This emergency light has the following two advantages:
1. It turns on automatically when the mains power fails, so you need not search it in the dark.
2. Its battery starts charging as soon as the mains resumes.
Operation of the circuit is quite straightforward. Mains supply is stepped down by transformer X1, rectified by a full-wave rectifier comprising diodes D1 and D2, filtered by capacitor C1 and fed to relay coil RL1. The relay energises to connect the bat-tery to the charging circuit through its normally-opened (N/O) contacts. Free-wheeling diode D3 acts as a spike buster for the relay.
The charging circuit is built around npn transistor BD139 (T1). The trans-former output is fed to the collector of transistor T1, which provides a fixed bias voltage of 6.8V to charge the battery. When the battery is fully charged, the battery voltage becomes equal to the breakdown voltage of the zener diode (ZD1). Zener diode ZD1 conducts to provide an alternative path for the current to ground and battery charging stops.
When mains fails, relay RL1 de-energizes. The battery now gets connected to the white LED array (comprising LED1 through LED6) through current-limiting resistor R2. The LEDs glow to light up the room. To increase the brightness in your room, you can increase the number of white LEDs after reducing the value of resistor R2 and also use a reflector assembly.
Just don’t think that this is yet another addition to other emergency light circuits published in this site earlier. This circuit is a hit different. Its main features are:
1. Very reliable operation.
2. As transformer is not used, it is compact and cost-effective.
3. The torch bulb glows automatically at power off and goes out on restoration of power.
4. Since Ni-Cd battery is used, no maintenance is required. Also, battery life is very long, nearly 4-5 years (though this depends on frequency of usage and also on ampere-hour rating of the battery used).
Sounds interesting, doesn’t it? Read on then. The circuit is very simple, comprising just a handful of components. This implies that the circuit operation also is very simple. The circuit consists of two parts:
1. Power supply for charging the Ni-Cd battery.
2. Switchover circuit which detects mains failure and switches the bulb ‘on’.
In the power supply section, capacitors C1 and C2 function as non-dissipating, re-active impedance which limit the current to a safe value. With the values of capacitors as shown, the maximum current that can be drawn is limited to about 70 mA at 230V AC. Resistor R2 limits the initial surge current and resistor R1 assists in discharging the capacitors after switch off. Diodes D1 through D4 form a conventional bridge rectifier while capacitor C3 is the filter capacitor. Fuse F1 is for protection and is very helpful in the event of any component giving up the ghost. This supply charges the battery as long as mains is present.
In the ‘switchover’ section, transistor T1 is used as switch. Normally, when AC mains supply is present, the rectifer output charges the battery through resistor R4 and LED D5 combination at about 50mA rate. The glowing LED (D5) also gives an indication of mains presence. Further, due to the LED (D5), base of transistor T1 is about 1.6V (drop across D5) more positive than its emitter. This voltage is more than suffcient to keep the transistor at cut-off.
As soon as the mains voltage fails, the base of transistor T1 is pulled low through resistor R3 which drives transistor T1 to saturation thereby turning the bulb ‘on’. Since the transistor is in its saturated state, the voltage drop across it is very low. Hence the bulb glows with full brilliance. The bulb can be switched off by the ON/OFF switch, when not required. With this bulb (2.2V, 250mA) the torch can work continuously for about two hours. The batteries should be charged for about 14 hours after they are discharged. You can verify following voltages in the circuit:
1. Base voltage of the transistor must be 1.8V to 2.0V, i.e. about 0.6V less than the battery voltage.
2. Emitter voltage must be equal to the battery voltage. 3. Collector voltage must be 2.0V to 2.2V, i.e. nearly equal to the battery voltage.
All above voltages should be checked with AC mains off. If any of the above mentioned voltages is absent it indicates that the transistor is bad and it should be replaced by a good one.
