Tuesday, February 4, 2014

Simple Flashing LED

The circuit is designed to use very little current to prolong battery life so that it can be left on permanently. A superbright’ red LED is used because this provides a bright flash with a low current.

If you want to use 4.5V supply by connecting 3 Alkaline cells or any other source, change the resistor along with LED from 3.3k to 1k for a better flash.
Note that AA cells will last longer than a 9V PP3 battery 

To flash two LEDs alternatively we have to increase the clock pulse speed to possibly fastest to have exact proper

Parts Required:
  1. 100k potentiometer
  2. 10k and 3.3k
  3. 10mF
  4. LED
  5. 555 Timer
  6. 9v Battery

Circuit Diagram:
There are three different modes to flash an LED using 555 Timer

and their bread board arrangements
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Friday, January 10, 2014

Heat Sink Basics

As power transistor handle large currents, they always heat up during operation. Since transistor is a temperature dependent device, the heat generated must be dissipated to the surrounding in order to keep the temperature within permissible limits. Generally, the transistor is fixed on a metal sheet (usually aluminum) so that additional heat is transferred to the Aluminum sheet. The metal sheet that serves to dissipate the additional heat from the power transistor is known as heat sink.

heat sinks
Fig-1:  Heat Sink (Aluminum Sheet)

aluminum heat sinks
Fig-2: Heat Sink (Aluminum Sheet)

Heat Sink with IC
Fig-3: Heat Sink with Transistor/IC

Heat Sink with Transistor
Fig-4: Heat Sink with Transistor/IC

Most of the within the transistor is produced at the collector junction. The heat sink increases the surface area and allows heat to escape from the collector junction easily. The result is that temperature of the transistor is sufficiently lowered. Thus almost the entire heat in a transistor is produced at the collector-base junction. If the temperature exceeds the permissible limit, this junction is destroyed and the transistor is rendered useless.
Most of power is dissipated at the collector-base junction. This is because collector-base voltage is much greater than the base-emitter voltage, although currents through the two junctions are almost the same.

Heat sink is a direct practical means of combating the undesirable thermal effects e.g. thermal runaway. It may be noted that the ability of any heat sink to transfer heat to the surrounding depends upon its material, volume, area, shape, contact between case and sink and movement of air around the sink. Finned aluminum heat sinks yield the best heat transfer per unit cost. It should be realized that the use of heat sink alone may not be sufficient to prevent thermal runaway under all conditions. In designing a transistor circuit, consideration should also be given to the choice of (i) operating point (ii) ambient temperatures which are likely to be encountered and (iii) the type of transistor e.g. metal case transistors are more readily cooled by conduction than plastic ones. Circuits may also be designed to compensate automatically for temperature changes and thus stabilize the operation of the transistor components.
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NTSC PAL TV Signal Identifier

This circuit is able to identify PAL and NTSC video signals. Its output is high for an NTSC signal and low if the signal is PAL. This output signal can be used, for example, to automatically switch in a colour subcarrier converter or some other device while an NTSC signal is being received. One application is for the reception from satellites of free-to-air TV signals, which in Australia generally contain a mixture of 625-line PAL and 525-line NTSC programs. Operation of the circuit is as follows.

IC1 is an LM1881 video sync separator which takes the video input signal and generates vertical synchronisation pulses. 
For an NTSC signal, these pulses are 16.66ms apart, corresponding to the 60Hz field rate, while for a PAL signal they are 20ms apart, corresponding to the 50Hz field rate. The vertical sync pulses are fed into IC2a, the first of two dual retriggerable monostable multivibrators in the 74HC123A. IC2a has a period of very close to 17.9ms, set by the 200kO resistor and 0.22µF capacitor at pins 14 & 15. Because the monostable is retriggerable, NTSC sync pulses arriving every 16.66ms will keep its Q output, at pin 13, high.

NTSC-PAL TV Signal Identifier Circuit diagram:

ntsc-pal-tv-signal-identifier Circuit

However PAL sync pulses arriving every 20ms will allow the Q output to go low after 17.9ms, before being triggered high again 2.1ms later. Thus an NTSC signal will give a constant high output while a PAL signal will result in a train of pulses 2.1ms wide. The Q output from IC2a is fed to the inverting input of IC2b, the second monostable, which has a period of about 0.5s, as set by the 270kO resistor and 4.7µF tantalum capacitor at pins 6 & 7. With its input constantly high, resulting from an NTSC signal, IC2b is not triggered and its Q output remains low.

