# Schmitt Trigger Synthesizer

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A Simple Synthesizer using a Schmitt trigger

For this circuit, you may need to connect the audio jack to a guitar amp. A regular stereo out may not have enough gain to hear the signal.

A Schmitt trigger is a type of threshold circuit with positive feedback. The circuit is named "trigger" because the output retains its value until the input changes sufficiently to trigger a change. A Schmitt trigger is a bistable multivibrator; in its inverting configuration it can be used as an oscillator. The integrated circuit chip we are using is called a hex Schmitt trigger because it has six inverters on one chip. For this exercise, you can use 74C14 or CD40106, they are both Hex Schmitt Triggers.

Single inverter

• pin 14 goes to voltage source
• pin 7 goes to ground
• R1 = 10k (resistor between pin 1 and pin 2)
• C1 = .1uF (capacitor between pin 1 and ground)
• The hot tip of the audio jack connects to pin 2, which is the OUTput signal
• The sleeve of the audio jack connects to ground

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## Step 4: Multimeter: Measure the Resistance of the Photoresistor and Potentiometer

Write down the range of resistance for your potentiometer and photoresistor.

Two inverters

• pin 14 goes to voltage source
• pin 7 goes to ground
• R1 = 10k (resistor between pin 1 and pin 2)
• R2 = 10k (resistor between pin 3 and pin 4)
• C1 = .1uF (capacitor between pin 1 and ground)
• C2 = .1uF (capacitor between pin 3 and ground)
• R3 = 10k (resistor between pin 2 and OUT)
• R4 = 10k (resistor between pin 4 and OUT)
• The hot tip of the audio jack connects to OUT
• The sleeve of the audio jack connects to ground

## Step 5: ​Use Two Inverters

To connect multple inverters to the same audio output, send each signal through a 10k resistor which all terminate at the hot tip of the audio jack. To play with the signal, could substitute R1 and/or R2 for variable resistors like a potentiometer or photoresistor.

## Step 7: Use Three Inverters

This time use a 10k resistor for inverter #1, a potentiometer for inverter #2, and a photoresistor for inverter #3.

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