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The following information is a single lesson in a larger project. Find more great projects here.

Project Overview:

This course teaches the basics of the Breadboard, Schematic, and PCB editing in 123D Circuits using a simple Schmitt trigger oscillating circuit.

Step 1: Getting Started

It can be very exciting to have a circuit do something all by itself, like make an LED blink on and off. It takes few electrical components to make this happen, but there’s actually quite a bit going on.

An LED will light up when a large enough current is flowing through it. It will be off when little or no current is running through it. So the challenge here is to come up with a way to have a circuit automatically switch between driving and not driving the current through the LED.

  1. Continue to the next step.
  2. Stuck? HINT: Please don’t delete the unconnected components,. You will need them in Step 12.

Step 2: What Is an Oscillator?

You will need a way to have your circuit change what it is doing over time, in a repeating way. Simple circuits that do this are called oscillators. To make this blinking LED circuit you will want an oscillator that turns it on, then off, then on, then off, … and so on.

There are countless ways to make oscillators in electronics. One possible way to do this is to use a resistor, and a capacitor, along with a device called an inverting Schmitt trigger.

In this lesson, you will make this oscillating circuit to blink a LED. Along the way, you will learn about oscillators, and charging and discharging of capacitors.

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Step 3: The Schmitt Trigger

A Schmitt trigger has one input and one output. When the voltage at the input is in a certain range, the output turns fully on. When it is in another range, the output turns fully off. Otherwise, the output doesn’t change.

The circuit to the left contains an inverting Schmitt trigger microchip, labelled 74HC14. This microchip actually contains 6 inverting Schmitt triggers, but you’re only going to need one of them for this circuit. So, there are only four pins of this microchip that need to be wired up. The input to this Schmitt trigger is at pin 1 (blue wire). The output is at pin 2 (green wire). Power comes through pins 14 (red wire connected to +4.5 volts from the battery), and pin 7 (black wire connected to the battery return).

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Step 4: Testing the Schmitt Trigger

The output is configured to drive a green LED. The LED will light up when the Schmitt trigger output is fully on, and is dark when it is fully off.

You can see the other end of the blue wire is connected to an adjustable control circuit made up of a potentiometer (variable resistor). The potentiometer is configured to be able to adjust the voltage on its center pin anywhere from 0 volts to the voltage supplied by the battery (4.5 volts).

  1. Press "Start Simulation".
  2. Adjust the knob on the potentiometer to see what happens.
  3. Continue on to the next step

Step 5: Test Results

You may have noticed that the LED is on when the knob is turned to the left, and is off when the knob is turned to the right.

This is true, but an inverting Schmitt trigger is a little more complicated than that. It turns out that the output turns on anytime the input voltage is below about 35% of the supply voltage at pin 14 (4.5 volts). The output turns off anytime the input is above about 55% of the supply voltage. The output will stay where it is when the input voltage is between 35% and 55%.

Note that these levels can be different for different Schmitt triggers. This one happens to be at 35% and 55%.

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Step 6: Trigger Points

The 35% and 55% levels are known as trigger points, thus the “trigger” part of the name “Inverting Schmitt Trigger”. The “Inverting” part means that the output goes on when the input is in the low voltage region, and goes off when the input is in the higher region. The “Schmitt” part of the name is a tribute to the inventor of this circuit, Otto Schmitt.

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Step 7: Locating the Trigger Points

  1. Press "Start Simulation".
  2. Carefully adjust the potentiometer to locate the trigger points.
  3. Continue on to the next step.

Step 8: What Is Feedback?

You are going to make use of both the “inverting” and “trigger” aspects of this circuit to turn this into a working oscillator. You will accomplish this by using the output of the Schmitt trigger to alternately charge and discharge a capacitor. The charge on this capacitor will then be used to drive the input.

When the output of a circuit drives the input, this is called feedback.

When the input voltage is low, the output of the inverting Schmitt trigger is fully on. If this output is connected through a resistor to a capacitor to ground, then the capacitor will charge up until the voltage across it eventually matches the Schmitt trigger output voltage.

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Step 9: Capacitor Charging

The resistor is there to limit the current that is charging the capacitor. This acts to slow things down. The larger the resistor, the less current is allowed to flow to the capacitor.

Similarly, the larger the capacitance of the capacitor, the longer it takes to charge it because it can hold more electrical energy.

So changing the resistance or the capacitance will change the charging and discharging time of the capacitor.

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Step 10: Feedback + Capacitor Charge

What happens if you use the voltage on the capacitor to drive the input to the Schmitt trigger? From before, you know that when the voltage reaches the upper trigger level (around 55% of the battery voltage), the inverting Schmitt trigger output will turn off (drop to 0 volts).

At this point, the electrical energy that has built up on the capacitor will start flowing back through the resistor in the opposite direction. In other words, the voltage across the capacitor will start to drop. When the voltage across the capacitor drops to the lower trigger level of the Schmitt trigger (around 35% of the battery voltage), the output will again turn fully on, charging the capacitor, and the process will start all over again.

  1. Continue to the next step.

Step 11: How This Becomes an Oscillator

The result is that the output of the Schmitt trigger will turn on and off as the charge on the capacitor alternates between the two trigger levels. As was mentioned earlier, the amount of resistance and capacitance affects how long it takes for the capacitor to charge and discharge.

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Step 12: Making the Oscillator

It’s time to create the capacitor charging portion of the circuit and try it out.

  1. Remove the black wire below the left leg of the potentiometer by selecting it and then clicking the garbage can icon.
  2. Remove the red wire below the right leg of the potentiometer by selecting it and then clicking the garbage can icon.
  3. Move the extra resistor below the breadboard up so that the left side connects to the breadboard column below the right leg of the potentiometer, and the right side connects to the column below the second pin from the left (pin 2) of the Schmitt trigger microchip.
  4. Move the extra capacitor below the breadboard straight up so that its left pin connects to the breadboard column above the black wire, and its right pin connects to the column above the end of the blue wire.
  5. Press “Start Simulation”.
  6. Continue on to the next step

Step 13: Blink Speed

By following the blue wires, you can see that the charge on the capacitor is, indeed what is driving the input (pin 1) of the Schmitt trigger microchip.

You may have noticed that the potentiometer was reused. By itself, this potentiometer can vary from 0 Ohms to 100 kOhms. If it was the only resistor used to limit the current that is charging (and discharging) the capacitor, then adjusting the resistance to 0 Ohms would result in the capacitor charging and discharging very, very quickly.

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Step 14: Adjusting the Blink Speed

The value of resistors that are connected end-to-end (in series) simply add together. The 27 kOhm resistor that we connected in the previous step makes the combined resistance of the potentiometer variable between 0 + 27 = 27 kOhms and 100 + 27 = 127 kOhms. This makes sure that the LED isn’t blinking so fast that it just looks like it is always on.

The blinking of the LED can be made faster by turning the potentiometer to the right, and slower by turning it to the left. Try it.

  1. Press “Start Simulation”.
  2. Adjust the potentiometer to change the LED blink rate.
  3. Continue on to the next step.

Step 15: Finishing Up

You should now have an understanding of one way to create an oscillator using a Schmitt trigger. You have also learned something about the charging and discharging of capacitors.

Feel free to experiment with changing the values of the resistors and the capacitor, and seeing what happens to the speed of the LED blinking. Have fun!

In the next lesson you will learn to make a schematic for the circuit!

Next Lesson:Schematic - Schmitt Trigger Oscillator

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