Mar 31, 2013


Here is a circuit that helps you test cable continuity without requiring any physical contact with the bare cable. This circuit  detects AC signal frequencies and gives an LED indication if the cable is conducting. This circuit is highly sensitive and can detect signals from the surface of the cable itself and thus no direct contact with the bare cable is necessary. The circuit can be used to test other wires, including modem, audio/video and dish antenna cables to name a few.

Fig. 1: Circuit of cable tester
TP1- 9V
TP2- Amplified output corresponding to the input signal

Mar 7, 2013


Amplifier Circuit

The circuit described here is of a  Class-B audio amplifier based on operational amplifier TL082, transistors TIP41 and TIP42. LM833 is a dual operational amplifier with  very high slew rate and low noise distortion particularly designed for audio applications. This audio amplifier circuit can delivers upto 15 watt audio output into an 8 ohm speaker at +12/-12V DC dual supply. Both operational amplifiers in the IC(LM833) are used here. IC1a is connected as a buffer and capacitor C3 decouples the input. Ic1b is connected in the inverting mode and it provides negative feedback. Complementary power transistors TIP41 and TIP42 are connected in the Class B push pull scheme and they drives the loud speaker. Diode D1 provides 0.7V bias voltage for the push pull pair and capacitor C2 protects the 0.7V bias voltage across D1 from heavy voltage swings at the IC1b’s output.

 Key Points:

  1. Assemble the audio amplifier circuit on a good quality Board and Use a holder for every IC using here. Variable Resistor R2 can be used for controlling the volume.
  2. +12/-12V dual supply must be use for powering the amplifier. 
  3. TIP42 and 41 can supplied maximum of 6A.
  4. Maximum supply voltage for IC1 is +16/-16 V DC.

Feb 28, 2013


Digital Thermometer generally used in wide variety of scientific and engineering applications, especially measurement we present a highly reliable digital thermometer which can be used in aquarium and measuring temperatures around +85°C.This project was born from the need to easily check the water temperature at a glance. It features three 2.2” large LED digits that are easy to read from across the room and a precision DS1822 temperature sensor. Temperatures can be selected to display in Centigrade, as well as Fahrenheit.Of course, a digital thermometer with a large LED readout and a remote temperature probe is not just limited to aquarium owners. This project will appeal to anyone that wants accurate digital temperature measurements. Home beer brewers, hydroponic gardeners, amateur weather watchers, or folks just interested in energy management will all find this to be a very useful device.


“  This article describes a simple UPS circuit that you can incorporate into the design of your own low power DC project to ensure continued operation during short term power failures  “.
An uninterruptible power supply (UPS) ensures the continuous operation of critical electronic equipment.They are especially necessary if you live in an area where there are frequent power failures.They are manufactured to meet a wide range of power requirements, from backing up your personal computer to keeping your entire home office (or workshop) going during a power failure. Most UPS systems are designed to transparently maintain AC power to your equipment.They provide for a smooth transition from main power to backup power and back again. There are a number of applications that have relatively low power requirements and run on DC rather than AC voltage but must also remain operational in the event of a main power failure.These include small security sensor modules, data acquisition, and status monitoring devices among others.


Want to switch mains appliances on and off remotely? This UHF Remote Mains Switch can do it for you. It’s operated using a handheld UHF transmitter, and an in-built timer also enables the unit to turn off automatically after a preset period.
 There are many instances when it would be convenient to switch an appliance on or off remotely, rather than switching it manually. Such circumstances include switching on pathway lights when you arrive home, switching garden and/or pool lighting on or off, and switching power to water pumps. remote switching can also be very convenient for appliances that are difficult to access, eg, in a factory. This unit was originally designed to switch mains-powered water pumps on and off in response to signals transmitted by a water tank level meter base station. however, we soon realised that by adding a separate hand held transmitter to control the unit, it could also be used as a stand-alone unit for lots of other applications. Commercial remote control mains-operated switches are readily available for switching appliances rated up to about 1000W. however, if you want to switch devices rated over 1000W, or control water pumps, then you need the UhF remote Mains Switch described here. It can switch devices rated at up to 2500W over a range of up to 200m. That’s 10 times the range typically available from the low-cost commercial units!
Main Features

• Switches loads of up to 1875W (or 2500W using 10A mains wiring)
• Up to 10 units can be used with the transmitter, each with a separate
• 16 encoder selections
• Over 200m range
• Unit is operated using a separate handheld UHF transmitter
• On and off switching via remote transmitter or local switch
• Timer operates from one minute to four hours in 15 ranges,
plus a continuously on selection
• Brownout detection switching
• Optional power-on variation
• Not suitable for security or safety-critical applications.


