glowing of tubes depends on the switch positions as follows:
1. If both switches S2 and S3 are closed, transistor T2 conducts and provides ground path to the base of transistor T1. Thus the inverter does not oscillate and L1 (for emergency light) and L2 (for ultraviolet light) tubes don’t  glow.
2. If switch S2 is closed and switch S3 is open, transistor T2 does not conduct and hence transistor T1 oscillates. Thus the inverter oscillates and L1 and L2 tubes glow.
When mains supply fails and inverter switch S2 is closed, irrespective of the position of switch S3, transistor T1 starts oscillating and inverter transformer X2 provides output voltage at its secondary, which causes glowing of tube L1 or L2 depending on the position of switch S1.

Switch S2 is used to switch off the emergency light or ultraviolet tube when it is not in use. Switch S1 is  used to connect L1 or L2 tube according to your requirement. Glowing of LED1 indicates that the inverter is ‘on,’ while glowing of LED2 indicates that mains is available.

Ultraviolet light can be used for counterfeit currency detection. The fluorescent fibres of the proper currency notes clearly reflect when the currency note is illuminated by ultraviolet light.

Warning: UV light is very dangerous to the eyes and causes damage to the skin cells. So avoid direct exposure and looking directly into a UV light source.