Led circuits

In the early '80s, I read some articles on LED arrays. They described using arrays of 64 or more LEDs as crude oscilloscopes and other interesting things. I decided I'd make one myself - a really big array, 256 LEDs arranged as 16 columns by 16 rows. In 1983, while I was in my final semester of electronics classes at Monterey Peninsula College, I designed my array. I wanted a high-density array, in part to make the PC board smaller, but mainly to give a better appearance. So, I used the smallest LEDs I could find, 2mm x 2.5mm. I was lucky and found that All Electronics was selling them in 200-packs for something like $20. I bought two packs - a fair sum for me at the time.

This circuit uses a set of 13 differently colored LEDs to generate a full color spectrum. The photo does not fully represent the colors generated due to camera limitations. The real-world display is very eye-catching. If you want to "trick out" your PC, this circuit is for you. Forget about those boring blue PC light displays.

There are many 9V chaser circuits that seem to waste about 7V when driving LEDs that are only about 2V. This project is unique, because it uses only two inexpensive alkaline battery cells totaling 3V for power. Since most of the waste is eliminated, the cells last a long time. Unlike the other circuits, this one flashes the LEDs for only about 30ms each, further extending the battery life. For user convenience, it has a stepper speed control and a brightness control. At slower speeds and with reduced brightness, the battery life is further extended considerably. Mounted in a circle, the LEDs appear to rotate as they step from one to the next.

This sign I designed uses no microprocessor. It has an eprom and multiple counters. As in most electric signs, the LEDs are matrixed, and strobed very quickly to make it possible for all 70 LEDs to appear lit. This sign is strobed horizontally, unlike most large signs which are strobed vertically. I did it this way because electrically it was simpler. The eprom has 8 outputs, of which I used 7 of them to drive the 7 horizontal rows. The eprom outputs are not strong, so they are buffered.

The circuit in Figure 1 uses a PIC16C55 µP to read 10-bit binary data and directly display the data in decimal format on three common -cathode LED displays. If value of the data exceeds 999, the displays show three hyphens (---) to represent overflow. The PIC16C55 has high drive capability. Each output pin can source 20 mA and sink 25 mA; the outputs can thus directly drive the LED display.

The 8-lead plastic mini-DIP LM3909 IC was developed by National Semiconductor in the mid 'seventies of the past century. It was a monolithic oscillator specifically designed to flash Light Emitting Diodes. By using the timing capacitor for voltage boost, it delivered pulses of 2 or more volts to the LED while operating on a supply of 1.5V or less. The circuit was inherently self-starting, and required addition of only a battery and capacitor to function as an LED flasher. Unfortunately, since 1998, the manufacturer discontinued the production of this chip. For this reason, and on request of some correspondents, I tried to emulate this IC operation using common discrete components, obtaining unexpected satisfactory results.

The circuit presented here uses bicolour LEDs to generate a display in three colours, namely, red, green, and yellowish green. Transistors T1 through T20 form a grid to which common-cathode bicolour LEDs (LED1 through LED10) are connected. Transistors T1 through T10 have their collector terminals connected to the emitter of transistor T21. Similarly, transistors T11 through T20 have their collector terminals connected to the emitter of transistor T22.

This project flashes eight LEDs in an apparently random manner. It uses a 4060 combined counter and display driver IC which is designed for driving 7-segment LED displays. The sequence is not really random because seven of the LEDs would normally be the display segments, the eighth LED is driven by an output that is normally used for driving further counters. The table below shows the sequence for the LEDs. You can use less than eight LEDs if you wish and the table may help you decide which ones to use for your purpose.

This circuit adopts the rather unusual Bowes/White emitter coupled multivibrator circuit. The oscillation frequency is about 1Hz and is set by C1 value. The LED starts flashing when the photo resistor is scarcely illuminated. The onset of flashing can be set by trimming R2.

A very simple LED flasher circuit. With HT-2014L the circuit produces 1 flash per second and the HT-2014M produces 2 flashes per second.

Many published circuits that flash LEDs need 3 volts or more. This circuit uses only a single inexpensive C-MOS IC and flashes the LED for a full year on a single 1.5 volt AA alkaline battery cell. The circuit uses a charge pump technique to provide the LED the needed voltage.

To squeeze even more energy from a alkaline battery cell, this circuit adds two transistors to a circuit similar to the above design to boost the efficiency. A small 1.5 volt alkaline N cell should flash the LED for a full year. It too uses a "charge pump" technique to provide a LED the needed voltage.

Although white LEDs are common in a variety of lighting applications, their 3 to 4V forward-voltage drop makes low-voltage applications challenging. Charge pumps and other ICs are available for driving white LEDs, but they generally don't work with the low supply voltage of 1.5V in single-cell-battery applications. The low-voltage circuit of Figure 1 provides a current-regulated output suitable for driving white LEDs. The boost converter, IC1, can supply load currents to 62 mA with input voltages as low as 1.2V, making it suitable for use with a 1.5V, single-cell battery.

I use this circuit for general use to power white LEDs, like a headset light I use on my desk and a keyboard light for my computer. I built it into a tiny Radio Shack project box (270-288), which they no longer sell. I put heat-shrink tubing around the female output connector (RS P/N 273-1743), so that the contacts are not exposed, but you can still plug and unplug it.

This is a spectacular but completely useless project. It lights Ultra-Bright LEDs in a sequence and each LED flashes brightly very briefly. The LEDs light-up going around and around since they are mounted in a circle (on a CD), then they pause before chasing again. The very brief flash of each LED (15ms) and the pauses (1 second) reduce the average current so the battery should last a long time.

A Very Simple circuit and some Variations. Play with it. All parts are cheap and should be easily obtained. None are very critical.

This circuit is designed to test visible and infrared LEDs in pulsed mode operations. It can drive the LED with peak currents in excess of 10 amps. A light detector nearby can monitor the response time and intensity of the LED under test.

The completed board may be driven by voltages between .8 and 3 Volts. While the basic design goal was ‘candle like light from a single cell’, the values used were chosen to allow safe operation from 3 Volts so you can, for example, use it to ‘drain the last bit of energy’ out of used Lithium based cells from flashlights. This means that you can also drive it from a fresh 123 cell at full brightness (for 16 hours or so) or two NiMH, alkaline or other cells in series. While the current consumption (and therefore light output) goes down with supply voltage, some light is produced even with very low inputs.

When the circuit is first powered on LED's D2 and D4 light just to indicate the circuit is operating. When the start button (SW2) is pressed all the LED's turn off. They then illuminate sequentially at one second intervals until all five LED's are on. After a random interval between 0 and about 7 seconds the LED's extinguish, signaling the start of the race. Once the LED's have extinguished simply press the start button again to initiate another race start sequence.

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