However, the pulse train from a PAL signal will constantly retrigger it, so its Q output will remain high. The period of IC2b also effectively makes it a low-pass filter which removes spurious switching due to any input glitches. The output signal is taken from the Q-bar (inverted) output, so that an NTSC signal gives a high output, while PAL gives low. For the particular application for which the circuit was developed, diode D1 and the resistor network shown drive the base of an NPN switching transistor and relay. A dual-colour 3-lead LED can also be fitted to indicate NTSC (red) or PAL (green). Note that with no video input, the output signal is high and will indicate NTSC.

Source :
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Top Bench Power supply Circuit Diagram

This is the Top Bench Power supply Circuit Diagram. A tapped transformer drives a diode bridge (D1-D4) and two 2500 µ¥ filter capacitors (Cl and C2), that provide a no-load voltage of 37 or 47 volts, depending upon the position of switch S2a. The unregulated dc is then fed to a pre-regulator stage composed of Ql and D5. 

Those components protect IC1 (the 723) from an over-voltage condition; the 723 cant handle more than 40 volts. The LED (LED1) and its 2.2 k current-limiting resistor (Rl) provide on/off indication. The current through the LED varies slightly according to the transformer tap selected, but thats of no real consequence.

 Top Bench Power supply Circuit Diagram

 Top Bench Power supply Circuit Diagram

 The series-pass transistor in IC1 drives voltage-follower Q2, providing current amplification. The transistor can handle lots of power. It has a maximum collector current of 15 amps and a maximum VCE of 70 V, both of which are more than adequate for our supply.
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Friday, December 27, 2013

Invisible Broken Wire Detector

Portable loads such as video cameras, halogen flood lights, electrical irons, hand drillers, grinders, and cutters are powered by connecting long 2- or 3-core cables to the mains plug. Due to prolonged usage, the power cord wires are subjected to mechanical strain and stress, which can lead to internal snapping of wires at any point. In such a case most people go for replacing the core/cable, as finding the exact location of a broken wire is difficult.

In 3-core cables, it appears almost impossible to detect a broken wire and the point of break without physically disturbing all the three wires that are concealed in a PVC jacket. The circuit presented here can easily and quickly detect a broken/faulty wire and its breakage point in 1-core, 2-core, and 3-core cables without physically disturbing wires. It is built using hex inverter CMOS CD4069.

Gates N3 and N4 are used as a pulse generator that oscillates at around 1000 Hz in audio range. The frequency is determined by timing components comprising resistors R3 and R4, and capacitor C1. Gates N1 and N2 are used to sense the presence of 230V AC field around the live wire and buffer weak AC voltage picked from the test probe. The voltage at output pin 10 of gate N2 can enable or inhibit the oscillator circuit.

When the test probe is away from any high-voltage AC field, output pin 10 of gate N2 remains low. As a result, diode D3 conducts and inhibits the oscillator circuit from oscillating. Simultaneously, the output of gate N3 at pin 6 goes ‘low’ to cut off transistor T1. As a result, LED1 goes off. When the test probe is moved closer to 230V AC, 50Hz mains live wire, during every positive half-cycle, output pin 10 of gate N2 goes high.

Thus during every positive half-cycle of the mains frequency, the oscillator circuit is allowed to oscillate at around 1 kHz, making red LED (LED1) to blink. (Due to the persistence of vision, the LED appears to be glowing continuously.) This type of blinking reduces consumption of the current from button cells used for power supply. A 3V DC supply is sufficient for powering the whole circuit.

Circuit diagram:

Invisible Broken Wire Detector Circuit Diagram

AG13 or LR44 type button cells, which are also used inside laser pointers or in LED-based continuity testers, can be used for the circuit. The circuit consumes 3 mA during the sensing of AC mains voltage. For audio-visual indication, one may use a small buzzer (usually built inside quartz alarm time pieces) in parallel with one small (3mm) LCD in place of LED1 and resistor R5. In such a case, the current consumption of the circuit will be around 7 mA.

Alternatively, one may use two 1.5V R6- or AA-type batteries. Using this gadget, one can also quickly detect fused small filament bulbs in serial loops powered by 230V AC mains.
The whole circuit can be accommodated in a small PVC pipe and used as a handy broken-wire detector. Before detecting broken faulty wires, take out any connected load and find out the faulty wire first by continuity method using any multimeter or continuity tester.

Then connect 230V AC mains live wire at one end of the faulty wire, leaving the other end free. Connect neutral terminal of the mains AC to the remaining wires at one end. However, if any of the remaining wires is also found to be faulty, then both ends of these wires are connected to neutral. For single-wire testing, connecting neutral only to the live wire at one end is sufficient to detect the breakage point.

