Introduction: Oscillator Circuits for the 555 Timer

By
About: Jack passed away May 20, 2018 after a long battle with cancer. His Instructables site will be kept active and questions will be answered by our son-in-law, Terry Pilling. Most of Jack's instructables are tuto…

In my first Instructable about the 555 timer chip (Introduction to the 555 timer) I demonstrated a 555 in an astable oscillator.

I would like to thank Mike63 for sending me a couple of useful formulas to calculate the frequency and the low time.

  • Frequency = 1 / (0.693 X C1 X (R1 + 2 X R2))
  • Low Time = 0.693 X C1 X R2

I googled the formulas and found them on the internet just as he sent them to me, but the answer seemed to have the decimal point in the wrong place. The formulas expect to have the capacitance in Farads. When I put multipliers in the formulas I got the right answers.

In this instructable I will be exploring the astable oscillator and demonstrating a couple more interesting 555 oscillator circuits.

To construct these circuits you will need:

.

* A 10 uF capacitor is enough to get you started, but you will probably want to experiment with different values. This capacitor kit will give you plenty of different values. Or use what you have. I just grabbed a handful of different capacitors from my parts bin.

The resistor and capacitor values I show in all these circuits are only starting points. Try different values to see what effect they have on the frequency of the oscillator. Don't be afraid to experiment.

If you don't have a meter or an oscilloscope don't let that stop you. You can still make the circuits and blink the LEDS at different frequencies. The high frequency oscillator will light both LEDs continuously.

Step 1: Measuring and Calculating the Frequency of an Astable Oscillator

To build the circuit follow the diagram in the first picture. The wires at the top are the meter or oscilloscope leads. The alignment dot on the chip is to the upper right.

The resistor mounted between pin 7 and the VCC rail is R1 and the 10 K pot is R2.

Start with a 10 uF electrolytic capacitor and R1 is 4.7K. Adjusting the pot will give you a frequency range of about 5 - 30 Hz.

You will notice that the red LED indicating that the output pin is low lights up for a shorter time than the green one.

Plug in your meter and/or oscilloscope. Go across the green LED and it's resistor.

You can measure the frequency. If you have an oscilloscope you can see the wave form like in the second picture..

The LibreOffice Calc spreadsheet named 555.ods is a tool to calculate the frequency.

Download it, run it and you can see the spreadsheet says the frequency range should be 5 - 30 Hz.

The calculated frequency is only approximate, capacitors can have a wide tolerance.

Experiment with different capacitors and resistors and notice the difference in the frequency. At frequencies above 30-40 Hz both LEDs will appear to be on at the same time, but it is still oscillating.

The fourth picture shows the pinouts of the 555 chip and an explanation of how the circuit works.

Attachments

Step 2: High Frequency Oscillator

The advertised maximum frequency of the 555 is about 360 KHz. This Oscillator will sometimes work at a slightly greater frequency.

This circuit is very similar to the first circuit.

Replace the 10 uF electrolytic with a 1 nF disk capacitor. This is a very small capacitor. The marking on it says 102.

Add a 100 nF disk capacitor between pin 5 and ground. This is another very small capacitor. The markings on it say 104. You may need a magnifying glass to read the markings on small disk capacitors, I do.

Replace the 4.7 K resistor (R1) with a 2.2 K resistor.

You now have a high frequency oscillator. With my parts I could adjust the frequency from 64 KHz to 395 KHz. Your results may vary. Capacitors can have a wide tolerance.

Step 3: Voltage Controlled Oscillator

In this oscillator the discharge pin (7) is not used and a potentiometer is connected between the VCC (8) and Ground (1) pins. The middle pin is connected to the control (5) pin. The capacitor is discharged through the output pin when it goes low.

The potentiometer connected to pin 5 controls the frequency. It also changes the high and low time. When the voltage is high the high time is greater and the frequency is lower. When it is low the low time is greater and the frequency is higher. Without the 2.2K and 1.5 K resistors the high and low times will vary from zero to one hundred percent and the oscillation will stop at both ends of the potentiometer travel. Try it, replace the resistors with wires.

With the 1 K resistor and the 10 uF capacitor the two 10 K pots can vary the frequency from about 2.5 Hz to 100 Hz. The effect of changing the resistance and/or capacitance on the frequency is linear.

Build the circuit according to the diagram and experiment with it. The wires at the top of the diagram are the meter or oscilloscope leads.