LED Chasers

 

 

 

Above. This is the schematic for a very basic 10 LED chaser I built. I prefer my "chasers" to run more slowly than most people and used a 10 uF capacitor for C1. The 10K pot can reduce the flash speed from not moving to whatever minimum time constant is possible with the C1 value you choose. Don't bother with ultra-bright LEDs for these "chaser" projects as cheaper, lower millicandela (mcd) LEDS work fine. I favor blue and green LEDS. The 4017 decade counter is a fabulous part and can be driven to flash a row of LEDs with a 555 timer chip or a discrete BJT multivibrator

 

 

LED Flasher Projects

Introduction

 

LED flashers are great fun for the novice or seasoned builder!.
In modern times, analog circuits such as the astable BJT multivibrator or a 555 timer IC driving 4017 decade counter(s) are being replaced with small microprocessor-based circuits such as the PIC Microcontroller. The QRP/SWL HomeBuilder web site has featured a number of multivibrator and LED flasher circuits over the past decade and each year at least 40 - 50 emails are received requesting help, specific designs or calling for more projects. Thus, another page of LED flasher circuits was added and building these circuits was quite enjoyable.

Lower millicandela LEDS were used for all the experiments. The 2N3904 transistor was used exclusively and the schematics do not indicate this part. By request, this web page will avoid using a lot of math

 

 

 

 

 

Basic Multivibrator Flasher

 

To the left, in schematic 1 is the basic 2 LED astable multivibrator flasher built by Mary. She chose to use 2 different colored LEDs and the red LED is clear when unlit. It is quite bright when lit compared to the yellow LED despite the fact that it only draws 0.5 mA more. The bread boarded test circuit was powered by a new 9 volt battery and was regulated by a L78L05 (in a TO-92 package as shown in the schematic). The 5 volt regulator was used to avoid exceeding the reverse breakdown voltage of the 2N3904. This topic will be discussed a little later on.

470 ohm current dropping resistors were chosen to keep the collector current draw less than 10 mA. The LEDS were bright enough to see well in dim lighting. You may change this resistor "R" value (lower R = brighter), but do not exceed the maximum current rating for the LED or transistor (this is more applicable to higher voltage multivibrators). You may also place 2 or more LEDs in series on each half of a multivibrator, however, the current dropping resistor may need to be reduced to maintain brightness. Consider using a power supply as opposed to battery power for your flashers.

To change the pulse (oscillation) frequency, you can change the base resistor or the timing capacitor values. For example, increasing the capacitor or the base resistor values will increase the time OFF per cycle and thus reduce the oscillation frequency. The oscillation frequency is 60 divided by the sum of the time OFF for each half of the multivibrator. Do not feel you have to use the same timing capacitor for each 1/2 of the multivibrator. Multivibrators with different timing components on each 1/2 are termed asymmetrical.

Over time, some builders sent me emails that they could not get their multivibrator to run. I problem-solved with them and discovered many problems including bad parts, bread boarding errors, the oscillation frequency was too fast to observe, transistors were not saturated during their ON time and failure of the transistors due to excessive current or perhaps even reverse emitter-base breakdown.

 

 

 

Basic Astable 555 Timer IC Flasher

For a lower parts count than the 2 transistor multivibrators, 2 LEDs can be alternately flashed with a 555 integrated circuit configured as shown in Schematic 2.  I chose the combination of a 2K2 and a 47K resistor to determine the oscillation frequency along with the 10 uF capacitor connected to pins 2 and 6. You can practically change the (R Speed) 47K value to between 10K and 100K or more. Greater resistance = lower speed. You may also wish to connect up a 100K or so potentiometer instead of the 47K resistor for a variable speed version. Additionally, the 10 uF capacitor value can be changed. Feel free to experiment.

Although, alternately flashing LEDs  is great for the beginner to electronics, the basic one ON, one OFF circuit gets boring quickly. In the next section, we will try to improve the look and try to approximate a flash like a police car (within limits).