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555 Timer Circuits

Learn About 555 Timer Circuits and DIY Electronics

The 555 timer can be used for more than just timing circuits. The 8-pin IC is extremely versatile because it has built in two comparators, a three-resistor voltage divider, and an SR (set/reset) flip-flop. This allows for the 555 timer to be used as a Schmitt trigger in the right wiring configuration. The reason the 555 timer can thus produce a DC square wave from a triggering charging/discharging cycle of a capacitor is why it is also referred to as a Schmitt inverter. The rising and falling input of the saw-tooth waveform produced by the capacitor triggers the output alternate between max voltage and min voltage. This produces a steady “high” and “low” digital sequence that can be seen on the oscilloscope in the video below. The LM556 and LM558 are really just the dual and quad versions of the LM555 respectively and help consolidate your schematic layout when multiple timers are needed.

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Because we change the time constant of the circuits with the potentiometer by varying the resistance, we are able to adjust the frequency of the 555 timer circuits to increase and decrease the frequency of the output. The output is represented by a blinking LED and the digital square wave on the oscilloscope.


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555 Timers as Schmitt Triggers for PWM Circuits

Duty CycleBecause DC motors (also referred to as digital motors) only have the ability to be either on or off, the matter of variable speed becomes tricky unless a proper pulse width modulation (PWM) circuit is used. The 555 timer can be used in these circuits for this purpose very easily. Because you need to be able to vary your duty cycle to change the speed of a DC motor, the LM555 works well for this because of its built in Schmitt Trigger circuit. The duty cycle is what changes the amount of voltage the DC motor “perceives” and thus alters its angular velocity. Pulse width modulation is really a way to trick the motor into thinking it is receiving a voltage in between the “high” and “low” voltages. If you hook a motor directly to a 12V battery, you can have the motor turned off completely or spin at top speed only. Varying the pulses of 12V and 0V very rapidly (with a certain PWM frequency) is what “tricks” the motor into thinking it has a certain voltage across it in between the 12V and 0V values. The duty cycle is the ratio of “on” vs. “off”  or “high” vs. “low” period for each cycle.

In order to use the LM555 timer for PWM circuits, we need to take advantage of its Schmitt trigger capabilities as a function generator. We need the LM555 to output a digital sequence of highs and lows as seen on the oscilloscope, but we need to be able to vary the duty cycle instead of the frequency. This requires different types of 555 timer circuits where we utilize the Schmitt trigger as an oscillator where we can vary the percentage of time the output is high vs. when it is low.

The schematic below illustrates how we can utilize its properties to control the speed of a DC motor with the Schmitt trigger generating a PWM signal.

LM555 as a motor driver

If we have a 10% duty cycle (or ~10% the speed of the motor) and we have a 1 KHz PWM frequency, the motor will receive a 12V voltage across is for 10% of the time at 1000 times per second. The other 90% of the time, the motor is off and receives no voltage across it, hence the motor is spinning quite slow, but you can begin to see how we can now alter the speed of the motor to whatever works for our application. The ability to vary the speed of DC motors is critically important in the field of robotics, but it has its advantages in a myriad of applications. There are complete motor driver chips, such as the L298N (buy them here), that are much better and more specialized for this purpose than the LM555 chip because of built-in diode over-current and back EMF protection, PWM relay, and driving two motors forwards and backwards per L298N chip. However, using the 555 timer for this circuits application definitely helps with the understanding of how PWM and signal generation works.

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Check out the parts list here.

What You Need For Your 555 Timer Circuits With Flashing LED

  • 555 Timer IC
  • 10VDC Power Supply
  • 10uF Electrolytic Capacitor (16V or higher)
  • 1 kΩ Resistor
  • 470 Ω Resistor
  • 9 kΩ Potentiometer (or any variable resistor)
  • LED (whatever color you like!)
  • Red and Black Wires (solid core – 22 AWG)
Click here to find out where to get these items quickly for the best price.

How To Hook It Up

For those who want a quick explanation, just hook up the pins of the 555 timer illustrated in the basic schematic below. Match up the numbers of the pins with the numbered leads of the circuit elements as shown in this diagram. You will find a more detailed explanation and image of the finished breadboard below. You can also reference the LM555 datasheet for better explanations of the pin-outs.

The Schematic Of The Circuit

The semi-circular notch on the IC chip indicates the top of the chip. The pins are numbered from 1 to 8 in a counter-clockwise fashion starting with 1 at the top left (as shown).

555 Timer Circuit Schematic

The Fundamentals

You want to hook up this circuit exactly as shown and with the exact same components I used because I have experienced problems quickly if you stray from the specified components. You can make it work with other resistor and capacitor values, but you may need to do some calculations to see whether it will work. You will need to calculate your time constant (T = RC) to determine how your capacitor will charge and discharge to get the appropriate frequency of flashing. In the video, I explain that your time constant equals your total resistance multiplied by your total capacitance. Resistances add in series and in parallel you have:

Resistors In Parallel Equation

Where this equation gives you your total resistance. The resistor connected to the LED does not affect your time constant, but it is only there to keep your LED from frying!

Capacitors actually add in the exact opposite manner to resistors. Capacitance just add in parallel, and you can use the same equation above to add capacitance in series.

The 555 timer actually produces a square wave, like a serial clock used in digital logic, where a series of transistors inside the IC chip produce a sequence of on and off voltages at a certain frequency. Thus, 555 timer circuits are frequently used in digital logic.

Pin 1 on the timer chip is the ground (GND) pin that is connected to the common ground on your power supply. Pin 8 is where your power is supplied, and in our case it is 10VDC. The rest of the pins deal with the way that your voltage signals are timed, and this is impacted by your resistors and capacitors. Here is an illustration of what each pin is responsible for:

Pin Configuration
When you have everything properly hooked up, your 555 timer circuit should resemble something like the one shown below. If you are having issues, please remember to start by building it exactly the same way that is shown in this tutorial. Once you have gotten your circuit to work, you can experiment with changing the values of the resistors and capacitors, but 555 timers are finicky. If you change something and it stops working, you need to first draw out your schematic, make the appropriate calculations, and then see why your LED is not coming on or is not flashing. If you are getting frustrated with it, remember that these chips can burn out, so grab your DMM and test your pins for continuity! You want to test your pins for continuity on a 555 timer chip that you know works (like a shiny brand new one), and then write down what pins are connected to each other internally with very low impedance. Remember that this IC chip consists of several transistors on the inside.

Finished 555 Circuit
As mentioned in the video, 555 timer circuits are used for a variety of tasks. They play a big role in timing circuits in general, but they are also used for many other projects. Projects that you would normally think require a microcontroller can sometimes take advantage of only a 555 timer and some simple electronics components. What projects can you think of that you would like to build with them?

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