SOME modern freezers contain alarms which sound if you leave the door open and allow the internal space to warm up. However, they do not work if the freezer suffers a power failure, which is a bit of a drawback. Making a temperature-sensitive circuit which can sound an alarm is not too difficult but what is required here is a lowcost circuit which can run on batteries for a very long time. This design uses a circuit based on a PIC, using a feature about which little has been written, namely the ability to send it to sleep! The circuit is extremely simple, and the software uses several techniques which could be useful in other projects.

Circuit Description
If you are the sort of person who enjoys the challenge of constructing complex circuits, you will be disappointed! The complete circuit contains only five components, as shown in Fig.2. The clever stuff, of course, is provided by the PIC. The temperature sensor used is a lowcost disc thermistor, R1, which can be attached via a length of 2-core cable. A small preset variable resistor, VR1 is used to set the operating point, the temperature threshold at which the alarm sounds. Capacitor C1 is used to make the input circuit time-dependant, as described in the next section. For the alarm, a piezo sounder (WD1) is used because it can make a relatively large amount of noise whilst using a very small amount of electrical power. The whole circuit will conveniently run off a 6V battery.
Construction is very simple. The suggested stripboard component layout and      track cut details are shown in Fig.10. The thermistor can be soldered to a short length of wire such as thin audio coax. An improvement would be to waterproof the thermistor connections by dunking it in polyurethane varnish. The wire can be fed into the freezer via the door seal. It is important to resist the temptation to add a light emitting diode as a battery indicator – the l.e.d. would take about a thousand times more power than the rest of the circuit! The PIC should be plugged into the board via an 8-pin d.i.l. socket. The circuit and batteries can be housed in a plastic box to sit outside the freezer, a small hole being provided to glue the piezo sounder behind. You should not need to replace batteries very often.
The circuit will work quite happily at room temperature. Once the batteries are connected (it seems to work well on 6V although this is higher than the maximum recommended). Gently rotate preset VR1 until the threshold is found between the alarm bleating or not. Set it so that the alarm is just off. Then hold the thermistor in your fingers to warm it up, and the alarm should sound; let go to allow the thermistor to cool again to room temperature, and the alarm should stop. Once you are convinced all is well, put the thermistor in the freezer, and after allowing time for the temperature to stabilise, increase the resistance on the preset so that the alarm threshold is set where you would like it. In fact, the best way to find out if the batteries are OK is to let the thermistor warm up a bit when you open the freezer – if it is working and the alarm sounds, the batteries are fine!


” It just takes 20 seconds to recharge !!! “

Here is an interesting project which uses capacitors to store energy instead of chemical,sit uses an different type of capacitors called Goldcap capacitors,GoldCap capacitors offer an interesting alternative power source when compared to conventional disposable or even rechargeable batteries. They can be charged very rapidly and can also deliver a high peak output current. Their voltage rating however is quite low so a little electronic assistance is necessary to raise the output voltage to a more useful level.PP3 (6F22) type 9 V batteries are often used in small portable equipment that require very little current and may only be used intermittently. Under these conditions its often the case that the battery is flat just when you urgently need to use the equipment. NiCd rechargeable cells are not a good choice in these applications because their self-discharge characteristics are much worse than dry cells and often there is no charge left after a long time in storage 


