LM3410 LED Driver

The LM3410 IC is a constant current LED driver useful in either boost con-verter or SEPIC design applications. A SEPIC (Single Ended Primary Induct-ance Conver ter) design allows the power supply’s output voltage to be set above, below or equal to its input voltage. In this application the chip is configured as a boost-converter (i.e. the output voltage is greater than the input voltage).

LM3410 LED Driver Circuit Diagram

The LM3410 is available in two fixed-frequency variants. Using either the 525 kHz or 1.6 MHz clock version it is possi-ble to build a ver y compact LED driver. The output stage can supply up to 2.8 A, allowing several high-power LEDs to be driven from a rechargeable Lithium cell or several 1.5 V bat-teries. The chip also features a dimmer input giving simple PWM brightness control.Output current is defined by an external shunt resistor. To keep losses low the LM3410 uses an internal voltage reference of just 190 mV.

Power dissipation in the shunt resistor is therefore low. Using the desired value of LED current the value and power dissipation of the shunt resistor is given by:
R_Shunt = 0.19 V/I_LED
P_Shunt = 0.19 V*I_LED

A 10 µH coil (L1) will be suf ficient for most applications providing it has a suitable satu-ration current rating. The Input and output capacitors should be 10 µF ceramic t ypes with a low value of E SR . Many distributor s including Farnell stock these component s. The Diode should beaSchottky type (as in all switching regulators). The author has developed a PCB for this design; the corresponding Eagle files can be freely downloaded from www.elektor.com/090850. In sum-mar y the most important features of the LM3410 are:

• Integrated 2.8 A MOSFET driver.
• Input voltage range from 2.7 V to 5.5 V.
• Capability to drive up to six series connected LEDs (maximum output 24 V).
• Up to 88 % efficiency.
• Available is 525 kHz and 1.6 MHz versions.
• Allows both boost and SEPIC designs.
• Available in 5 pin SOT23 or 6 pin LLP outline.

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220V AC Powered White Led Lamp

This is the simple version of a white LED lamp that can be directly powered from mains. It can give ample light even for reading purpose. Capacitor CX along with diodes D1 through D4 forms the AC step down circuit. CX reduces high voltage AC from mains to a low voltage AC which is rectified by the diodes D1-D4.

220V AC Powered White Led Lamp Circuit Diagram

Capacitor C1 removes ripples from AC so that low voltage DC is available to power the LEDs.CX is the X rated AC capacitor that reduces AC voltage through capacitive rectance property. Resistor R1 is very important to remove the stored voltage from CX when power is switched off. This prevents lethal shock. Resistor R2 limits the inrush current.

More LEDs can be added by reducing the value of R2.Since the circuit is directly connected to mains, take utmost care to avoid shock. No components should be touched when it is connected to mains.

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LED Light Pen Schematic

Physicians and repair engineers often use small light pens for visual examination purposes. Rugged and expensive as these pens may be, their weak point is the bulb, which is a ‘serviceable’ part. In practice, that nearly always equates to ‘expensive’ and / or ‘impossible to find’ when you need one.

LEDs have a much longer life than bulbs and the latest ultra bright white ones also offer higher energy-to-light conversion efficiency. On the down side, LEDs require a small electronic helper circuit called ‘constant-current source’ to get the most out of them. 

LED Light Pen Circuit Diagram

Here, T1 and R1 switch on the LED. R2 acts as a current sensor with T2 shunting off (most of) T1’s base bias current when the voltage developed across R2 exceeds about 0.65 V. The constant current through the white LED is calculated from

R2 = 0.65 / ILED

With some skill the complete circuit can be built such that its size is equal to an AA battery. The four button cells take the place of the other AA battery that used to be inside the light pen.

Author: Myo Min – Copyright: Elektor

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White LED Lamp

Did it ever occur to you that an array of white LEDs can be used as a small lamp for the living room? If not, read on. LED lamps are available ready-made, look exactly the same as standard halogen lamps and can be fitted in a standard 230-V light fitting.

