Auto Brightness Adjustment to Seven Segment LED Displays

Raj from Embedded Lab shows in his latest tutorial guide how to implement adaptive brightness control to seven segment LED displays for optimum readability in all illumination conditions. The technique has been demonstrated by constructing a temperature and humidity meter that adapts the brightness of the seven segment LED displays to the surrounding lighting conditions.The project uses a general purpose LDR to sense the surrounding illumination and MAX7219 to drive the LED display.

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2MHz Square Wave generator Circuit Diagram

With the values shown the circuit generates a 2-MHz symmetrical square wave. Changing capacitors Cl and C2 to 0.01 µ¥ results in a frequency of 500 Hz. For the particular integrated circuits and power supply voltages (5.0 V), the reliable operating range of Rl = R2 is 2 k ohm to 4 k ohm.

2MHz Square Wave generator Circuit Diagram

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15 step digital power supply

     Here is a simple circuit to obtain variable DC voltage from 1.25V to 15.19V in reasonably small steps as shown in the table.

     The input voltage may lie anywhere between 20V and 35V. The first section of the circuit comprises a digital up-down counter built around IC1— a quad 2-input NAND schmitt trigger (4093), followed by IC2— a binary up-down counter (4029). Two gates of IC 4093 are used to generate up-down logic using push buttons S1 and S2, respectively, while the other two gates form an oscillator to provide clock pulses to IC2 (4029).

     The frequency of oscillations can be varied by changing the value of capacitor C1 or preset VR1. IC2 receives clock pulses from the oscillator and produces a sequential binary output. As long as its pin 5 is low, the counter continues to count at the rising edge of each clock pulse, but stops counting as soon as its pin 5 is brought to logic 1. Logic 1 at pin 10 makes the counter to count upwards, while logic 0 makes it count downwards. Therefore the counter counts up by closing switch S1 and counts down by closing switch S2.

      The output of counter IC2 is used to realise a digitally variable resistor. This section consists of four N/O reed relays that need just about 5mA current for their operation. ( The original circuit containing quad bilateral switch IC 4066 has been replaced by reed relays operated by transistorised switches because of unreliable operation of the former.) The switching action is performed using BC548 transistors. External resistors are connected in parallel with the reed relay contacts. If particular relay contacts are opened by the control input at the base of a transistor, the corresponding resistor across the relay contacts gets connected to the circuit.

     The table shows the theoretical output for various digital input combinations.

     The measured output is nearly equal to the theoretically calculated output across regulator IC3 (LM317). The output voltage is governed by the following relationship as long as the input-to-output differential is greater than or equal to 2.5V: Vout = 1.25(1+R2/R1) Where, R1 = R15 = 270 ohms (fixed) and R2 = R11 + R12 + R13 + R14 = 220 + 470 + 820 +1500 ohms = 3,010 ohms (with all relays energised) One can use either the binary weighted LED display as indicated by LED1 through LED4 in the circuit or a 74LS154 IC in conjunction with LED5 through LED20 to indicate one of the 16 selected voltage steps of Table I. The input for IC4 is to be tapped from points marked ‘A’ through ‘D’ in the figure. This arrangement can be used to replace the LED arrangement at points A, B, C, and D. This 74LS154 IC is a decoder/demultiplexer that senses the output of IC2 and accordingly activates only one of its 16 outputs in accordance with the count value. LEDs at the output of this IC can be arranged in a circular way along side the corresponding voltages.

      When the power is switched on, IC2 resets itself, and hence the output at pins 6, 11, 14, and 12 is equivalent to binary zero, i.e. ‘0000’. The corresponding DC output of the circuit is minimum (1.25V). As count-up switch S1 is pressed, the binary count of IC2 increases and the output starts increasing too. At the highest count output of 1111, the output voltage is 15.19V (assuming the in-circuit resistance of preset VR2 as zero). Preset VR2 can be used for trimming the output voltage as desired. To decrease the output voltage within the range of 1.25V to 15.2V, count-down switch S2 is to be depressed.

