Electronics

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## Introduction

This tutorial will teach you how to make an LED blink using only electronic components, specifically by using a 555 precision timer chip. A basic understanding of electronics is expected.

## Background Information

In previous articles, we made an LED blink by writing software running on a microprocessor based development board. We used the C++ like Sketch programming language in the Blink: Making An LED Blink On An Arduino Uno article. In the Blink: Making An LED Blink On A Raspberry Pi article, we used the Python, C, C++, and Swift languages. This time we will make an LED blink using only electronic components and no software.

We will be using a 555 precision timer integrated circuit, configured in the astable mode, to create an oscillator that will drive our blinking LED. See below for the schematic diagram.

In the astable mode, the frequency of the rectangular pulse generated by the 555 timer, along with the time spent at high and low voltage levels, is based on the values of R1, R2, and C1 that are determined by the following equations.

Output pulse frequency and period:

$$\large f=\frac{1}{ln(2)C_{1}(R_{1}+2R_{2})}$$

$$\large T=ln(2)C_{1}(R_{1}+2R_{2})$$

Time spent at high voltage:

$$\large T_{h}=ln(2)C_{1}(R_{1}+R_{2})$$

Time spent at low voltage:

$$\large T_{l}=ln(2)C_{1}R_{2}$$

If we select the component values as shown in the schematic of R1 = 1 KΩ, R2 = 680 KΩ, and C1 = 1 µF, we get a frequency (f) of 1.06 Hz, a period (T) of 943 ms, a high time (Th) of 472 ms, and a low time (Tl) of 471 ms. This means that the LED will blink about every second and will be lit about half the time.

For more information regarding 555 precision timer IC operation, please see this Wikipedia page. In addition, the 555 Astable Circuit Calculator is a good resource that can be used to refine the resistor and capacitor component values to suit your particular needs or preferences.

## Building The Circuit

Place the 555 chip and other components on a breadboard and wire the circuit according to the schematic shown above.

Specifically, attach a jumper wire connecting the positive bus strip on top of the breadboard to the positive bus strip on the bottom. Attach the negative bus strips in the same fashion. Now, place the 555 chip in the middle of the breadboard across the center gap. Connect a 1 KΩ resistor between pins 4 and 7 of the 555 chip. Connect the 680 KΩ resistor between pins 6 and 7 of the chip. Attach the positive side (longer lead) of the 1 µF electrolytic capacitor to pin 2 and the negative side (with minus sign) to the negative bus strip. Connect either side of the 10 nF ceramic capacitor to pin 5 and the other side to the negative bus strip. Attach jumper wires to connect pins 4 and 8 to the positive bus strip. Connect pin 1 to the negative bus strip. Connect pins 2 and 6 together with a jumper wire.

Attach the red LED to the breadboard by connecting the anode of the LED to one of the breadboard’s empty terminal strips and the cathode to the ground bus strip. The anode (positive terminal) of an LED is longer than the cathode (negative terminal). Connect one end of a 1 KΩ resistor to the terminal strip containing the red LED’s anode and the other end of the resistor to pin 3 of the 555 IC.

Connect the positive side (red) of the 9 V battery clip to one of the positive bus strips and the negative side (black) to one of the negative bus strips.

The circuit should look similar to the one below when completed.

Now let’s see if our circuit works. Connect a 9 V battery to the battery clip and our LED should start blinking. Nice!

Play around with the equations and test some other component values if you have them within the circuit. For instance, if we switch out the 680 KΩ resistor used above with a 470 KΩ resistor, the LED will blink at a faster rate with a period of around 652 ms. Make sure to disconnect the battery before making any modifications.

## Summary

In this tutorial we learned how to blink an LED with only electronic parts and no software was required. We used a 555 precision timer configured with a couple of resistors and capacitors in the astable mode (continuous oscillation) to drive an LED. We also learned how to adjust the timing of the 555 timer oscillation with a set of equations.

Thank you for joining me in this journey and I hope you enjoyed the experience. Please feel free to share your thoughts in the comments section below.

#### John Woolsey

John is an electrical engineer who loves science, math, and technology and teaching it to others even more.

He knew he wanted to work with electronics from an early age, building his first robot when he was in 8th grade. His first computer was a Timex/Sinclair 2068 followed by the Tandy 1000 TL (aka really old stuff).

He put himself through college (The University of Texas at Austin) by working at Motorola where he worked for many years afterward in the Semiconductor Products Sector in Research and Development.

John started developing mobile app software in 2010 for himself and for other companies. He has also taught programming to kids for summer school and enjoyed years of judging kids science projects at the Austin Energy Regional Science Festival.

Electronics, software, and teaching all culminate in his new venture to learn, make, and teach others via the Woolsey Workshop website.

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