Battle of the Bands: Theremin, Organ, & APC Synthesizer!

A 555 timer is an integrated circuit (IC) chip that is used to generate a square wave. It can be used in timer/flip-flop applications or as an oscillator. In this instance, we are using it to generate square waves at various frequencies to create the different notes of the instruments. By altering the RC circuit that we are feeding into the 555 timer, we can change the frequency of the output square wave to play different notes. All of the instrument circuits are fundamentally the same. They just use different means of changing the RC circuitry we are providing as an input. In these circuits, we will be using the 555 timers in astable and monostable configurations (more on this later).

Before we dive fully into the 555 timer, there are some fundamental building blocks we need to understand. The first is a comparator shown in the red block and the yellow block. A comparator has two inputs and one output. When the V+ input is greater than the V- input (denoted with the bubble), it drives the output high. When the V+ input is less than the V- input, it drives the output low. There are two comparators inside the 555 timer circuitry. These comparators are used to turn the output of the 555 timer on or off based on the threshold value and the trigger value.

The second circuit component we need to understand is the SR (set-reset) Flip-Flop shown in the purple. The truth table for the implementation logic of the SR flip-flop is shown in the table above. The SR flip-flop sets the state of the output based on the state of the comparators. The threshold comparator is connected to the Reset pin of the flip-flop and the trigger comparator is connected to the Set pin of the flip-flop.

The push-pull output driver shown in pink is used to source current for the output. The transistor shown in blue is used to connect the discharge pin to ground, typically to discharge an external capacitor. The 3 resistors shown in green are of equal value. They divide the supply voltage, VCC, into 2/3 and 1/3 of its value. These values are used as the reference voltages for the comparators.

Although it looks complicated at first glance, the 555 timer essentially just charges and discharges an external capacitor and toggles the output based on the state of the capacitor. Starting out, the external capacitor is discharged meaning that the trigger voltage is less than 1/3 Vcc. This turns on the trigger/set comparator, driving the output high. The external capacitor continues to charge until it gets above 2/3 Vcc. At that point, the threshold/reset comparator turns on, driving the output low. The discharge transistor connects the external capacitor to ground causing it to discharge. Once the external capacitor drops below 1/3 Vcc, the entire process repeats over and over again creating our neverending square wave.

In case it still doesn't click, you can check out these simulations courtesy of Paul Falstad. I am a fairly visual learner, so seeing circuit simulations really helps me understand what is going on in a circuit.

555 timer square wave simulation - http://www.falstad.com/circuit/e-555square.html

555 timer internals simulation - http://www.falstad.com/circuit/e-555int.html

We will predominantly be using the 555 timers in astable mode. This means the circuit is fed by an RC circuit and is free-running to generate an output square wave. The Atari Punk Console uses the first 555 timer in astable mode and the second 555 timer in monostable mode. Monostable mode means that it is fed by an external source and outputs a one-shot pulse. In the Atari Punk Console, we are using an astable 555 timer to provide a signal to the monostable 555 timer.

Now it's time to make your theremin! Instead of using proximity like a typical theremin, this theremin uses light intensity to determine the note's frequency. This theremin has two photoresistors in the RC circuit feeding the 555 timer circuit to affect the rate of charging/discharging of the capacitor. The resistance of a photoresistor decreases as the light intensity decreases meaning the capacitor will take longer to charge and discharge. So by reducing the amount of light getting to your photoresistors, you reduce the frequency being played. To increase the frequency, increase the light intensity.

To build your theremin, start by placing the 555 timer chip on a breadboard. You can build the circuit any way you want. I typically start by placing components in sequential order starting at pin 1 of the IC chip and moving all the way around to pin 8. Make sure you place some distance between your photoresistors so that you can control the amount of light getting to each photoresistor separately. Be careful noting the polarity on your capacitors and the speaker. R5 is a potentiometer that is used for volume control.

Once you have built the circuit, go around the IC from pin 1 to pin 8 again to double-check that everything is connected properly. To confirm nothing is shorted, you can connect a multimeter between pin 1 and pin 8 on your 555 timer. You should see a fair amount of resistance between these two points. Once you are sure everything is connected properly, you can connect the 9V battery. Your theremin should start making noise! If you don't hear anything or the sound is faint, adjust R5 to finetune the volume to your liking.