 

We opened one, and as expected, a capacitor has been used to drop the voltage from 230 V to the voltage suitable for the LEDs. This method is cheaper and smaller compared to using a transformer. The lamp uses only 1 watt and therefore also gives off less light than, say, a 20 W halogen lamp. The light is also somewhat bluer.

White LED Lamp Circuit Diagram

The circuit operates in the following manner: C1 behaves as a voltage dropping ‘resistor’ and ensures that the current is not too high (about 12 mA). The bridge rectifier turns the AC voltage into a DC voltage. LEDs can only operate from a DC voltage. They will even fail when the negative voltage is greater then 5 V. The electrolytic capacitor has a double function: it ensures that there is sufficient voltage to light the LEDs when the mains voltage is less than the forward voltage of the LEDs and it takes care of the inrush current peak that occurs when the mains is switched on. This current pulse could otherwise damage the LEDs. Then there is the 560-ohm resistor, it ensures that the cur-rent through the LED is more constant and therefore the light output is more uniform. There is a voltage drop of 6.7 V across the 560-Ωresistor, that is, 12 mA flows through the LEDs. This is a safe value. The total voltage drop across the LEDs is there-fore 15 LEDs times 3 V or about 45 V. The voltage across the electrolytic capacitor is a little more than 52V.

To understand how C1 functions, we can calculate the impedance (that is, resistance to AC voltage) as follows: 1/(2π·f·C), or: 1/(2·3.14·50·220·10-9)=14k4.

When we multiply this with 12 mA, we get a voltage drop across the capacitor of 173 V. This works quite well, since the 173-V capacitor voltage plus the 52-V LED voltage equals 225 V. Close enough to the mains voltage, which is officially 230 V. Moreover, the latter calculation is not very accurate because the mains volt-age is in practice not quite sinusoidal. Furthermore, the mains voltage from which 50-V DC has been removed is far from sinusoidal.

Finally, if you need lots of white LEDs then it is worth considering buying one of these lamps and smashing the bulb with a hammer (with a cloth or bag around the bulb to prevent flying glass!) and salvaging the LEDs from it. This can be much cheaper than buying individual LEDs.

 

Author: Unknown - Copyright: Elektor

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Piezo Powered Lamp Schematic

Energy is becoming ever more expensive, and some fresh ideas are needed. There are already human-powered devices on the market, most of which employ a dynamo to generate power. It is also possible to recover energy from a piezo crystal of the sort found, for example, in the loudspeakers in greetings cards. Making use of this device is relatively straightforward. 

Piezo Powered Lamp Schematic Circuit Diagram

 

Piezo crystals can generate voltages of many tens of volts when given a firm enough prod with a finger to bend the baseplate. The charge moved, however,is relatively small and the crystal is effectively a capacitor with a capacitance of only around 20 nF to 50 nF. This means that we need larger-scale storage in the form of an electrolytic capacitor.

The piezo crystal can be treated as an alternating current source. We therefore need a rectifier and a reservoir capacitor. Pressing the metal surface of the transducer ten or twenty times with a finger will charge the electrolytic in steps to the point where it has enough charge to drive a LED. The circuit is a ‘charge pump’ in the full sense of the term.

When the button is pressed the electrolytic discharges into the LED, which emits a brief, but bright, flash of light.

 

Author :Burkhard Kainka  – Copyright :  Elektor

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Automatic Mooring Light

Integrated-circuit U1-an LF351 or 741 op amp-is used as a comparator to control the light. Resistors R2 and R3 provide a reference voltage of about 2.5 volts at pin 3 of U1. When daylight falls on light-dependent resistor LDR1, its resistance is low: about 1000 ohms. In darkness, the LDR's resistance rises to about 1 megohm.

Automatic Mooring Light Circuit Diagram

Since R1 is 100,000 ohms, and the LDR in daylight is 1000 ohms, the voltage ratio is 100 to 1; the voltage drop across the LDR is less than the 2.5 volt reference voltage and pin 2 of U1 is held at that voltage. In that state, the output at pin 6 of U1 is positive at about 4.5 volts, a value that reverse-biases Q1 to cutoff, which in turn holds Q2 in cutoff, thereby keeping lamp I1 off.  