Notes.

1. When relay contacts across a particular resistor are opened, the corresponding LED glows.

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Build a Low Ripple Power Supply Circuit Diagram

How to build a low ripple power supply circuit diagram. This simple low ripple power supply circuit diagram may be used where a high current is required with a low ripple voltage (such as in a high powered class AB amplifier when high quality reproduction is necessary) , Ql, Q2, and R2 may be regarded as a power darlington transistor.

ZDl and Rl provide a reference voltage at the base of Ql. ZDl should be chosen thus: ZDl = Von-1. C2 can be chosen for the degree of smoothness as its value is effectively multiplied by the combined gains of Q1/Q2, if 100 µF is chosen for C2, assuming minimum hfe for Ql and Q2, C = 100 x 15(Q1) x 25(Q2) = 37,000 µf.

Low Ripple Power Supply Circuit Diagram

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Subwoofer Lowpass Filter uA741 Single Op Amp

This is the simplest Sub woofer Low Pass filter Circuit using uA741 single op amp ic. The circuit is very low cost with respect to their work. The cut off frequency of this circuit is 25Hz to 80Hz maximum. Using this circuit , you can easily design a 2.1 Sub-woofer Speaker System at your own Home. The circuit contains very few components. In Pakistan, the cost of this circuit with PCB is Rs:45 The same circuit is working in my own hand made sub-woofer system. So Try this [Link]

Parts List:

R1,R3,R4 = 10K 1/4W
R2=100K 1/4W
CY1,CY2 = 0.22uF Polyester
C1,C2 = 10uF/25V Electrolytic
IC1 = uA741A Single Op-Amp Ic + 8 Pin Ic Socket
3 Pin Male & Female Connector x 2
2 Pin Male & Female Connector x 1
PCB as in required size 4.5 cm x 3.4 cm

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Simple 500W Inverter 12 Volt to 220 Volt Circuit Diagram

This is the Simple 500W Inverter 12 Volt to 220 Volt Circuit Diagram about the the inverter, because like working outdoors, or to backup storage to use when necessary. Most of this is circuit low power, which is not suitable for practical applications. My friends said that he would be about 500 Watt. It is a good size. Use with television receivers and light bulbs as well. When looking for circuit. I get headaches. 

500W Inverter 12 Volt to 220 Volt Circuit Diagram

 

If you are a beginner or I can not buy expensive good quality circuits. Requires only one transistor. Or if you have free time. I want to build old circuit is alive again. This circuit will accommodate all your needs. It is a simple circuit. The same principle, I take battery voltage 12V to produce a oscillator about 100 Hz and pass to a two frequency divider circuit is only 50HZ. and drive a 10 ampere transformer with 10 x 2N3055 transistor in parallel. 

By a single transistor has 2A, when I use 10 transistors or 5 pairs of drive high current output. The complexity of circuit, but the principle is not it, and it is the number of transistors on a basic, easy to buy. You may be modified 100 watt power inverter To the size of transistors and transformers as well. 

Note:

If you think that This circuit is not good enough. For your work. It is hard to find equipment. You do not have it now. These circuits may be viewed below. It may be appropriate for you.


Source: leksound project

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Build a Isolated 15V To 2500V Power Supply Circuit Diagram

Build a Isolated 15V To 2500V Power Supply Circuit Diagram. A dc-dc converter using a 74HC04 drives Tl. Tl is a ferrite-core transformer using a Fair-Rite, Inc. P/N 5975000201 (uo + 5000) and has a 7-turn primary and a 25-turn secondary. Kynar, #30 wirewrap wire is used. With Tl, the circuit isolation is good to 2500 V.

Isolated 15V To 2500V Power Supply Circuit Diagram

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Ground free DVM Module Supply from 5 V

The majority of hand-held digital volt meters use an LCD screen and are powered from a nine volt battery. Inside is most probably an ICL7106 chip (or something compatible). This takes care of measuring the input and driving the LCD. This IC is very popular and can be found in other laboratory and homebrew equipment where it offers a simple solution for both measuring current/voltage and driving the display.

