Synth Circuit

Today we’ll build simple square wave synthesizers out of Hex Schmitt Trigger Integrated Circuits (IC’s). Feel free to read about Schmitt Trigger circuits, however, we will be “misusing” these little chips to create simple square waves. a square wave is typically represented like this…

In a sense it is just alternating between on and off, or full voltage and no voltage in a circuit. Using resistors and a capacitors we can coax this behavior from a Hex Schmitt Trigger IC not once, but 6 times! Meaning from one Hex Schmitt Trigger we can get 6 voice polyphony. There are various ways to soften the edges of the square waves using filter and modulation circuits, but today we’ll just focus on producing is basic chip tune-esque tunable square waves.

To start, make sure you have these components…

  1. 1 breadboard
  2. 1 hex schmitt IC chip (integrated Circuit)
  3. 1 photoresistor
  4. 1 potentiometer
  5. 1 button
  6. a few resistors
  7. a few capacitors
  8. a battery clip and a 9V battery OR a variable power supply
  9. a 1/4″ output jack
  10. 386 amplifier IC (optional)

Ok… let’s start with the breadboard. Breadboards are great tools for prototyping circuits. You can plug components right into the holes without having to solder them. It is important to understand how a breadboard works…

 

 

In the breadboard above, all the holes in the green rows are connected and all the holes in the blue rows are connected. Let’s call the long rows on the sides “rails”. The rails run alongside of the rows and they bring power (+) and ground (-) up and down your breadboard. The middle grey space between the blue and green rows breaks the connection between the blue and green sections. It is also conveniently the perfect size for placing an IC chip across that middle gap. Let’s start by placing our Hex Schmitt IC on the breadboard like this…

 

 

Also, add two wires between the positive and negative rails on either side of the board. This will allow us to connect power to one side of the breadboard and be able to access it on both sides. Note the subtle notch in the chip and be sure to point that to the left. The numbers don’t really matter and they might get confusing if people have different types of breadboards, just focus on the general orientation for now. This particular IC chip is a 6 voice hex inverter circuit. The pinouts for the chip are as follows…

Starting at the notch, the pin directly left of the notch is pin 1, and the numbering continues in a horseshow end with pin 14 at the right side of the notch. This numbering system is true for most ICs. Looking at the pinouts above, note which pins are pairs (1&2, 3&4, etc). Even of these pairs will control one our synthesis voices. Pin 14 (VD0) is the power pin, where we give the chip power, and Pin 7 (VSS) is the ground pin, which connects to ground. Let’s add power and ground to our chip…

 

 

Next we’ll add a 10K resistor, small capacitor, and a photocell to complete one voice of our synthesizer. All three of these components work together to produce the tone. The capacitor is a component that periodically stores and discharges electrons which triggers the IC to switch between high and low states. The size of the capacitor determines the range of frequencies for that voice of the synthesizer. The photoresistor changes the rate at which the capacitor charges, and will be how you dynamically change the frequency of the square wave (within the range set by the capacitor). The fixed 10K resistor conditions the output signal to send to an amplifier or powered speaker. Add the 10K resistor, small capacitor, and a photocell as pictured below using pins 3&4 on the IC to give us plenty of room…

 

 

In order to hear the tone we need to power our bread board and add a jack that allows us to connect our circuit to a speaker. Once you’ve added those components, connect a guitar cable between your powered speaker and the output jack and cover your ears!