When darkness falls, the LDR's resistance rises above R1's value and the voltage at pin 2 of U1 rises above the reference voltage of 2.5 volts. U1's output terminal (pin 6) falls to less than a volt and Q1 is biased on. The base-to-emitter current flow turns Q2 on, which causes current to flow through the lamp. When daylight arrives, the LDR's resistance falls sharply, which causes the lamp to be turned off, ready to repeat the next night/day cycle.

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Power LED Driver

If you want to operate power LEDS with a truly constant current which significantly prolongs the lifetime of the lamp and avoid the power loss resulting from using a constant voltage supply with a series resistor, you need a suit-able constant current source. However, the only way to achieve really good efficiency is to use a switching regulator. Altogether, this means that you need a switching regulator designed to generate a constant current instead of a constant voltage.

With this in mind, the author started working on the development of a LED pocket torch with especially high efficiency. Along with using high-capacity rechargeable batteries to maximise operating life, it’s worthwhile to be able to reduce the brightness, and therefore the operating current of the LEDs, when you don’t need full power. Accordingly, the author incorporated a dimming function in the design, based on operation in PWM mode in to reduce power losses to an absolute minimum.

Power LED Driver Circuit Diagram

As you can see from the circuit diagram, the author chose an LT3518 switching regulator IC, which is a buck/boost converter optimised for LED operation. Here it is used as a down converter (buck mode). This IC can achieve better than 90% efficiency in this mode, depending on the input voltage. According to the typical application circuit on the data sheet [1], its switching frequency can be set to approximately 170 kHz by selecting a value of 82 kΩ for R1. To maximise overall efficiency with this type of IC, the volt-age drop over the sense resistor used to measure the current flowing through the LED should be as low as possible. This particular device operates with a voltage drop of 100 mV, corresponding to a current of just under 1.5 A with the specified value of 68 mΩ for R2. This value proved to be suitable for the Cree LED used by the author. At this current level, a diode with a power rating of at least 6 W should be used for D1.

IC1 has an additional property that is ideal for this application: the connect-ed LED can be dimmed by applying a PWM signal to pin 7 of the IC, with the brightness depending on the duty cycle. Obviously, the PWM frequency must be lower than the switching frequency. The PWM signal is provided by IC2, a special voltage-controlled PWM generator (type LTC6992 [2]). The duty cycle is controlled by the volt-age applied to the MOD input on pin1 (range 0–1 V). The resistor connected to pin 3 determines the internal clock frequency of the IC according to the formula f= 1 MHz × (50 kΩ/R3). This yields a frequency of approximately 73.5 kHz with R3 set to 680 kΩ, which is much too high for controlling IC1.

However, the PWM IC has an internal frequency divider with a division factor controlled by the voltage applied to pin 4, which in this circuit is taken from voltage divider R4/R5. The division factor can be adjusted over the range of 1 to 16,384. The division factor with the specified component values is 64, resulting in a PWM frequency of around 1,150 Hz. If you want to be able to generate a PWM signal with an adjust-able duty cycle over the full range of 0 to 100%, you must use the LTC6992-1 option. The -4 option, which provides a range from 5 to 100%, might be an acceptable alternative.To prevent the duty cycle (and thus the brightness of the LED) from depending on the battery voltage, which gradually drops as the battery discharges, IC3 generates a stabilised 1.24 V control voltage for potentiometer P1. Series resistor R7 reduces the voltage over P1 to 1V, which exactly matches the input voltage range of the LTC6992.

All capacitors should preferably be ceramic types, in particular due to their low effective series resistance (ESR) as well as other favourable characteristics. However, only capacitors with X5R or X7R dielectric should be used; capacitors with type Y dielectric have very poor temperature characteristics.The supply voltage is limited to 5.5V by the maximum rated supply voltage of IC2. The author used four NiMH re-chargeable cells connected in series, which yields a voltage that is just within spec. With an operating voltage in the range of 4.5 V to 5.5 V, you must use an LED that can operate at less than 4V.

