40106 – Hex Schmitt Trigger Inverter

The 40106 has six (hex) inverter gates, built into one chip. An inverter is just something that inverts the signal of the input. If the input is LOW the output will be HIGH and vice versa.

All of the inverters on the chip are Schmitt triggers, which essentially means that they use hysteresis to filter out a noisy input signal into a clean and clear LOW/HIGH output.

Comparison of the action of an ordinary comparator and a Schmitt trigger on a noisy analog input signal (U). The output of the comparator is A and the output of the Schmitt trigger is B. The green dotted lines are the circuit’s switching thresholds. The Schmitt trigger tends to remove noise from the signal.

By FDominec – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=1860547

The actual part number we are using is CD40106BE.


The image below illustrates the symbol for a single Schmitt Trigger. The white circle on the output means that it is an inverting Schmitt trigger.

Simple LED Blinker with the 40106

Each of the six inverters of the chip can be used separately. We can very easily build an oscillator with just one capacitor and one resistor. A larger value capacitor will create a longer delay which is useful for controlling LEDs. A smaller capacitor will be more useful for creating sounds.


Click the image for an interactive simulation

The parts you will need:

  • a breadboard
  • some jumper wires
  • power supply (we use 5 AA batteries, which is about 7.5V when fully charged)
  • 40106 chip
  • 10 μF capacitor (10 microfarads)
  • 100 kΩ potentiometer (this could be also any other fixed or variable resistor)
  • LED (use whatever colour or shape you like)
  • 470 Ω resistor (the value of this depends on the LED an the power supply voltage)

#1 Power connections

  1. Insert the chip (40106 to your breadboard. Make sure to have the little notch pointing to the left as shown in the image.
  2. Connect pin 14 to VCC (the positive terminal of your power supply, in our case it’s +7.5V DC)
  3. Connect pin 7 to GND (ground/the negative terminal of your power supply. In our case it will be 0V DC)
  4. Connect both sides of the power rails of the breadboard together. (Plus to plus and minus to minus)
  5. The red and black wires on the top right corner go to the battery pack or whatever power supply you are using. (But don’t connect them yet. Let’s finish the circuit and double check everything is correct before turning on the power)

#2 Capacitor

  1. Connect the 10 μF capacitor between pin 1 and the ground.
  2. Make sure the side of the capacitor marked with a stripe goes to ground. If you connect it backwards, it will pop like a popcorn when you turn on the power, and the magic smoke that makes all electronics work will escape.

#3 Potentiometer

  1. Connect the two legs (one side and the middle) of the potentiometer to pins 1 and 2 of the chip.
  2. Make sure you check where the legs of the potentiometers actually connect to. You might need to also adjust the capacitor placement a little bit.

#4 LED

  1. Connect the LED. The long leg (anode) of the LED should connect to the output of the oscillator (pin2).
  2. The short leg (cathode) of the LED should connect to the GND through the 470Ω resistor.
  3. Connect the power supply and try it out! You should be able to control the blinking using the

Improved LED Blinker with the 40106

The previous circuit will work, but if you take a closer look at the signal being generated with an oscilloscope, you will notice two problems.

  1. If you turn the potentiometer all the way down, the output freaks out. This is because the resistor value is all the way down to zero in that position.
  2. The output is not actually a square wave if you have the LED connected directly to the output. You can try comparing the signal with and without the LED to see the difference. This doesn’t really matter in this case, but later we are going to want to make a clock signal for a sequencer with an LED for feedback from the same output. In that situation it would be nicer to have a cleaner output.

Both of these issues can be easily fixed:

  1. Add a fixed resistor in series with the potentiometer to have some resistance even when the potentiometer is al the way down. I used a 10kΩ resistor
  2. Feed the output from pin 2 of the chip to pin 3, and the connect the LED to pin 4. This inverts the signal, but also converts it into a nice square wave for the LED.

Audio Oscillator with the 40106

For an audio oscillator you just need to increase the frequency by switcing the capacitor to a lower value (I’m using 1 μF here, but try different values).

Combining Multiple Oscillators

You can also start creating multiple oscillators and combining them together in various ways. For example, we could use one low frequency oscillator (LFO) to modulate another oscillator.


This is just a basic example. There are lots of other ways to use the simple oscillators. See the links below for some possibilities.

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