In this circuit, a 5cm (2-inch) long, thick, single-strand wire is used as the test probe. To detect the breakage point, turn on switch S1 and slowly move the test probe closer to the faulty wire, beginning with the input point of the live wire and proceeding towards its other end. LED1 starts glowing during the presence of AC voltage in faulty wire. When the breakage point is reached, LED1 immediately extinguishes due to the non-availability of mains AC voltage.

The point where LED1 is turned off is the exact broken-wire point. While testing a broken 3-core rounded cable wire, bend the probe’s edge in the form of ‘J’ to increase its sensitivity and move the bent edge of the test probe closer over the cable. During testing avoid any strong electric field close to the circuit to avoid false detection.
Author: K. Udhaya Kumaran
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Thursday, December 26, 2013

A Handy Pen Torch

This easy to construct “Handy pen torch” electronic circuit and low component count, uses two power white LEDs for lighting. Low volt (4.8V dc) supply available from the built in rechargeable Ni-Cd battery pack is first converted into two channel (independent) constant current sources by two pieces of the renowned precision adjustable shunt regulator chip LM334 (IC1 and IC2). Around 25mA at 3.6 volt dc is available at the output of these ICs.

A regulated dc supply is used to drive two power white LEDs D4 and D6. Resistors R3 and R5 limits the output current (and hence the light output) of IC1 and IC2 circuits respectively. Besides these components, one red color LED (D2) is included in the main circuit which works as a battery charging supply input indicator. Resistor R1 limits the operating current of this LED.

Pen Torch Electronic Circuit Schematic

Circuit Project: Handy Pen Torch circuit

Diode D1 works as an input polarity guard cum reverse current flow preventer. Capacitor C1 is a simple buffer for circuit stabilization. After succesful construction, preferably on a small piece of general purpose PCB, enclose the whole circuit in a suitable and attractive pen torch cabinet. If necessary, drill suitable holes in the cabinet to attach the dc socket, on/off switch and the input indicator etc.

In prototype, commonly available 4.8 volt/500mah Ni-Cd battery pack (for cordless telephones) is used. One very simple but reliable ac mains powered battery charger circuit for the handy pen torch is also included here. Basically the pen torch circuit is a constant current charger wired around Transistor T1 (BC636), powered by a 12v/350mA step down transformer and associated componentsD1, D2 and C1.

AC mains powered battery charger for the pen torch

Circuit Project: Handy Pen Torch circuit

Unregulated 12 volt dc available from the input power converter circuit, comprising step down transformer(TRF), rectifier diodes (D1,D2) and filter capacitor (C1), is fed to T1 through a current limiting resistor R1. Grounded base PNP transistor T1 here works as a constant current generator. With 22 ohm resistor for R1, the charging current available at the output of the charger is near 50mA.

Red LED (D3) provides a fixed voltage reference to the base of T1, with the help of resistor R2. (During charging process, Diode D1 in the main circuit prevent reverse current flow from the battery pack when charging input supply is absent.) After construction of the pen torch circuit, fit the assembled unit inside a small plastic enclosure for safety and convenience.

Circuit Source: DIY Electronics Projects
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Wednesday, December 25, 2013

LED Bike Light

On my mountain bike I always used to have one of those well-known flashing LED lights from the high street shop. These often gave me trouble with flat batteries and lights that fell off. As an electronics student I thought: “this can be done better”. First I bought another front wheel, one which has a dynamo already built in the hub. This supplied a nice sine wave of 30 Vpp (at no load). 

With this knowledge I designed a simple power supply. The transistors that are used are type BD911.These are a bit of an over-kill, but there were plenty of these at my school, so that is why I used them. Something a little smaller will also work. The power supply is connected to an astable multi-vibrator. This alternately drives the front light and the rear light. The frequency is determined by the RC time-constant of R3 and C3, and R2 and C4. This time can be calculated with the formula: t = R3×C3 = 20×103×10×10-6 = 0.2 s You can use a 22k (common value) for R2 and R3, that doesn’t make much difference. On a small piece of prototyping board are six LEDs with a voltage dropping resistor in series with each pair of LEDs.

LED Bike Light Circuit Diagram

Such a PCB is used for both the front and the rear of the bike. Of course, you use white LEDs for the front and red ones for the rear. The PCB with the main circuit is mounted under the seat, where it is safe and has been working for more than a year now. There are a few things I would change for the next revision. An on/off switch would be nice. And if the whole circuit was built with SMD parts it could be mounted near the front light. This would also be more convenient when routing the wiring. Now the cable from the dynamo goes all the way to the seat and from there to the front and rear lights.

Source :

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