So far so good, there is how-ever one feature of this device which needs careful consideration. The power supply to the chip (both the positive and negative connection) must not have any direct connection to either of the two measuring input terminals. It requires floating supplies. This is not a problem for battery powered equipment but needs more thought when the ICL7106 is fitted into mains powered equipment.

Ground-free DVM Module Supply from 5 V Circuit Diagram :

The simplest, most expensive solution is to use two independent power supplies in the equipment. A battery could also be used as an isolated supply but in a mains powered device it would seem a bit out of place and inconvenient.

In this case the term ‘floating supplies’ means that it is possible to have two separate DC levels. This level of isolation can be achieved with capacitors to separate the two DC supplies. Back in 2003 we published a circuit in the July/August edition of Elektor (circuit number 75) which used a NE555 IC. Unfortunately this design required a supply voltage upwards of 10 V. If the DVM module is fitted to equipment which only uses a 5 V supply (as is often the case) the circuit will not be of much use.The author has solved the problem by modifying the original circuit, using a hex Schmitt trigger inverter type 74HC14N instead of the NE555. One of the inverters generates a square wave of about 75 KHz. The remaining five inverters are wired in parallel to pro-vide more output drive current for the voltage multiplier stage.

DC isolation is provided by capacitors C2 and C3. A classic voltage multiplier configuration is made up of capacitors and diodes. The circuit generates an output of around 8.5 V at a load current of 1 mA. This is sufficient to power the DVM chip. The 5 V supply for the circuit must be stabilised.

The values of the input voltage divider resistors R2 and R3 are independent of the chip’s power supply and must be selected according to the desired measurement range.

Source :  http://www.ecircuitslab.com/2012/09/ground-free-dvm-module-supply-from-5-v.html

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Motorcycle Alarm With Transistor Circuit Diagram

This is a simple Motorcycle Alarm With Transistor Circuit Diagram. Its designed to work at 12-volts. But - if you change the relay for one with a 6-volt coil - itll protect your "Classic Bike". The standby current is virtually zero - so it wont drain your battery.

Motorcycle Alarm With Transistor Circuit Diagram

Any number of normally-open switches may be used. Fit the mercury switches so that they close when the steering is moved or when the bike is lifted off its side-stand or pushed forward off its centre-stand. Use micro-switches to protect removable panels and the lids of panniers etc. While at least one switch remains closed - the siren will sound.

About one minute after all of the switches have been opened again - the alarm will reset. How long it takes to switch off depends on the characteristics of the actual parts youve used. You can adjust the time to suit your requirements by changing the value of C1 and/or R3.

The circuit is designed to use an electronic Siren drawing 300 to 400mA. Its not usually a good idea to use the bikes own Horn because it can be easily located and disconnected. However, if you choose to use the Horn, remember that the alarm relay is too small to carry the necessary current. Connect the coil of a suitably rated relay to the Siren output - and use its contacts to sound the horn.

The circuit board and switches must be protected from the elements. Dampness or condensation will cause malfunction. Without its terminal blocks, the board is small. Ideally, you should try to find a siren with enough spare space inside to accommodate it. Fit a 1-amp in-line fuse as close as possible to the power source. This is Very Important. The fuse is there to protect the wiring - not the circuit board. Instead of using a key-switch you can use a hidden switch; or you could use the normally-closed contacts of a small relay. Wire the relay coil so that its energized while the ignition is on. Then every time you turn the ignition off - the alarm will set itself.

When its not sounding, the circuit uses virtually no current. This should make it useful in other circumstances. For example, powered by dry batteries and with the relay and siren voltages to suit, it could be fitted inside a computer or anything else thats in danger of being picked up and carried away. The low standby current and automatic reset means that for this sort of application an external on/off switch may not be necessary.

