Sculpt-O-Sound presents: Sequential-Switch-O-Matic
Summary
This is my take on a sequential switch which can switch CV or audio signals from one input to one of 4 outputs and vice versa.
Design
I was watching a video on youtube the other day and saw a demonstration of the Doepfer A-151 sequential switch. The switch allows a signal on one ‘input’ port to be connected/send to one of four ‘output’ ports. Note the ports are bidirectional, so you can use the switch in the opposite direction. The sequential switch selects the next output depending on a trigger pulse (via TRIG IN). While watching the video I gathered that the circuit would probably consist of some counter chip and an quad analog switch and not much more. As I have lots of CD4066s lying around I started designing a circuit and building it on a bread board. After some experimentation I ended up with this schematic:
Note all ports are connected to gnd via a 100k resistor via the jack connector when not patched. This is drawn only once but applies to all I/O and O/I ports.
The LM358 will change the incoming signal into a trigger for the counter. The counter outputs a one on one of its ports and zeros on all the others. Q0 … Q3 will activate a switch. The counting sequence length will depend on the setting of the switch (bottom right). An external reset signal can also be used to restart the counter.
The 4066 is powered with a positive and negative supply voltage. The voltage however must not exceed 20V, so in a eurorack setting (where power spans 24 Volts) I had to add some trickery to lower the supply voltages. I used LEDs as voltage droppers to achieve that. The current the circuit needs are small, so the LEDs did the trick. Not however that I use 2 for the negative voltage. While experimenting with this I noticed that the 4066’s switches will only switch properly if the negative voltage is about 2 Volts higher than the absolute value of the positive supply voltage. I ended up with a supply voltage of +8.6 V and about -6.7 V. These are used as power voltages throughout the circuit.
Build
I used a vero board for my implementation and 3D-printed the front panel. You can see 2 of the LEDs in this picture. I used some dark paint to block the light coming from the LEDs. The Doepfer A-151 uses a 3 position switch (for sequence length 2, 3 and 4) which I did not have available. I did however have a 4 position switch and did not want the 4th position to be unused. So I added output Q5 to the switch. This makes it possible to have a sequence length of 5 steps (the 5th step not connecting the input to any output(and v.v.)). This 5th step may be interesting to use a ‘skip’ to the sequence.
Usage
Either connect an input signal to the O/I port and expect an output on the I/O[1234] ports or the other way around. Use a trigger signal to step forward and activate the next switch. Select the sequence length using the 4 position switch.
Demo
The video below contains a short demo of the switch:
