Introduction: Parallel Sequencer Synth

About: Hobbyist and electronics enthusiast.

This is a guide for creating a simple sequencer. A sequencer is a device that cyclically produces a series of steps that then drive an oscillator. Each step can be assigned to a different tone and thus create interesting sequences or audio effects. I called it a parallel sequencer because it is not driven by one oscillator at each step, but by two oscillators at the same time.

Step 1: Block Diagram

Let's start with the block diagram.

The device will be powered by a 9 volt battery and the controller will reduce this voltage to 5 volts.

A separate oscillator will generate a low frequency, ie the tempo, which will serve as a clock for the sequencer. It will be possible to adjust the tempo using the potentiometer.

In the sequencer, it will be possible to set the reset step and sequence mode using the toggle switches.

The output of the sequencer will be 4 steps, which will then control two oscillators connected in parallel, the frequencies of which will be set with potentiometers. Each step will be represented by one LED. For oscillators, it will be possible to switch between two frequency ranges.

The output volume will be regulated by a potentiometer.

Step 2: Breadboard

I first designed the circuit on a breadboard. I tried a few alternative versions of the tempo oscillator with different circuits, as well as several configurations with a decimal or binary sequencer with a demultiplexer. The oscilloscope is helpful in design as well as in troubleshooting.

Step 3: Schematics

*link to HQ Image Schematics

*If you find an explanation of the schematics unnecessary, you can proceed to the next step - Parts List (BOM)

Power from the 9V battery is transmitted to the circuit via the main switch S1, which will be located on the panel. The voltage of approximately 9V is reduced to 5V by the linear regulator IC1.
It is also possible to use a DC-DC buck converter to reduce the voltage, the disadvantage may be the high-frequency noise introduced into the system. Capacitors C1, C3, C15 and C16 help to attenuate the interference and C2 smooth the output voltage.

The tempo oscillator / low frequency oscillator (LFO) is generated using a schmitt-trigger inverter IC 40106 (IC2). The VR9 potentiometer provides an adjustable output frequency. By combining C5 and VR9, it is possible to select the desired range (in this case from about 0.2Hz to 50Hz). The output frequency can be increased by selecting a smaller potentiometer VR9, or by decreasing the value of capacitor C5. R2 limits the upper frequency range if the potentiometer is set to approx. 0 ohms. Unused gates of IC 40106 must be tied to ground.

The LFO generator can also be an IC 4093, 555 or an operational amplifier.

The LFO, or clock signal, is fed to a decimal sequencer 4017. The CLK and RST inputs are secured against interference by pull-down resistors R39 and R5. The ENA pin must be tied to ground to allow the sequencer to run. The sequencer works as follows:
Each time the CLK changes from low to high, the sequencer turns on one of the output pins in the order Q0, Q1, Q2 ... Q9. Only one of the output pins Q0 - Q9 is always active. Thus, the sequencer cyclically repeats these ten states. However, any output can be connected to the RST pin to reset the sequencer in this step. For example, if we connect Q4 to the RST pin, the sequence will be as follows: (Q) 0,1,2,3,0,1,2,3,0,1,2,3... This feature of the IC is used with the three-position switch S2, which provides either 10 steps (middle position, reset tied only to ground), or reset to Q4 (4 steps), or reset to Q6 (6 steps) mode. Since the device will be a 4-step sequencer, resetting the IC on step 4 will result in a continuous sequence without a pause, resetting the IC on step 6 will result in a sequence of 4 steps and a pause of 2 steps, and finally the third option will be resetting the IC on step 10. This results in a sequence of 4 steps and a 6 step pause. The pause provided by switch S2 is always added only after the sequence of steps (1234 __, 1234 __... or 1234 ______, 1234 ______...) has been performed.

However, if we want to add a pause between the steps themselves, we must reorganize the order in which the oscillators will be powered. This is taken care of by switch S3. When turned on in the right position, the sequencer operates as described above. However, if it is switched to the opposite side (left), step 4 of the IC sequencer becomes the third input to the oscillator and step 7 becomes the fourth input to the oscillator. The sequence will therefore look like this (S2 in the middle position): 12_3__4___, 12_3__4 ___, ...

The table below describes all the sequence options that can be generated by both switches:

Switch S2 positionSwitch S3 positionCyclic sequence
(_ means pause)

One LED (LED3 to LED6) is assigned to each step, for clarity.

The parallel oscillators are formed in the NE556 circuit, in an astable configuration. The capacitors selected by switches S4 and S5 are charged and discharged through resistors R6 and R31 and potentiometers VR1 to VR8. The sequencer switches transistors Q1 to Q8 in pairs (Q1 and Q5, Q2 and Q6, Q3 and Q7, Q4 and Q8, repeatedly) and thus allows the capacitors to be charged and discharged via variously set potentiometers. The internal logic of the IC4 circuit, based on the voltage of the capacitors, turns on and off the output pins (pins 5 and 9). The frequency range of the individual steps can be adjusted by changing the values of the potentiometers and also by changing the values of the capacitors C8 to C13. Between each emitter and the corresponding potentiometer, a 1k resistor (R8, R11, R14 ...) is added for upper frequency limitation. Resistors connected to the base of transistors (R9, R12, R15 ...) ensure the operation of transistors in the saturation state. The outputs of both oscillators are connected via a voltage divider VR10 (volume pot) to the output jack.

