Controlling the CK9561 alarm IC (16 Song Music Box)

     A friend made me aware of the CK9561 (aka KD9561) alarm IC. I wanted to understand it better so I could use it in my projects. I started by studying a popular kit that uses the chip. It goes by multiple names, but usually includes the following keywords "16 Song Music Box". Here is a link to one of them.    

16 Song Music Box

    The circuit uses switches to select one of four pitches, and one of four "presets". Together you have 16 combinations. There exists a schematic for the board, but it's not very revealing. I redrew it so I could make sense of it.

16 Song schematic

    The pitch circuit uses CMOS switches to connect different combinations of resistors across two pins of the 9561 chip. This results in different effective resistances that set the rate of the internal oscillator.

    The preset circuit uses some TTL and Diode-Resistor-Logic to select the different alarm types. The 9561 is odd because it requires pin F1 to be "open" (floating, hi-z, etc) to select one of the modes. This means the pin has to be disconnected from both power and ground. Another CMOS switch is used to connect and disconnect this F1 pin.

The New Control Circuit

    I took what I learned from the 16 Song board, and started thinking about how I could control this chip with CV instead.

    I came up with a totem of comparators that would select the "presets". Their logic is very similar to the TTL/DTL circuit of the original. A CMOS switch is used identically. The main difference is that a single control voltage can be used to select from the four alarms.

    The pitch control is very different though. I realized that a clock signal could be injected into one of the pins for the pitch resistor. I found this easier than voltage controlling the internal oscillator, so I selected a common chip that includes a VCO: the CD4046. Two transistors form an exponential current sink that gives the expected voltage to pitch relationship.

    Now it's possible to voltage control the 9561 alarm IC.

New CV control schematic