This eliminates devices with several chips connected in series on a carrier, which is very often the case with power LEDS rated at over 5 W. These devices require a correspondingly higher supply voltage, which means more cells connected in series. This is only possible if the supply voltage for IC2 is reduced by a 5 V voltage regulator or other means, and of course R4 must also be connected to this lower supply voltage.

Finally, a few words about soldering. An exposed thermal pad must be provided on the PCB for the LT3518, and the rear face of the IC must be soldered to this pad. The author obtained good results by dimensioning the exposed pad large enough to extend beyond the outline of the IC. When assembling the board, first tin the pad and the rear face of the IC. Then heat the pad with a soldering iron. When the solder melts, withdraw the tip of the soldering iron to the edge of the pad and simultaneously place the IC on the pad and align it. After this the pins can be soldered.

Author : Burkhard Kainka Copyright: Elektor

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Infrared Emitter and Detector using IC 74LS14

This circuit have applied to line detection of robot project, Good match between the transmitter and the detector is important for proper operation, especially if the hole is large. Robot with a simple object or obstacle detection. Infrared Transmitter detector pair sensors are relatively easy to implement, although involved some degree of testing and calibration in order to make correct. They can for the impediment, motion detection, transmitters, encoders are used, and the color detection.

Infrared Emitter and Detector using IC 74LS14 Circuit Diagram:

This can be done with a piece of rope stretched between and in accordance with LED and phototransistor. A length of stiff wire or plugs can be used to set the alignment. Another method that can be used for long distances is a laser pointer shone through a hole.

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Light Sensitive and Differential Temperature Switch

Fig.1 Precision Light- Sensitive Switch Circuit Diagram:

 

In Fig. 1 see a precision light- sensitive switch that activates when the sensed quantities go above or below pre-set values. The LDR can be any cadmium sulfide unit that has a resistance in the range 500R to 20k at the required trip level. The RV1 adjust LDR at normal light level.

Fig. 2 Differential Temperature Switch Circuit Diagram:

In Fig. 2 see a differential temperature switch circuit using ordinary silicon diodes as temperature sensing elements and responding to differentials of a fraction of degree. RV2 can be used to apply an effective offset of several degrees to the two diodes. To adjust the circuit, apply the required differential temperature to the diodes and then adjust RV2 so that the relay just turns on. The circuit responds to the relative temperatures, rather than the absolute temperatures, of the two diodes.

Part List :

R1=LDR *see text
R2-3=10Kohm
R4=2.2Kohm
R5-6-7-10=4.7Kohm
R8-9=2.7Kohm
RV1=22Kohm  pot.
RV2=1Kohm  pot.
D1-4=1N4001
D3-4=1N4148
Q1-2=BC214L
IC1-2=LM741
RL1-2=RELAY 12V >120 ohm

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Wireless LED Driver

There are times when you want to control a LED indicator light through the side of a plastic box, without wires and without drilling a hole in the box.  One example where this may be needed is in data collection systems.  These are often used out of doors in harsh environments and have to be hermetically sealed. Holes drilled in the side of the box for panel mounted LEDs or light pipes can often leak.

Wireless LED Driver Circuit Diagram :



The circuit below solves this problem by sending power to the LED through the plastic, using a magnetic coupling technique. The circuit below can route power through plastic enclosures as thick as ¼ inch.  The circuit will not work through metal boxes.  An expensive inductor, driven by a series resonant mode 125KHz oscillator, forms the power transmitter.  A similar inductor, wired as a 125KHz parallel resonant circuit, forms the power receiver.  A voltage doubler circuit at the receiver efficiently converts the collected AC into DC.  The circuit will operate over a wide 3v to 6v supply range.