Here is a word of caution now. Since the circuit is not isolated from AC mains. it may be hazardous to touch any component when the mains supply is on, especially if the supply wires (live and neutral) get interchanged. It is strongly recommended to use an all-plastic enclosure (including the reflector for the bulb) for the circuit. Also the ON/OFF switch used should have a plastic lever. Take proper care and precautions while building, testing and using the circuit, and never ever allow the supply wires to interchange. It is advisable to provide a plug for the mains input on the box itself so that it can be plugged directly into a mains outlet. This reduces the chances of mains supply wires getting interchanged.
With proper precautions and a little care, it is hoped that this small circuit will help make life a bit more comfortable.
Using the circuit of direct-conversion receiver described here, one can listen to amateur radio QSO signals in CW as well as in SSB mode in the 40-metre band.
The circuit makes use of three n-channel FETs (BFW10). The first FET (T1) performs the function of ant./RF amplifer-cum-product detector, while the second and third FETs (T2 and T3) together form a VFO (variable frequency oscillator) whose output is injected into the gate of frst FET (T1) through 10pF capacitor C16. The VFO is tuned to a frequency which differs from the incoming CW signal frequency by about 1 kHz to produce a beat frequency note in the audio range at the output of transformer X1, which is an audio driver transformer of the type used in transistor radios.
The audio output from transformer X1 is connected to the input of audio amplifier built around IC1 (TBA820M) via volume control VR1. An audio output from the AF amplifier is connected to an 8-ohm, 1-watt speaker. The receiver can be powered by a 12-volt power-supply, capable of sourcing around 250mA current.
Audio-output stage can be substituted with a ready made L-plate audio output circuit used in transistor amplifiers, if desired. The necessary data regarding the coils used in the circuit is given in the circuit diagram itself.
The circuit of a 7MHz CW/AM QRP transmitter described here can be used to transmit either CW or audio frequency modulated signal over a 7MHz carrier.
The carrier frequency oscillator is crystal controlled using 7MHz crystal in its fundamental mode. The tank circuit comprises a shortwave oscillator coil which can be tuned to 7MHz frequency with the help of ½J gang capacitor VC1.
Transistor T2 (with identical tank circuit connected at its collector as in case of transistor T1) serves as a power amplifer. The RF output from oscillator stage is inductively coupled to the power amplifer stage. The output from power amplifier is routed via capacitor C3 and inductor L3 to a half-wave dipole using a 75-ohm coaxial cable. ½J gang capacitor VC3 along with inductor L3 forms an antenna tuning and matching network between the output of power amplifier stage and coaxial transmis-sion line for maximum power transfer. Suitable heatsink should be used for transistor T2.
Tuning adjustments may be accomplished using a 6-volt torch bulb. Connect the bulb to the collector of transistor T1 frst through a coupling capacitor and tune ½J gang VC1 for maximum brilliance. (Note: the bulb would light according to intensity of RF energy.) Same procedure may be repeated for power amplifier stage and antenna tuning network for ensuring maximum power transfer. For CW operation, switch S1 is to be kept on for bypassing the audio driver transformer and Morse key is used for on/off-type modulation. CW would be generated during key depressions. For AF modulation, Morse key points should be closed and switch S1 should be fipped to ‘off’ position. Any suitable mic. amplifer may be used to feed audio input to the audio driver transformer X1. (For transformer X1 you may use the transistor-radio type AF driver transformer.)
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. Inchangeover mode, exchange line is disconnected from telephone 3 and gets connected to telephone 2.
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 ring-tone-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.
Here is a simple, versatile circuit which indicates the level of water in a tank. This circuit produces alarm when water level is below the lowest level L1 and also when water just touches the highest level L12. The circuit is designed to display 12 different levels. However, these display levels can be increased or decreased depending upon the level resolution required. This can be done by increasing or decreasing the number of level detector metal strips (L1 through L12) and their associated components.
In the circuit, diodes D1, D2 and D13 form half-wave rectifers. The rectifed output is fltered using capacitors C1 through C3 respectively.