Unused designators: R1, R3, R7, R10, R13, R16, R19, R22, R25, R28, R36, LED1

Step 4: Parts List (BOM)

  • 5x LED
  • 1x Stereo Jack 6.35
  • 1x 100k Linear Potentiometer
  • 1x 50k Linear Potentiometer
  • 8x 10k Linear Potentiometer
  • 12x 100n Ceramic Capacitor
  • 1x 470R Resistor
  • 2x 100k Resistor
  • 2x 10k Resistor
  • 23x 1k Resistor
  • 2x 1uF Electrolytic Capacitor
  • 1x 47uF Electrolytic Capacitor
  • 1x 470uF Electrolytic Capacitor
  • 8x 2N3904 NPN Transistor
  • 1x IC 40106
  • 1x IC 4017N
  • 1x IC NE556N
  • 1x Linear Regulator 7805
  • 3x 2 Position 1 Pole Toggle Switch
  • 1x 2 Position 2 Pole Toggle Switch
  • 1x 3 Position 1 Pole Toggle Switch
  • Prototype Board
  • Wires (24 awg)
  • IC sockets (optional)
  • 9V Battery
  • 9V Battery Clip

Tools for soldering and woodworking:

  • Soldering Iron
  • Soldering Solder
  • Pliers
  • Marker
  • Multimeter
  • Caliper
  • Tweezers
  • Wire stripping pliers
  • Plastic Cable Ties
  • Caliper
  • Sanding Paper or Needle File
  • Paint Brushes
  • Watercolor Paints

Step 5: Wooden Box

I decided to build the device into a wooden box. The choice is yours, you can use a plastic or aluminum box, or print your own using a 3D printer. I chose a box measuring 16 x 12.5 x 4.5 cm (approximately 6.3 x 4.9 x 1.8 in), with a pull-out opening. I got the box in a local hobby shop, it is made by KNORR Prandell (link).

Step 6: Parts Layout and Preparation for Drilling

I arranged the potentiometers, ice holders and switch nuts on the box and arranged them the way I liked them. I took the layout and then I covered the box with masking tape from above and from one side, where there will be a hole for a 6.35mm jack. I marked the positions of the holes and their size on the masking tape.

Step 7: Drilling

The top wall of the box was relatively thin, so I drilled slowly and gradually widened the drills. After drilling the holes, it was necessary to treat them with sandpaper or needle files.

Step 8: The Base Coat

As the first coat of paint - the base coat - I applied green. The base layer will be covered with a light brown color and orange color. I used watercolors. After each layer, I let the box dry for a few hours, as the wood soaked up enough water.

Step 9: The Second Layer of Paint

I applied a combination of light brown and soft orange to the green base layer. I spread the paint with horizontal movements and where I wanted to achieve more pronounced stains, I applied as little water and more paint (less diluted paint).

* The colors in the images in this step are different from the other photos, because the color on them has not yet dried.

Step 10: Making the Circuit Board

I decided to create a printed circuit board on a universal board. It's much faster than waiting for a shipment of custom-made pcbs, and as a prototype, that's enough. If anyone is interested, I can also create and add complete gerber files.

From the universal printed circuit board, I cut out a narrow, longer strip that fit the length of the box. I soldered the circuit gradually, in smaller parts. I marked the places where the wires will be connected with black circles.

Step 11: Troubleshooting and Clear Circuit Board Making Process

Not getting lost when creating a printed circuit board is sometimes difficult. I've learned a few tricks that help me.

Components that are mounted on the panel or off the board are marked inside the blue (black) rectangles in the schematics. This ensures clarity in the preparation of wires or connectors and their location. Each line that intersects a rectangle, therefore, means one wire that needs to be connected later.

It is also helpful to note the connections and mounting of those components that have already been installed. (I use a yellow highlighter for that). This will clearly distinguish which parts and connections already exist and which still need to be done.

Step 12: PCB

For those who want to make or order a pcb, I am attaching a .brd file. The printed circuit board has dimensions of 127 x 25mm, I added two holes for M3 screws. You can create your own files according to the desired gerber format.

Step 13: Mounting Parts in the Box

I inserted and secured the components that will be on the top panel - potentiometers, switches, LEDs and output jack. The LEDs were placed on plastic holders, which I secured with the help of hot glue.

It is advisable to add the potentiometers knobs later so that they are not scratched when soldering the contacts and handling the box.

Step 14: Wiring

The wires were soldered in parts. I always stripped and tinned the wires first before connecting them to the components on the panel. I proceeded from top to bottom so that the wires did not get stuck during work and I also secured the wire bundles with cable ties.

Step 15: Inserting the Battery and the Board Inside the Box

I put the circuit board inside the box and insulated it from the front panel with a thin piece of foam. To keep the cables from bending and holding everything tight, I tied the bundles with a cable tie. Finally, I connected a 9V battery to the circuit and closed the box.

Step 16: Mounting Potentiometer Knobs

The last step is to install the knobs on the potentiometers. Instead of the ones I chose for the parts layout, I mounted metal, silver-black knobs. Overall, I liked it more than the plastic ones, with a bright yellow matte color.

Step 17: Project Completed

The parallel sequencer synth is now complete. Have a lot of fun generating various sound effects.

Stay healthy and safe.

Audio Challenge 2020

Runner Up in the
Audio Challenge 2020