With a 5v supply, the circuit draws about 25ma of current.  However, by gating the oscillator on for a brief 20ms period, with a 0.5Hz rate, the average power can be reduced to about 250 microamps.  If you want to extend the range of operation out to ½ inch, try using a 74C14 (CD4069) with a 12v supply.  Using surface mounted components; the complete LED assembly can be encapsulated and glued to the outside surface of the box.  Tiny unshielded surface mounted inductors can be used to reduce the size of the transmitter and receiver.  However, smaller parts will reduce the power transfer range to perhaps only a 1/8 inch separation.
A very nice bright green LED, which works great for this circuit, is one from Kingbright, available from Digikey, part number 754-1089-1.

Source : discovercircuits.com

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Ultra Low Power LED Flasher

The efficiency of some newer LEDs is amazing.  Some of the latest green LEDs can launch blinding light with just one milliamp of current.  I take advantage of one of these newer devices in the circuit below.  The flashing circuit uses a classic multivibrator oscillator, made from a tiny National Semiconductor’s LMC7215 low power voltage comparator.

Ultra Low Power LED Flasher Circuit Diagram :

The circuit produces a short 10ms pulse every two seconds, drawing power from a 3v supply.  I suggest using a surface mounted green LED from Kingbright.  Although the LED peak current is restricted to just one milliamp, this part generates a very bright flash of light.  The circuit draws and average DC current of only 6 microamps from a 3v supply. 

When powered by a small lithium coin cell with a 100ma-hour rating, it will flash for about two years.  This flasher might be a great add-on circuit for a flashlight, a key chain or even attached to a cell phone, so the thing can be found in the dark.  Another application might be for a fake car alarm indicator.  The circuit might also be used in conjunction with some other battery powered product, to let the user know when it is in operation.  The small 6uA current drain would not tax even the most stingy power budget. Link

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Outdoor Lighting Controller

When you step out of your brightly-lit house  into the darkness, it takes a while for your  vision to adjust. A solution to this problem  is this outdoor light with automatic switch-off. As a bonus, it will also make it a little bit  easier to find the keyhole when returning  late at night. Often no mains neutral connection is avail-able at the point where the switch-off timer  is to be installed, which makes many circuit  arrangements impractical. However, the circuit here is designed to work in this situation. The design eschews bulky components such as transformers and the whole unit can  be built into a flush-mounted fitting. The circuit also features low quiescent current consumption.

Outdoor Lighting Controller Circuit Diagram :


The circuit is star ted by closing switch (or  pushbutton) S1. The lamp then immediately receives power via the bridge rectifier. The drop across diodes D5 to D10 is 4.2 V, which provides the power supply for the delay circuit itself, built around the CD4060 binary  counter.

When the switch is opened the lighting sup-ply current continues to flow through Tri1. The NPN optocoupler in the triac drive circuit detects when the triac is active, with antiparallel LED D1 keeping the drive sym-metrical. The NPN phototransistor inside the  coupler creates a reset pulse via T1, driving  pin 12 of the counter. This means that the  full time period will run even if the circuit is retriggered. The CD4060 counts at the AC grid frequency.  Pin 3 goes high after 213clocks, which corresponds to about 2.5 minutes. If this is not long  enough, a further CD4060 counter can be cascaded. T2 then turns on and shorts the internal LED of opto-triac IC2; this causes Tri1 to  be deprived of its trigger current and the light  goes out. The circuit remains without power until next triggered.

The circuit is only suitable for use with resistive loads. With the components shown (in particular in the bridge rectifier and D5 to  D10) the maximum total power of the connected bulb(s) is 200 watts. As is well known, the filament of the bulb is most likely to fail at the moment power is applied. There is little risk to Tri1 at this point as it is bridged by  the switch. The most likely consequence of overload is that one of diodes D1 to D6 will  fail. In the prototype no fuse was used, as it would not in any case have been easy to change. However, that is not necessarily recommended practice!

Circuits at AC line potential should only be constructed by suitably experienced persons and all relevant safety precautions and  applicable regulations must be observed during construction and installation.

