ACS MIDI CV/Gate : Schematics

 

The use of 12 bit DAC MCP 4822 from Microchip makes the schematic very simple.

 

 

 

 

 

 

 

 

 

 

 

 

 

In the new version 2.1 with the 2 polyphonic modes, a switch is connected beetween the pin 12 (RB6) and the ground.

 

Theory of operation

 

 

 MIDI data are read by the optocoupler 6N137 and then sent on the Rx pin (RB2) of the UART of the PIC. The model used here is a 16F88, but it can be replaced by a 16F87 (unlike the 87, the 88 has an ADC which is not used in this module). The PIC uses a SPI bus to send the data to the two MCP4822 Digital to Analog Converters. These chips are then connected to 4 Op Amplifiers CA 3140EZ which are enhanced versions of the very known 741. The max output voltage of the DAC's is 4.096 V and it has to be multiplied by 2.604 to reach the value of 1 V/Octave.
A 0.1 µF capacitor is used to filter the analog signal. Each CA3140 has 2 trimmers. The first one (noted A) is used to tune precisely the CV output. The note A at 1760 Hz can be tuned to within about 1 Hz. The second one (B) is used to cancel the offset of the AOP.
The Trig/Gate and LED signals are taken directly on the pin of the PIC. A NPN transistor BC 547 C is used to feed the  4 LED's.

 

 

 

 

 

"LITE" version

 In this version, the 4 AOP have been removed. So, the module just needs a single +5 V power supply. On the PCB a 78L05 voltage regulator  can be added to use the module with a 7 to 25 V power supply.