LED Beat Frequency Project

This is an old-and-new project. I wanted to build a tuning reference with visual feedback. This allows me to make some crazy non-melodic stuff with VCOs and after that some melodic lines that are in specific musical key without listening the VCO. The display circuit takes two inputs (f1,f2) and displays the difference frequency between these two frequencies with four LEDs arranged in a circle. Only one LED conducts at a time so what is displayed is a dot of light. The dot rotates clockwise when f1 is greater than f2 and counterclockwise when f1 is smaller. The rate of rotation is the beat frequency between f1 and f2. When f1 equals f2, the dot remains stationary. Originally I generated the reference frequency f1 with XR2206 function generator chip. The input frequency f2 is compared against this reference that can be chosen with a switch from C to B. There was a trimmer for every note in the even-tempered musical scale. Even though the trimmers are low PPM (15 PPM/C) precision trimmers the tuning wanders too much with temperature. I decided to replace the XR2206 tuning reference with an top-octave-generator that is clocked from a 2MHz crystal. FlatKeys (www.flatkeys.co.uk) makes a replacement chip for the original MK50240 top octave divider. Their FK50240 chip has 12 real synthesisers that tune from the master clock to the desired notes, with a worst case error of 0.05 milliHertz (0.00005 Hz). If you have a Polymoog or other 70s and 80s polyphonic top-octave-generator based synthesiser you can change the tuning accuracy to match the expectations of the 21st century with this Flatkeys chip. Original MK50240 chips are also rare and difficult to find so this is good news to keep those older instruments going. Pictured is the FK50240 chip in its own plastic box and the old version of this circuit.