Since the ambient light intensity can vary greatly based on where you are playing your theremin, you may have to swap out C3. If the frequency range on your theremin is too high pitched, try switching C3 for a capacitor with a slightly larger capacitance value. If the frequency is too low, try a slightly lower value capacitor.

Now it's time to make your organ! The organ uses a 555 timer circuit in astable mode. The tactile switches are used to switch in different capacitors to determine the frequency of the tone that is played. You can add as many different switches and capacitors in parallel as you want. You can also combine them together by pressing two buttons or keys at the same time (combining capacitors in parallel results in an equivalent capacitance value equal to the sum of the two capacitances which will produce a sound that's frequency is lower than either of the two switches on their own).

To calculate the specific tone of each of your keys, you can use the following equation. R1 is going to be the value of your potentiometer based on the position of the wiper and R2 is 10kOhm. C is the value of the capacitor that you are switching in. f is the resulting frequency in Hertz. 0.693 is the natural log of 2.

f = 1 / (0.693×C×(R1 + 2×R2))

As a starting point, above is a table of some common capacitance values with their corresponding calculated frequencies assuming that R1 is 100 kOhm. Remember that if you don't have the exact capacitance value you are looking for, you can combine capacitors using the following equations.

For capacitors in series: 1/C_equivalent = 1/C1 + 1/C2 + 1/C3

For capacitors in parallel: C_equivalent = C1 + C2 + C3

To more easily figure out how you can combine capacitors you already have to create a specific capacitance value, check out this Instructable and try out my Stocked Resistor/Capacitor Equivalence Calculator.

Build your organ in the same way you built your theremin. Start by placing the 555 timer chip on the breadboard. Remember you need will need to save a lot of space for the pushbuttons. Start placing the rest of the components beginning with the parts connected to pin 1 of the IC and moving all the way around to pin 8. I placed the buttons and their corresponding capacitors after connecting everything else.

Be careful noting the polarity on your capacitors and the speaker. As before, double-check the circuit and confirm nothing is shorted. Once you are sure everything is connected properly, you can connect the 9V battery. Press the keys and your organ should start playing! If you don't hear anything or the sound is faint, adjust R3 to finetune the volume to your liking.

Now it's time to make your Atari Punk Console! The Atari Punk Console (APC) is essentially an astable 555 timer circuit generating a square wave to drive a monostable 555 timer circuit which creates a single square pulse. There are two potentiometers for control: one for the frequency of the oscillator (R2) and one for the pulse width (R3). The APC is considered one of the most iconic simple DIY synthesizers.

Build your Atari Punk Console in the same way you built your other instruments. I would begin by building the circuit for the first 555 timer (U1) before moving on to the second 555 timer (U2). Start placing the rest of the components beginning with the parts connected to pin 1 of the IC and moving all the way around to pin 8. Once you have finished the circuitry surrounding U1, add U2 to the board and construct the circuitry surrounding U2.

As before, be careful noting the polarity on your capacitors and the speaker. Double-check the circuit and confirm nothing is shorted. Once you are sure everything is connected properly, you can connect the 9V battery. Your APC should start playing! Start twisting the frequency and pulse-width knobs and playing your very own synthesizer. If you don't hear anything or the sound is faint, adjust R4 to finetune the volume to your liking.

Now you've got all three instruments and you're ready to assemble your band! In my class, I gave about a day and a half to go through the 555 timer, build the circuits, assemble bands, come up with a name and logo, write songs, and prepare a performance. The end result was a lot of fun. There were bands with perfectly tuned instruments who played near-perfect renditions of "Lavender Town" and other bands who went with more of a free jazz, improvisational approach. All-in-all, these are really fun circuits for anyone at any level to play around with.

You can also try remixing these circuits even more! Try swapping the potentiometers in the APC for photoresistors. Try combining the organ or theremin with the second stage of the APC. Try using thermistors or pressure-sensitive resistors in place of the photoresistors. Try using fixed resistors and variable capacitors instead of fixed capacitors and variable resistors. There are so many different directions you can take these instruments. Have fun!

For more projects, visit my pages:

https://dargenio.dev/

https://github.com/mjdargen

https://www.instructables.com/member/mjdargen/