Inspired by a new soldering iron and a surplus of orange LEDs, I decided that I wanted to build an LED flasher circuit. We’ve all seen two LEDs flash in an alternating pattern, but what about something a little more interesting? Oddly enough I found my answer on YouTube.
That looks pretty cool. There’s even a schematic, kind of. It’s actually just another video with a low resolution schematic picture. Here I present to you both a high rez image, and an Eagle schematic:
But I’m not done. I’ll even explain how the circuit works so someone might actually learn something.
The circuit requires 12 volts, but I ran it with 9 on my breadboard. The 555 IC is a timer, the speed of which is controlled by C1. The original schematic said 1uF, but I felt it was too fast and put in a 3.3uF cap instead. The output of the 555 (pin 3) goes into the clock input of the 4017 (pin 14.) The 4017 IC is a decade counter. It has 10 outputs (deca is the prefix for 10) which activate according to the clock input. The output pins are, in order: 3, 2, 4, 7, 10, 1, 5, 6, 9, 11. Pin 12 is a divide by 10 output, meaning that it is active (high) for the first half of the decade and inactive (low) for the second half. In this circuit, pin 12 isn’t used.
Pin 13 is the enable pin, meaning that the counter won’t count unless 13 is grounded. Pin 15 is reset, which does just what it sounds like. In our circuit, reset and 11 are connected together, so that the counter is self-resetting.
Q1 and Q2 are BD139 transistors. They are used as switches, so that the LEDs can draw lots of current and not fry the ICs. I plan on hooking up banks of 20 LEDs to each output. H1 and H2 (in the picture) are headers for this connection. You can replace these with LEDs as I have in the schematic. R7 also won’t be necessary for me since the LED banks will contain their own resistors.
When you plug it in, the 555 sends a timing signal to the 4017, which starts counting. The first pin to activate, 3, is attached to LED 1. The counter moves to pin 2, which isn’t attached to anything, so the LED turns off. Then it activates pin 4, which is also attached to LED 1, so it turns on again. Pin 7 is empty, so no light, so on and so forth. When the count reaches pin 9, it activates the reset, sending the count back to pin 3.
The charging of C1 affects the 555’s speed, and consequently, the speed of the 4017. To slow the speed, increase the value of C1. 10uF is really slow. 100uF is like watching paint dry. 0.47uF will give you seizures. 3.3 seemed just right for me, but feel free to experiment.
This schematic could easily be modified so that every output from the 4017 had an LED on it, and then you’d have an LED chaser. The 4017 and 555 are great ICs to have around, and can be had for less than a dollar each.