Initially, when water level is below strip L1, the mains supply frequency oscillations are not transferred to diode D1. Thus its output is low and LED1 does not glow. Also, since base voltage of transistor T1 is low, it is in cut-off state and its collector voltage is high, which enables melody generating IC1 (UM66) and alarm is sounded.
When water just touches level detector strip L1, the supply frequency oscillations are transferred to diode D1. It rectifes the supply voltage and a positive DC voltage develops across capacitor C1, which lights up LED1. At the same time base voltage for transistor T1 becomes high, which makes it forward biased and its collector voltage falls to near-ground potential. This disables IC1 (UM66) and alarm is inhibited.
Depending upon quantity of water present in the tank, corresponding level indicating LEDs glow. It thus displays intermediate water levels in the tank in bar-graph style. When water in the tank just touches the highest level detector strip L12, the DC voltage is developed across capacitor C2. This enables melody generating IC1 (UM66) and alarm is again sounded.
In most houses, water is frst stored in an underground tank (UGT) and from there it is pumped up to the overhead tank (OHT) located on the roof. People generally switch on the pump when their taps go dry and switch off the pump when the overhead tank starts overflowing. This results in the unnecessary wastage and sometimes non-availability of water in the case of emergency.
The simple circuit presented here makes this system automatic, i.e. it switches on the pump when the water level in the overhead tank goes low and switches it off as soon as the water level reaches a pre-determined level. It also prevents ‘dry run’ of the pump in case the level in the underground tank goes below the suction level.
In the fgure, the common probes connecting the underground tank and the overhead tank to +9V supply are marked ‘C’. The other probe in underground tank, which is slightly above the ‘dry run’ level, is marked ‘S’. The low-level and high-level probes in the overhead tank are marked ‘L’ and ‘H’, respectively.
When there is enough water in the underground tank, probes C and S are connected through water. As a result, transistor T1 gets forward biased and starts conducting. This, in turn, switches transistor T2 on. Initially, when the overhead tank is empty, transistors T3 and T5 are in cut-off state and hence pnp transistors T4 and T6 get forward biased via resistors R5 and R6, respectively.
As all series-connected transistors T2, T4, and T6 are forward biased, they conduct to energise relay RL1 (which is also connected in series with transistors T2, T4, and T6). Thus the supply to the pump motor gets completed via the lower set of relay contacts (assuming that switch S2 is on) and the pump starts flling the overhead tank.
Once the relay has energised, transistor T6 is bypassed via the upper set of contacts of the relay. As soon as the water level touches probe L in the overhead tank, transistor T5 gets forward biased and starts conducting. This, in turn, reverse biases transistor T6, which then cuts off. But since transistor T6 is bypassed through the relay contacts, the pump continues to run. The level of water continues to rise.
When the water level touches probe H, transistor T3 gets forward biased and starts conducting. This causes reverse biasing of transistor T4 and it gets cut off. As a result, the relay de-energises and the pump stops. Transistors T4 and T6 will be turned on again only when the water level drops below the position of L probe.
Presets VR1, VR2, and VR3 are to be adjusted in such a way that transistors T1, T3, and T5 are turned on when the water level touches probe pairs C-S, C-H, and C-L, respectively. Resistor R4 ensures that transistor T2 is ‘off’ in the absence of any base voltage. Similarly, resistors R5 and R6 ensure that transistors T4 and T6 are ‘on’ in the absence of any base voltage. Switches S1 and S2 can be used to switch on and switch off, respectively, the pump manually.
You can make and install probes on your own as per the requirement and facilities available. However, we are describing here how the probes were made
for this prototype.
The author used a piece of non-metallic conduit pipe (generally used for domestic wiring) slightly longer than the depth of the overhead tank. The common wire C goes up to the end of the pipe through the conduit. The wire for probes L and H goes along with the conduit from the outside and enters the conduit through two small holes bored into it as shown in Fig. 2.
Care has to be taken to ensure that probes H and L do not touch wire C directly. Insulation of wires is to be removed from the points shown. The same arrangement can be followed for the underground tank also. To avoid any false triggering due to interference, a shielded wire may be used.