Author : Harald Schad - Copyright : Elektor

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Simple Smooth Flasher

Ordinary LED flashers turn the LED on and off abruptly, which can get a little irritating after a while.

Circuit diagram :

Simple Smooth Flasher Circuit Diagram

The circuit shown here is  more gentle on the eyes: the light intensity changes very slowly and sinusoidally, helping to generate a relaxed mood. The circuit shows a phase-shift oscillator with an adjustable current source at its out-put. The circuit is capable of driving two LEDs in series without affecting the current. The frequency is set by three RC networks, each of which consists of a 100 µF capacitor and a 22 kΩ resistor.

Operation is largely independent of supply voltage, and the average LED current is set at about 10 mA. The circuit adjusts the voltage across the emitter resistor so that it matches the base voltage of the first transistor (around 0.6 V). The phase shifting network gives rise to the oscillation around this average value. In the prototype of this circuit we used an ultra-bright red LED.

Author : Burkhard Kainka - Copyright: Elektor

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Lighting Governor Using 555 Timer IC

This circuit is very handy as a timer circuit for a lamp, for lighting a staircase, for example, but can also be used as indicator for the front doorbell. A significant advantage of this circuit is that the circuit draws almost no current when in the inactive state. The circuit is activated with push button (S1), after which IC5 (a 555 timer IC) starts to count down the set time. During this  time the triac continues to conduct and the lamp is turned on. The ‘on’ time of the  lamp is on is determined by the combination of R1 and C2 and can be changed as  required by your application or personal  preference. R2 and C3 have been added because the  555 expects a ‘negative’ pulse at its trigger  input. When the power supply is turned on,  C3 holds the TR input of the 555 Low for a short time, which triggers the timer IC.

Circuit diagram :

Lighting Governor Circuit Diagram

Depending on the exact type (brand) of  555, the value of C4 (330 nF) may have to be changed to ensure a high enough  power supply voltage when in the active  state. Note also that you shouldn’t use a  ‘too heavy’ version of the triac. The circuit will drive at the most just a little more than 5 mA into the gate of the triac. The circuit worked properly when tested with a TIC206  and the slightly bigger TIC216.

When selecting push button S1, take into  account the switching current of the lamp. The switch must be able to handle that  safely. In the event of a defective part, a 15-V zener diode is connected across the power supply for protection (D3). R6 and R7 have been added so that C4 will be discharged. In this way no dangerous voltage can remain when the circuit is unplugged. When large values for C2 are used, such  as the 470 µF shown here, a good quality  capacitor is required for C4. Any potential  leakage resistance will then have no influence on the set time. Because of an inferior  capacitor in our prototype the time was  considerably longer than expected.

Author : Peter Jansen - Copyright : Elektor

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Simple Automatic Street Light

There have been lot of problems in street lights. Major problem in some places is every evening a person has to come and switch ON the street light and it should be again switched off in morning. Yes, this may not be the situation in everywhere but exists in many places. So this problem can be overcome by using a simple circuit. Below shown circuit will be automatically switched ON and OFF during night and morning times respectively.

Circuit diagram :

Simple Automatic street light Circuit Diagram

In above circuit R1 can be used to adjust the sensitivity. And the working of the circuit is very simple. The LDR will have very low resistance during day time so the transistor Q1 will be in OFF condition. And during night time the resistance will be very high so automatically the transistor Q1 will be ON. The Q1 is PNP transistor and the emitter of Q1 is given to base of Q2. So the Q2 transistor will be ON only if the transistor Q1 is ON. The TRIAC is used in the circuit to make is circuit complete. As the TRIAC will allow voltage to pass from either directions only when there is a certain threshold voltage in gate terminal. And the gate of TRIAC is controlled by transistor Q2.

So totally the lamp will be ON during night time and will be again switched off during day light. To change the sensitivity of the circuit to light adjust R2.

