30 LED Projects

30 LED Projects

(Parte 2 de 3)

These 7 circuits flash a LED using a supply from 1.5v to 12v. They all have a different value of efficiency and current consumption. You will find at least one to suit your requirements. The simplest way to flash a LED is to buy a FLASHING LED as shown in figure A. It will work on 3v to 9v but it is not very bright - mainly because the LED is not high-efficiency. A Flashing LED can be used to flash a super-bright red LED, as shown in figure B. Figure C shows a flashing LED driving a buffer transistor to flash a white LED. The circuit needs 4.5v - 6v. Figure D produces a very bright flash for a very short period of time - for a red, green, orange or white LED. Figure E uses 2 transistors to produce a brief flash - for a red, green, orange or white LED. Figure F uses a single cell and a voltage multiplying arrangement to flash a red or green LED. Figure G flashes a white LED on a 3v supply.


This circuit will illuminate a white LED using a single cell. See LED Torch Circuits article for more details.


This circuit will illuminate two white LEDs using a single cell. See LED Torch Circuits article for more details.

This circuit will flash a white LEDs using a single cell. See LED Torch Circuits article for more details.

In the diagram, it looks like the coils sit on the “table” while the magnet has its edge on the table. This is just a diagram to show how the parts are connected. The coils actually sit flat against the slide (against the side of the magnet) as shown in the diagram: The output voltage depends on how quickly the magnet passes from one end of the slide to the other. That's why a rapid shaking produces a higher voltage. You must get the end of the magnet to fully pass though the coil so the voltage will be a maximum. That’s why the slide extends past the coils at the top and bottom of the diagram.

The circuit consists of two 600-turn coils in series, driving a voltage doubler. Each coil produces a positive and negative pulse, each time the magnet passes from one end of the slide to the other. The positive pulse charges the top electrolytic via the top diode and the negative pulse charges the lower electrolytic, via the lower diode. The voltage across each electrolytic is combined to produce a voltage for the white LED. When the combined voltage is greater than 3.2v, the LED illuminates. The electrolytics help to keep the LED

illuminated while the magnet starts to make another pass

The LED in this circuit will detect light to turn on the oscillator. Ordinary red LEDs do not work. But green LEDs, yellow LEDs and high-bright white LEDs and high-bright red LEDs work very well. The output voltage of the LED is up to 600mV when detecting very bright illumination. When light is detected by the LED, its resistance decreases and a very small current flows into the base of the first transistor. The transistor amplifies this current about 200 times and the resistance between collector and emitter decreases. The 330k resistor on the collector is a current limiting resistor as the middle transistor only needs a very small current for the circuit to oscillate. If the current is too high, the circuit will "freeze." The piezo diaphragm does not contain any active components and relies on the circuit to drive it to produce the tone.


This circuit is so sensitive it will detect "mains hum." Simply move it across any wall and it will detect where the mains cable is located. It has a gain of about 200 x 200 x 200 = 8,0,0 and will also detect static electricity and the presence of your hand without any direct contact. You will be amazed what it detects! There is static electricity EVERYWHERE! The input of this circuit is classified as very high impedance.

Here is a photo of the circuit, produced by a constructor.

LEDs on 240v

I do not like any circuit connected directly to 240v mains. However Christmas tress lights have been connected directly to the mains for 30 years without any major problems. Insulation must be provided and the lights (LEDs) must be away from prying fingers. You need at least 50 LEDs in each string to prevent them being damaged via a surge through the 1k resistor - if the circuit is turned on at the peak of the waveform. As you add more LEDs to each string, the current will drop a very small amount until eventually, when you have 90 LEDs in each string, the current will be zero. For 50 LEDs in each string, the total characteristic voltage will be 180v so that the peak voltage will be 330v - 180v = 150v. Each LED will see less than 7mA peak during the half-cycle they are illuminated. The 1k resistor will drop 7v - since the RMS current is 7mA (7mA x 1,0 ohms = 7v). No rectifier diodes are needed. The LEDs are the "rectifiers." Very clever. You must have LEDs in both directions to charge and discharge the capacitor. The resistor is provided to take a heavy surge current through one of the strings of LEDs if the circuit is switched on when the mains is at a peak.