When you set the alarm - if one of the switches is closed - the siren will sound. This could cause annoyance late at night. A small modification will allow you to Monitor The State Of The Switches using LEDs. When the LEDs are all off - the switches are all open - and its safe to turn the alarm on.

Veroboard Layout

 

 

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Latch Up Alarm Using Opto Coupler

The latch-up alarm described here is based on single IC NE555, configured as an astable multivibrator. The timing components are selected such that the oscillation frequency of the multivibrator lies within the audio range. Instead of a flip-flop stage, an opto-coupler (MCT2E) is used for latching of the alarm.  Under normal condition, pin 4 of IC1 is pulled to ground via resistor R2, and its output at pin 3 is held  ‘low’. When switch S1 is pressed momentarily, transistor T1 conducts to bring reset pin 4 of 555 to logic  ‘high’. As a result, IC1 is activated and the alarm starts to sound.

 
Latch-Up Alarm Using Opto-Coupler Circuit Diagram

 

Simultaneously, the LED inside opto-coupler glows and the phototransistor conducts. As a result, trigger transistor T1 gets base bias via phototransistor and resistor R6. The alarm sounds continuously until reset switch S2 is pressed. When switch S2 is pressed, transistor T1 is switched  ‘off’ to bring pin 4 of IC1 to logic ‘low’ and the alarm is disabled.

Source:   http://www.ecircuitslab.com/2012/01/latch-up-alarm-using-opto-coupler.html

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It is unnecessary for the customer to stop the use

It is unnecessary for the customer to stop the use of cell phone jammer .
There are some mobile phones which are having many advanced features and so their cost is high. However, you can find several cheap mobile phones available in the UK market online mobile phone shops give you the latest brands from the prominent manufacturers at a very competitive price offer. The worst of it is that others use it to send objective answers to those in the examination hall, which may not be correct at the end of the day. This can end the students career if caught, as exam malpractice is a punishable offense.
In the end, I had to fork out a sum of $200 to clean up the mould and to get the mobile phone functioning again some 3 months later, because by the time it was repaired, I was back in Australia and had to get a friend to retrieve it for me.
Now, the lesson of this experience is simple - Read the fine print of your warranty that comes with every purchase of your mobile phone.Just because what you buy is an established brand, never ever assume that its design is built to keep out the moisture or that it would not become mouldy inside, not even if it is a Sony. I was told it was standard policy to treat mouldy conditions within the mobile phone as a result of improper use of the mobile phone, rendering any warranty void.
While the mobile phone was a newer model, and I had always considered it an important part of my professional tools, there was never any case of mis-use of the mobile phone that could have allowed water to seep inside the mobile phone. If there was any mould, it would only have been the change of weather from the country of purchase and the country of use.
Let my experience be your guide, and pay attention to the fine print of your warranty so that you are sure whether mouldy conditions are covered under the warranty especially if you are going to use a mobile phone purchased overseas where there are different climatic conditions.
Avoid buying a lemon when you are shopping for a newer mobile phone. Visit your local Australian and New Zealand mobile phones online store Aussie Mobile Phones Store [http://aussie-mobilephones.reviewadvisors.info] for free resources and to view the latest in mobile phones for the Australian and New Zealand region. Updated daily, the website has a vast array of mobile phones on offer for the discerning user, and provides an online shopping experience with a difference.
Secondary school is a term used to describe an educational institution where the final stage of schooling, known as secondary education and usually compulsory up to a specified age, takes place. It follows elementary or primary education, and may be followed by university (tertiary) education.

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Cat and Dog Repeller

Nowadays, just about every house has an outside lamp with a motion sensor. Such a device eliminates the need to feel your way to the front door, and it apparently also scares away intruders. The only problem is that free-running dogs and cats in the neighborhood have little regard for such lamps and continue to deposit their excrement in the garden, once they have found a habitual location there for this purpose. This gave rise to the idea of connecting a sort of siren in parallel with the outside lamp to clearly advise dogs and cats that they are not welcome.