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Solar Lamp using the PR4403

The PR4403 is an enhanced cousin of the PR4402 40 mA LED driver. It has an extra input called LS which can be taken low to  turn the LED on. This makes it very easy  to build an automatic LED lamp using a  rechargeable battery and a solar module. The LS input is connected directly to the solar cell, which allows the module to be  used as a light sensor at the same time as  it charges the battery via a diode. When  darkness falls so does the voltage across  the solar module: when it is below a thresh-old value the PR4403 switches on. During  the day the battery is charged and, with  the LED off, the driver only draws 100 µA.

Circuit Diagram :

Solar Lamp using the PR4403 Circuit Diagram

At night the energy stored in the battery is released into the LED. In contrast to similar designs, here we can make do with a single  1.2 V cell. The PR4403 is available in an SO-8 pack-age with a lead pitch of 1.27 mm. The  other components are a 1N4148 diode (or a Schottky 1N5819) and a 4.7 µH choke. Pin 2 is the LS enable input, connected directly to the solar module. According to the datasheet, it is possible to connect a series resistor at this point (typ. 1.2 M) to increase the effective threshold voltage. The LED will then turn on slightly earlier in the evening before it is not completely  dark. Pins 3 and 6 of the device must be connected together and together form the output of the circuit.

Author : Burkhard Kainka - Copyright : Elektor

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Green-Red Multi flasher

This circuit can be made to produce interesting and attractive light effects using just a cluster of red LEDs and one of green LEDs. One effect is first alternating bet ween red  and green, and then lighting red and green together. With the exception of the triple LED devices (Rapid Electronics # 56 - 0205  for green, # 56 - 0200 for red) all parts are cheap and easy to find, possibly even in your junkbox.

Circuit diagram :

Green-Red Multi flasher Circuit Diagram

The values of networks R3/C3, R4/C4 and R5/ C5 govern the length of the flashes. Using the  indicated values, these are about 18 seconds  with a 0.5 second interval.

Because the colours used do not have equal  luminous intensity (expressed in millican-delas) D1 and D2 are silicon diodes and D3 and D4, germanium, with Schottky devices (BAT82) as an alterative because they also exhibit a low forward drop of about 0.3 V. As germanium devices, look for the OA91, OA85 or AA119. If D1 and D2 are omitted, Green and Red are brighter by themselves than when on simultaneously.

MOSFE T T2 switches both LED devices on simultaneously arranging for roughly equal luminous output. The display has an integrated LDR that causes  the LEDs brightness to adapt automatically to darkness and bright light conditions. The circuit has lots of openings for experimentation and adaptation, for example, the flash rate is determined by the value of C1, while the link between the counter’s R (reset)  input and O3 output determines if a space is inserted after the last flash, or not. Colour-ful and lively effects may also be obtained by  using tri-colour LEDs with a common anode.

The power consumption of the circuit depends  largely on the LEDs used. With the Rapid LED  types shown, about 70 mA may be expected  at a 6 volts supply voltage.

Author : Ken Barry – Copyright : Elektor

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Solid-State Dark-Room Light

Light-emitting diodes are perfectly suitable for dark-room light, because they (a) obviate the need of filters; (b) emit cold light; (c) have a life that is not shortened by continuous on-off switching; and (d) do not radiate infrarays. The types used must, of  course, have a high light output; fortunately, there are nowadays LEDs with a luminous intensity of hundreds of millicandela.

Circuit diagram :

Solid-State Dark-Room Light Circuit Diagram

The sensitivity of photographic paper lies between wavelengths 300 nm and about 550 nm, whereas the wavelength of the light emitted by green LEDs is about 565 nm; that by amber types around 585 nm; and that by red LEDs about 640 nm. From  this, it is clear that all three types of LED may be used with impunity. None the less, in practice, it is best not to use green ones. Because of the special composition and high sensitivity of colour negative paper, only yellow LEDs with reduced light output should be used when processing this paper. The proposed light, therefore, has provision for reducing the emitted light. Note that since colour reversal paper is sensitive to all colours, it can only be processed  in  total  darkness. When working with orthochromatic paper, only red LEDs should be used. With reference to the diagram, each group of three LEDs is fed from a current source, Ti to T6 respectively. The current level, and consequently the light output of the LEDs, is determined by the setting of  Pti Zener diode D,9 provides the reference voltage for the current sources, ensuring that the light out-put of the lighting unit remains virtually constant over the life of the PP3 battery.