This can be as high as 330mA if only 1 LED is used, so the value of this resistor must be adjusted if a small number of LEDs are used. The LEDs above detect peak current. A 100n cap will deliver 7mA RMS or 10mA peak in full wave or 3.5mA RMS (10mA peak for half a cycle) in half-wave. (when only 1 LED is in each string).

The current-capability of a capacitor needs more explanation. In the diagram on the left we see a capacitor feeding a full-wave power supply. This is exactly the same as the LEDs on 240v circuit above. Imagine the LOAD resistor is removed. Two of the diodes will face down and two will face up. This is exactly the same as the LEDs facing up and facing down in the circuit above. The only difference is the mid-point is joined. Since the voltage on the mid-point of one string is the same as the voltage at the mid-point of the other string, the link can be removed and the circuit will operate the same. This means each 100n of capacitance will deliver 7mA RMS (10mA peak on each half-cycle). In the half-wave supply, the capacitor delivers 3.5mA RMS (10mA peak on each half-cycle, but one half-cycle is lost in the diode) for each 100n to the load, and during the other half-cycle the 10mA peak is lost in the diode that discharges the capacitor. You can use any LEDs and try to keep the total voltage-drop in each string equal. Each string is actually working on DC. It's not constant DC but varying DC. In fact is it zero current for 1/2 cycle then nothing until the voltage rises above the total characteristic voltage of all the LEDs, then a gradual increase in current over the remainder of the cycle, then a gradual decrease to zero over the falling portion of the cycle, then nothing for 1/2 cycle. Because the LEDs turn on and off, you may observe some flickering and that's why the two strings should be placed together.

SINGLE LED on 240v

A single LED can be illuminated by using a 100n or 220n capacitor with a rating of 400v. These capacitors are called "X2" and are designed to be connected to the mains.

happens to be at a peak

The LED will be 240v above earth if the active and neutral are swapped and this represents a shock of over 340v if anything is exposed. The power diode in the first diagram is designed to discharge the 0.22u during one half of the cycle so that the capacitor will charge during the other halfcycle and deliver energy to the LED. The 1k resistor limits the peak in-rush current when the circuit is first turned on and the mains

Two LEDs can be driven from the same circuit as one LED will be illuminated during the first half cycle and the other LED will be driven during the second half of the cycle.

LEDs can also be connected to the mains via a power diode and current-limiting resistor. But the wattage lost (dropped) in the resistor is about 2.5 watts and a 3 watt resistor will be needed to illuminate a 70mW white LED. This is an enormous waste of energy and a capacitor-fed supply shown above is the best solution.

The circuit illuminates a column of 10 white LEDs. The 10u prevents flicker and the 100R also reduces flicker.

FLASHING RAILROAD LIGHTS This circuit flashes two red LEDs for a model railway crossing.

This circuit will adjust the brightness of one or more LEDs from 5% to 95%.

Some 3-leaded LEDs produce red and green. This circuit alternatelyflashes a red/green bi-coloured LED:

Some 2-leaded LEDs produce red and green. These are called Bi-polar LEDs. This circuit alternatelyflashes a red/green bi-polar LED:

This is a simple driver circuit that drives the 3 LEDs in an RGB LED to produce a number of interesting colours. Even though the component values are identical in the three oscillators, the slight difference in tolerances will create a random display of colours and it will take a while for the pattern to repeat. The colours change abruptly from one colour to another as the circuit does not use Pulse Width Modulation to produce a gradual fading from one colour to another. This LED is called COMMON ANODE. This has been done so it can be connected to transistors or other devices that "SINK." The second circuit a common cathode LED. Note the different pinout.

This LED flashes at a fast rate then a slow rate. It only requires a current-limiting resistor of 100R for 4.5v to 6v supply or 470R for 7v to 12v supply. This LED is available from:http://alan-parekh.vstore.ca/flashing-5000mcd-p-8.html for 80 cents plus postage.

There are two different types of RGB LEDs. The RGB LED Driver circuit above uses an RGB LED with 4 leads and has 3 coloured chips inside and NOTHING ELSE. The LED described in the video has 2 leads and requires a dropper resistor so that about 20mA flows. The

LED also contains a microcontroller producing PWM signals. If you cannot get the 2-leaded LED, you can use a 4-leaded LED plus the circuit below. It is an analogue version of the circuit inside the self-flashing LED, for the slow-rate:

As with everything Chinese, the self-flashing LED is too gimmicky. It is better to produce your own colour-change via the circuit above. You can alter the rate by changing the value of the components and/or remove one or more of the 100u's. The circuit for a common cathode RGB LED is shown in the RGB LED Driver above.