House plants in general often have a pretty hard time of it compared to their garden bound cousins, which seem to get more than their fair share of watering, even if their owner forgets, thanks to the British weather. With so many other things to think about, the first reminder that many people get to water their plants is when it is noticed that one or two are wilting or the leaves are turning brown and dropping off! Modern central heating also ensures that the soil in pots dries out much faster, making regular watering more important, so that a little electronic help in remembering to do so should be most welcome.
Circuit Diagram
The circuit suggested here, and shown in Fig.20, drives a piezo sounder, WD1, to
provide a timely warning that the soil in the plant pot is almost dry. Hopefully, the plants will be watered regularly so the alarm will remain off but it may become active at any time and it is unlikely that the plants will be watered immediately as the owner may be out. It may therefore continue to sound all day before the plants are watered. To avoid having to replace the battery too often, it is important to ensure that the current drain in either condition is as low as possible. To minimise the current drain during the alarm condition, a complementary astable circuit built around transistors TR2 and TR3 is used. Its operation is beyond the scope of this article, but it oscillates with a frequency determined by resistor R2 and capacitor C1. With the component values given the frequency will be about 2kHz, producing a fairly loud sound from piezo sounder WD1. This device has a very high impedance and so a load resistor, R3, is provided for TR3. Since both transistors switch on and off together and remain off for a relatively long period (dependant on the value of R2) compared to the time when they are on, the average current drawn from the battery is very low, at about 1mA. The output consists of short positive going pulses which turn on the piezo sounder WD1. The operation of the oscillator is controlled by TR1. When this transistor is on, the base of TR2 is held low and the circuit cannot oscillate. The circuit relies on sensing the resistance of the soil between two metal probes which are inserted into the pot close to the plant. Completely dry soil will have a relatively high resistance but this will fall as the moisture content is increased.
                  The series resistance of the probes, resistor R1 and potentiometer VR1 form a potential divider across the supply. With the soil moist, the resistance of VR1 can be adjusted so that the voltage at the base of TR1 is at 0·6V, ensuring that this transistor is switched on and so disabling the oscillator. As the soil dries out, the base-emitter voltage of TR1 falls to a point at which it switches off sufficiently to allow the oscillatorto function, producing an audible warning.
As described earlier, this circuit produces short output pulses and therefore draws only a small current when it is oscillating (about 1mA). In the stand-by condition when the oscillator is switched off, the current drain on the battery is only 10mA, so the battery should last a long time.
The circuit is built on a piece of stripboard having 7 strips × 15 holes, as shown in Fig.21. Only one strip cut is required and there are no link wires. Care should be taken to ensure that the transistors and the sounder are connected the correct way around. The probes consist of two stiff metal wires the length of which is not
particularly important and will depend to a large extent on the size of the pot into which the unit is placed. Copper is perhaps the easiest wire to get hold of (and to solder). In the prototype, two 10cm lengths of 2·5mm diameter rigid wire of the type used in house wiring were used. These were soldered directly to the tracks at the positionsshown, the wire being too thick to pass through the holes in the board.Since these are liable to break off if the probes are pushed into hard earth, it is probably best to solder the wires directly to the copper tracks straddling several holes. This may then be strengthened by covering the joints and an adjacent area of the board with epoxy glue. Alternatively, the wires may be mounted a few millimetres apart on an insulating surface, such asthe plastic box in which the unit is to be placed, and connected to the board by flying leads.
Soundless Alarm
When completed, place the probes in moist soil close to the roots of the plant. Set VR1’s wiper to a fully anti-clockwise position, and then adjust it until the circuit just fails to oscillate. Should the alarm sound as the soil dries out but it is still judged to be too moist to require watering, VR1 should be turned further clockwise. In some situations, an audible alarm may not be desirable, in which case the sounder can be omitted, and an l.e.d. plus ballast resistor of about 470 can be wired in place of R3, with the anode (a) connected to transistor TR3’s collector, and the other side of the 470ohm resistor on the 0V line. Omit R3 itself. Do not use the l.e.d. without the ballast resistor as the current through it cannot be guaranteed to be within its limits, even though the current is pulsed. The sounder and l.e.d. may both be fitted, although this will result in a slightly increased current consumption and a slightly reduced sound output, but should still be adequate for most situations.


There are many places around the house where a small light would be useful but running a mains cable to the location is impractical. Corners of dark cupboards, over the telephone to light a note pad, by the front door to help find the keyhole at night, are just some of the applications which come to mind. None of these require very much light and high brightness light emitting diodes (l.e.d.s) can not only provide the illumination needed, but can also be readily fitted with a time delay circuit so that they switch off automatically, so saving battery power. The circuit described here offers such a solution. It is shown in Fig.1 To ensure that the circuit switches off after use and prevent having to change the battery too often, a timing circuit is required. For this a monostable configuration is used. A monostable has one stable state, in this case the off state.
                              When triggered into its on state, it will remain in that state for a preset period before switching off again. Some circuits of this type use two transistors (npn or pnp types) configured so that in the stable state one transistor is on while the other remains off. Following a trigger pulse, both transistors change state. A disadvantage of this circuit is that during the off state, one of the transistors is always turned on, and so consuming power. An alternative configuration is used here in which all transistors remain off when the circuit is in its stable state, so consuming virtually no current.