Maximum light output is set with the aid of P2. To this end, both Pi and P2 are first set to maximum resistance; after this, P2 is adjusted until a potential of 0.2 V is measured at point A. The maximum current through the LEDs is then about 20 mA. As the photograph shows, the unit has been constructed so that Si is easily operated. Since this switch is a press-to-make type, the light will switch off as soon as it is put aside, thus preserving the battery. It is possible to have the light on continuously by connecting an external battery to Uext. In that  case, Rio must be matched to this source according to Rio=(Uext-9)l0 [k4] but only if NiCd batteries are used. If standard cells are used, D2o and Rio must be omitted.

If a variety of photographic paper is processed, it may be useful to be able to switch between red and amber LEDs. For that purpose, each of the eighteen original yellow LEDs is duplicated by a red LED, shown in dashed lines. Switch S2 may be used to select the relevant bank of LEDs (red or yellow) as required for the specific application.

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Light Gate with Counter

The circuit described here counts the number of times that an infrared beam is interrupted. It could be used to count the number of people entering a room, for instance, or how often a ball or another object passes through an opening (handy for playing shuffleboard). The heart of the circuit consists of you guessed it a light gate! Diode D1 is an IR diode that normally illuminates IR transistor T1. The light falling on T1 causes it to conduct to a certain extent. The resulting voltage on the collector of T1 should be just low enough to prevent the following transistor (T2) from conducting. This voltage can be adjusted within certain limits using P1.

Circuit diagram :

Light Gate with Counter Circuit Diagram

As soon as an object comes between D1 and T1, the light shining on T1 will be partially or fully blocked, causing the IR transistor to conduct less current. As a result, the voltage on its collector will increase, producing a brief rise in the voltage on the base of T2. This will cause T2 to conduct and generate a negative edge at IC1. This negative edge will trigger the monostable multivibrator, which will then hold the output signal on pin 3 ‘high’ for a certain length of time (in this case, one second). Atthis point, two things will occur. First, a buzzer will be energised by the output of IC1 and produce a tone for approximately one second. When the buzzer stops, a negative edge will be applied to the clock input of IC2, causing the counter in IC2 to be incremented by 1. IC2 is conveniently equipped with an internal binary-to-BCD decoder, so its outputs only have to be buffered by IC3 and T3 to allow the state of the counter to be shown on the 7-segment display. Switch S1 can be used to reset the counter to zero.

If a one-second interval does not suit your wishes, you can modify the values of R3 or C1 to adjust the time. Increasing the value of R3 lengthens the interval, and decreasing it naturally shortens the interval. The same is true of C1. When building the circuit, make sure that T1 is well illuminated by the light from D1, while at the same time ensuring that T1 ‘sees’ as little ambient light as possible. This can best be done by fitting T1 in a small tube that is precisely aimed toward D1. The longer the tube, the less ambient light will reach T1. The sensitivity of the circuit can be adjusted using P1.

 

Author : T.Hareendran - Copyright : Elektor

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Simple Flashing Lights Schematic

This is a simple flashing lights circuit can be used as beacon. The assembly consists basically of two blinking steps that commands two light bulbs. With the help of P1 you can adjust the flashing frequency between some limits. There are 2 parts for the circuit, the second one works the same way as the other but with the help of a wire bridge or a switch you can choose different operating modes.A bridge between M and 3 means: 2 independent blinks.

Circuit diagram :

Flashing Lights Schematic Circuit Diagram

If there is a bridge between M and 2, then the lamps lights alternatively with a frequency that can be adjusted with P1. And finally there is one more possibility for M and 1, where the lamps blinks at the same time. The flashing lights circuit works with voltages between 3V and 15V. The lamps voltage must be 2/3 of working voltage. R5 and R10 are chosen so that the lamps are about to light.

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