Naturally, it would be nice to avoid startling the entire neighborhood with this alarm signal. Here we can take advantage of the fact that dogs and cats have a significantly better sense of hearing than people. Not only are their ears more sensitive, they can also perceive significantly higher frequencies. With people, the upper limit is around 18 kHz, but dogs and cats can hear frequencies in excess of 20 kHz. We can take advantage of this by building a siren that emits a frequency just above 20 kHz.

Cat and Dog Repeller Circuit diagram :

This will scare off dogs and cats, but people will simply not hear it. All we need for this is an oscillator with an amplifier stage and a tweeter that can reproduce such high frequencies, such as a piezoelectric tweeter. The schematic diagram shows how easily this can be implemented. The power supply for the entire circuit is formed by the components up to and including C2. The 230-V leads are connected in parallel with the motion-sensor lamp. C1 and R1 provide capacitive coupling to reduce the 230 V to an acceptable voltage. 

A DC voltage of approximately 9.1 V is generated from this voltage using a bridge rectifier and D1, filtered and buffered by C2. The oscillator is built around R3, C3 and IC1a.The frequency of this oscillator is rather dependent on the specific characteristics of IC1, so the values shown here should be regarded as guidelines. If the oscillator frequency is too high, it can be reduced by increasing the value of R3 and/or C3.

If the frequency is too low (which means that the siren tone it is audible), the value of R3 and/or C3 should be increased. The square-wave signal from the oscillator is applied to the input of an H bridge composed of several Schmitt triggers in combination with the final output stages (T1–T4). This approach causes the peak-to-peak value of the square wave signal to be twice the supply voltage. As a result, a respectable 18 V is obtained across the piezoelectric tweeter, which is sufficient to produce a quite loud whistle tone. When building the circuit, you should bear in mind that it is directly powered from 230 V and not electrically isolated from the mains network.

It is thus necessary to avoid contact with all of the components when the circuit is in use. In practice, this means that the circuit must be fitted into a well-insulated, waterproof box. If you want to test the circuit, it is a good idea to first discharge C1 using a resistor, since it can hold a dangerous charge. You must also ensure that components F1, C1, R1 and B1 all have a mutual insulation separation of at least 6 mm!

Source : www.ecircuitslab.com/2011/05/cat-and-dog-repeller.html

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3V Low Battery Voltage Flasher

Many battery powered devices use two AA alkaline cells.  Often you will not know when it is time to replace the batteries until the device powered by them actually stops operating.  The hobby circuit below can be connected to a 3v battery, to give you some warning when the battery is nearing its end of life. It will flash a LED when the battery voltage drops to about 2.4 volts.

The electronic circuit draws only 1ua of current in standby mode and jumps to only 20ua when flashing, so it can safely be included without depleting the battery energy. A voltage detector IC from Panasonic (Microchip also makes similar devices) is used to monitor the battery voltage. The device’s open drain output swings low, when the battery voltage is below 2.4 to 2.5 volts. This action turns on the two transistor oscillator circuit, which drives the LED with short current pulses lasting only 2ms.

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Bass and Treble Controller Audio Equalizer Circuit

An audio equalizer circuit is used to adjust the frequency response of an audio signal. This is a simple equalizer circuit  for controlling the bass and treble (tone) of an audio amplifier. For use this equalizer circuit in amplifier, equalizers output should be given in the input of amplifier. So that the main input  audio signal’s bass and treble could be controllable before the amplify section.

Circuit Diagram of Bass & Treble Controller- Audio Equalizer:

Fig: Audio equalizer circuit for Bass, Treble control

As shown here the bass and treble controller circuit  has two variable resistor(VR1 & VR2) to control  the bass and treble. VR1 for Bass Control and VR2 for Treble Control.