In the Knight Rider circuit, the 5 is wired as an oscillator. It can be adjusted to give the desired speed for the display. The output of the 5 is directly connected to the input of a Johnson Counter (CD 4017). The input of the counter is called the CLOCK line.

The 10 outputs Q0 to Q9 become active, one at a time, on the rising edge of the waveform from the 5.Each output can deliver about 20mA but a

LED should not be connected to the output without a current-limiting resistor (330R in the circuit above). The first 6 outputs of the chip are connected directly to the 6 LEDs and these "move" across the display. The next 4 outputs move the effect in the opposite direction and the cycle repeats. The animation above shows how the effect appears on the display. Using six 3mm LEDs, the display can be placed in the front of a model car to give a very realistic effect. The same outputs can be taken to driver transistors to produce a larger version of the display.

The Knight Rider circuit is available as a kit for less than $15.0 plus postage as Kitt Scanner.

Here is a simple Knight Rider circuit using resistors to drive the LEDs. This circuit consumes 22mA while only delivering 7mA to each LED. The outputs are "fighting" each other via the 100R resistors (except outputs Q0 and Q5).

Here's a clever circuit using two 5's to produce a set of traffic lights for a model layout. The animation shows the lighting sequence and this follows the Australian-standard. The red LED has an equal on-off period and when it is off, the first 5 delivers power to the second 5. This illuminates the Green LED and then the second 5 changes state to turn off the Green LED and turn on the Orange LED for a short period of time before the first 5 changes state to turn off the second 5 and turn on the red LED. A supply voltage of 9v to 12v is needed because the second 5 receives a supply of about 2v less than rail. This circuit also shows how to connect LEDs high and low to a 5 and also turn off the 5 by controlling the supply to pin 8. Connecting the LEDs high and low to pin 3 will not work and since pin 7 is in phase with pin 3, it can be used to advantage in this design.


This circuit produces traffic lights for a "4-way" intersection. The seemingly complex wiring to illuminate the lights is shown to be very simple.

to Index

The 5 is capable of sinking and sourcing up to 200mA, but it gets very hot when doing this on a 12v supply. The following circuit shows the maximum number of white LEDs that can be realistically driven from a 5 and we have limited the total current to about 130mA as each LED is designed to pass about 17mA to 22mA maximum. A white LED drops a characteristic 3.2v to 3.6v and this means only 3 LEDs can be placed in series.

3x3x3 CUBE

This circuit drives a 3x3x3 cube consisting of 27 white LEDs. The 4020 IC is a 14 stage binary counter and we have used 9 outputs. Each output drives 3 white LEDs in series and we have omitted a dropper resistor as the chip can only deliver a maximum of 15mA per output. The 4020 produces 512 different patterns before the sequence repeats and you have to build the project to see the effects it produces on the 3D cube.

These two circuits make a LED fade on and off. The first circuit charges a 100u and the transistor amplifies the current entering the 100u and delivers 100 times this value to the LED via the collector-emitter pins. The circuit needs 9v for operation since pin 2 of the 5 detects 2/3Vcc before changing the state of the output so we only have a maximum of 5.5v via a 220R resistor to illuminate the LED. The second circuit requires a very high value electrolytic to produce the same effect.


The circuit fades the LED ON and OFF at an equal rate. The 470k charging and 47k discharging resistors have been chosen to create equal on and off times.

This circuit can be used to indicate left and right turn on a motor-bike. Two identical circuits will be needed, one for left and one for right.

These three circuits flash the left LEDs 3 times then the right LEDs 3 times, then repeats. The only difference is the choice of chips.

LED DICE with Slow Down

This circuit produces a random number from 1 to 6 on LEDs that are similar to the pips on the side of a dice. When the two TOUCH WIRES are touched with a finger, the LEDs flash very quickly and when the finger is removed, they gradually slow down and come to a stop. LED Dice with Slow Down kit is available from Talking Electronics.

(Parte 2 de 3)