Touch Circuit
In the circuit diagram shown in Fig.1, transistors TR2 and TR3 form the monostable circuit, with capacitor C1 and resistor R2 determining the time for which the transistors remain on once the circuit has been triggered. This occurs when finger contact is made with touch pad TP1. The 50Hz mains “hum” normally present in all households will be induced into the circuit through the finger, causing transistor TR1 to turn on. This provides base current to TR3, turning it on, together with the l.e.d. (D2), whose negative-going pulse is generated acrosscurrent is buffered by resistor R3.When the collector of TR3 goes low, a capacitor C1, causing TR2 to turn on and provide more current to the base of TR3. When the contact with TP1 is broken, TR1 ceases to conduct, but TR3’s base continues to be held on via TR2. However, C1 starts to charge via resistor R2. Eventually, its charge rises to within less than 0·6V of the positive power supply, turning off TR2 and thus TR3 and the l.e.d. as well. Diode D1 inhibits any positive-going pulse generated across C1 when TR2 switches off. With the component values shown, the l.e.d. will remain on for about three minutes.Touch Down
It is worth noting that touch pad TP2 may be needed if the 50Hz mains “hum” introduced by finger contact with TP1 is not strong enough, or non-existent, as in a garden shed for example. Making finger contact between TP1 and TP2 causes a
small current to flow from the positive ine, though the finger and into the base of TR1. It is advisable to insert resistor R4 between TP2 and the positive line to prevent damage to TR1 should the two pads be shorted accidentally by an object with a low resistance. If the unit is found to be too sensitive, a high value resistor of about 10M can be connected from the base of TR1 to the battery negative. This will prevent the circuit from switching on inadvertently, especially in areas where the mains field is high.

The circuit is built on a small piece of stripboard having 12 holes by 8 strips, as shown in Fig.2. Only two track cuts need to be made and no wire links are required. Apart from the resistors, all other components must be inserted the correct way round. Power to the circuit should be supplied by a 9V battery. As the stand-by current is extremely low (basically the leakage current of the transistors), the expected life should be almost the shelf life of the battery, depending of course on how often it is switched on. Consequently, an on/off switch is not required. The finished unit should be mounted in an insulated plastic box of a size suitable for the battery and circuit board. The touch contact(s) can be made from any piece of metal such as a bolt or nail, but a drawing pin pushed through a suitable hole in the box and connected to the board via a short length of wire provides a neater, more attractive finish.

LED Considerations
When on, the total current is 6mA with the l.e.d. accounting for about 5·8mA. White l.e.d.s exhibit a forward voltage drop of around 4V, so two could be used in series to provide more light. Resistor R3 would then need to be reduced to 470ohmto maintain the l.e.d. current at around 5mA. The brightness of the l.e.d.(s) can be increased by reducing the value of R3 to increase the current flow. Do not allow the current to be greater than that permitted by the l.e.d., which should be stated in its data sheet and supplier’s catalogue. There appears to be little apparent increase in brightness beyond about 10mA. Data sheets normally quote an l.e.d. viewing angle and this describes the “off axis” brightness of the device. Unlike the filament in a light bulb, an l.e.d. chip emits light only from its surface, rather than all around, so the light comes mainly from the front of the device. This is modified to some extent by the plastic package and l.e.d.s are available with a more or less focused light beam. Depending on the use, a wider angled light pattern may be preferred.


The simple but very reliable monitor shown in Fig will be an asset to those who have difficulty finding their pulse in their wrist. It is also useful for checking the pulse rate immediately after exercise, which should be well above the normal rate of 60-80 beats per minute if any benefit from the exercise is to be derived.
Light Finger The device depends for its operation on variations in light intensity. When a finger is placed on a light dependent resistor R2, the l.d.r. detects the minute changes in light level caused by variations in blood flow as the heart pumps. These light changes are translated into minute voltage fluctuations that are subsequently amplified through a two-stage amplifier, a non-inverting op.amp (IC1a) and an inverting op.amp (IClb), by a gain of approximately 800 as determined by resistors R5, R7 and R10. At the output (pin 7 of IC1b), each heartbeat is reflected in the rhythmical swing of a meter needle across the dial of a milliammeter (ME1) or other suitable panel meter. No special lighting is needed as the l.d.r. is able to “see” through a finger tip in normal daylight.

The gain of the first op.amp is fed into the second and the overall gain is sufficient to obtain a healthy swing of the meter needle. Almost any moving coil meter can be pressed into service because we are not concerned with voltage or current measurement, only the needle deflections across the dial. However, be sure to fit a series limiting resistor R11 to suit the meter and prevent damage. A miniature button-type l.d.r. is preferred to the bulkier ORP12 so that the finger can completely cover the sensor surface and prevent stray lighting from reaching it. Two discrete 741 op.amps could be used in place of the LM358N if more readily available. Although not shown here, the prototype also housed a 30-second timer, using a 555 with an l.e.d. indicator.
When the timer is initiated, the needle movements are counted during the 30 second period, then doubled to obtain pulses per minute. The circuit could also be adapted as a front-end to more advanced monitoring systems. In use, after the unit is switched on, allow several seconds for the meter needle to stabilise somewhere about mid-scale. Place the fleshy part of the middle finger tip on the l.d.r. and rest the hand comfortably while keeping it still, then monitor the meter needle movement. If the meter needle responds by only a small amount, it is probably because your hand is excessively cold and the circulation is sluggish.