This Bass and Treble controller circuit needed a  12Volt power supply. I would have designed it  in that way because 12V is used in most of the audio amplifier circuit.  and since equalizer circuit is used with audio amplifier, so there no extra power supply will required for this Equalizer.  This audio equalizer circuit is very easy to build and has a very good quality.

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Jogging Timer

3V Battery powered, Beeps after a fixed minutes delay

This circuit was developed since a number of visitors of this website requested a timer capable of emitting a beep after one, two, three minutes and so on, for jogging purposes. As shown in the circuit diagram, SW1 is a 1 pole 9 ways Rotary Switch. Setting the switch in position 1, the Piezo sounder emits three short beeps every minute. In position 2 the same thing happens after a 2 minutes delay, and so on, reaching a maximum interval of 9 minutes in position 9.

Circuit diagram:

Parts:

R1_____________47K 1/4W Resistor
R2_____________10M 1/4W Resistor
R3______________1M 1/4W Resistor
R4_____________12K 1/4W Resistor (see notes)
C1,C3__________10µF 25V Electrolytic Capacitors
C2____________100nF 63V Polyester Capacitor
D1___________1N4148 75V 150mA Diode
IC1____________4093 Quad 2 input Schmitt NAND Gate IC
IC2____________4060 14 stage ripple counter and oscillator IC
IC3____________4017 Decade counter with 10 decoded outputs IC
Q1____________BC337 45V 800mA NPN Transistor
SW1___________1 pole 9 ways Rotary Switch (see notes)
SW2___________SPST Slider Switch
BZ1___________Piezo sounder (incorporating 3KHz oscillator)
B1____________3V Battery (two 1.5V AA or AAA cells in series etc.)

Notes:

1. Needing only one time set, rotary switch can be replaced by an hard-wired link.
2. A DIP-Switch can be used in place of the rotary type. Please pay attention to use only one switch at a time, or the device could be damaged.
3. Varying R4 from 10K to 15K you can obtain more or less than three short beeps after the preset time delay.
4. To obtain a one-second beep only, after the preset time delay, disconnect pin 9 of IC1C from pin 9 of IC2 and connect it to pin 8 of IC1C.

Source: www.RedCircuits.com

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Bipolar Power supply for Battery Instruments Circuits Diagram

Bipolar Power supply for Battery Instruments Circuits Diagram. To generate regulated ± 5-V supplies from a pair of dry batteries, the circuit of Fig. 1 is commonly used. In order to give protection from inadvertent reverse connection of a battery, a diode in series with each battery would produce an unacceptable voltage drop. The more effective approach is to fit diodes Dl and D2 as shown in Fig. 2, in parallel with each battery. 

When the supply is switched off, there is the risk of a reverse bias being applied across the regulators, if there is significant inductance or capacitance in the load circuit.Diodes across the regulators prevent damage. When the power supply is switched on, the two switches do not act in unison. There is a probability that one or the other regulators will be latched hard off by the other. To prevent this, D3 and D4 are Zener diodes so that ± 5-V rails are pulled up by the batteries until the regulators establish the correct levels.

Bipolar Power supply for Battery Instruments Circuits Diagram

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DC or AC Voltage Indicator

Detects 1.8 to 230 Volts DC or AC, Minimum parts counting

This circuit is not a novelty, but it proved so useful, simple and cheap that it is worth building. When the positive (Red) probe is connected to a DC positive voltage and the Black probe to the negative, the Red LED will illuminate. Reversing polarities the Green LED will illuminate. Connecting the probes to an AC source both LEDs will go on.

The bulb limits the LEDs current to 40mA @ 220V AC and its filament starts illuminating from about 30V, shining more brightly as voltage increases. Therefore, due to the bulb filament behavior, any voltage in the 1.8 to 230V range can be detected without changing component values.

Circuit diagram:

DC or AC Voltage Indicator Circuit Diagram

Parts:

P1 = Red Probe
P2 = Black Probe
D1 = 5 or 3mm. Red LED
D2 = 5 or 3mm. Green LED
LP = 1220V 6W Filament Lamp Bulb

Note:

• A two colors LED (Red and Green) can be used in place of D1 & D2.

Source: Red Free Circuit Design

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Laser Power Supply

If you have ever worked with lasers, you know how fun and interesting it can be, you also know how expensive it can be. The high voltage power supplies for the laser tubes are often more expensive then the tubes themselves. This supply can be built with common parts, most of which you probably already have in your junk box. The secret is the transformer used. It is a common 9V 1A unit, connected backwards for step up.

Parts

Part           Total Qty. Description            R1    1 10 Ohm 10W Or Greater Resistor R2    1 Ballast Resistor, See "Notes" D1, D2, D3    3 1N4007 Silicon Diode C1, C2, C3    3 0.1 uF 2000V Capacitor T1    1 9V 1A Transformer S1    1 115V 2A SPST Switch MISC    1 Case, Wire, Binding Posts (for output), Line Cord Notes T1 is an ordinary 9V 1A transformer connected backwards for step up. R1 MUST be installed on a LARGE heatsink. A good heatsink is the metal case the supply is built in. R2 Protects the laser tube from excess current. It should be soldered directly to the anode terminal on the tube. To find R2, start with a 500K 10W resistor and work down until the tube lights and remains stable. If you have trouble with the tube not starting easily, use a longer anode lead that is wrapped around the tube. Depending on the transformer you use, the circuit may or may not work. I cannot guarantee the operation of this circuit. Build at your own risk. Some transformers contain very few secondary windings which will quickly saturate the core and basically act like a direct short. The more secondary windings (that is, primary in this circuit) the better.

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Automatic Emergency Light

This is the simple circuit of automatic emergency light. It is called dark dependent circuit. A 555 timer IC and LDR is used for this configuration. When light fall on the LDR, its resistance is decrease and in dark position its resistance is going high. Using these characteristics of LDR, automatic emergency light is made.

Automatic Emergency Light Circuit diagram:

LDR is called Light Dependent Resistor. Its resistance is inversely proportional to the falling light on its surface. In dark, its resistance is approximately 7k to 10k. When light fall on its surface, its resistance is decrease to less than 1k. In the circuit, a voltage divider arrangement circuit is used here with LDR and 10K variable resistor.

When light fall on the surface of LDR, the resistance decrease to 1k. So the voltage of the timer pin-6 is greater than 2/3 of +Vcc. For this reason the output pin-3 goes low. The base voltage of transistor BC547 is less than 0.75 volt. The transistor is OFF state. So the light is OFF. In dark position the LDR resistance increase is high so the voltage of the trigger pin-2 decrease to less than 1/3 of +Vcc. The output pin-3 goes high so the base voltage of the transistor BC547 is greater than 0.75 volt. The transistor is ON state. So the light is ON and it emits light.

Source: http://www.ecircuitslab.com/2012/06/simple-automatic-emergency-light.html

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LM317 VARIABLE POWER SUPPLY Circuit Diagram

A truly timeless circuit. LM317 is a versatile and highly efficient 1.2-37V voltage regulator that can provide up to 1.5A of current with a large heat sink. Its ideal for just about any application. This was my first workbench power supply and I still use it.

 LM317 VARIABLE POWER SUPPLY Circuit Diagram

Since LM317 is protected against short-circuit, no fuse is necessary. Thanks to automatic thermal shutdown, it will turn off if heating excessively. All in all, a very powerful (and affordable!) package, indeed.

Although LM317 is capable of delivering up to 37V, the circuit pictured here is limited to 25V for the sake of safety and simplicity. Any higher output voltage would require additional components and a larger heat sink.
Make sure that the input voltage is at least a couple of Volts higher than the desired output. Its ok to use a trimmer if youre building a fixed-voltage supply.

Possible uses:
Variable workbench power supply, fixed-voltage supply... Just about any possible application when no more than 1.5A is necessary.

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