The Mini-3 Keyboard Interface

The keyboard interface provides the 1 volt per octive control voltage that controls the VCOs and VCF. It also provides the gate voltage to trigger the ADSRs. The resistor string and keyboard switches form a huge voltage divider connected across the current source. When a key is pressed the voltage at PIN 10 of the TLO74 goes low until it is equal to the point on the voltage divider pressed. It is normally held high by a 1M resistor connected to +12. PINS 8/9 of the TLO74 follows the input providing a buffer. PIN 6 of the TLO74 is the input of a comparitor, PIN 7 is -10 no key pressed +12 any key pressed. This voltage is tapped off to turn the 3080 OTA on and off for control voltage sample and hold and provide gate voltage to a NPN transistor switch. PINS 8/9 of the TLO74 are connected to the input of the 3080 trough two resistors and a 1mf cap that form a pre S&H curcit to help stability. The 3080 and FET buffer form the CV S&H curcit isolating the 1mf cap connected to PIN 6 of the 3080 when the comparitor is LOW and allowing it to charge to CV when the comparitor is HIGH. The 1M pot connected to PIN 3 of the TLO74 controls the charge/discharge time of the 1mf cap connected to PIN 3 providing a smooth voltage transition when notes are played. PINS 1/2 of the TLO74 are the CV out.

Testing The Keyboard Interface

Connect the Keyboard to the interface and a +- 12VDC supply, connect the Glide pot and turn the supply on.

Adjust the 10K 10-turn trimmer connected to PIN 13 of the TLO74 quad op-amp until the voltage at PIN 13 is 3 VDC for a 32 key or 4 VDC for a 48 key keyboard, the voltage at PIN 14 should be held at 0 by the presision 100K 1% resistors connected to PIN 12.

With the glide pot turned to minimum the voltage on PINS 1/2 should change and hold depending on the key pressed between 0-3VDC. The voltage on PIN 7 should change going HIGH on keypress and LOW on release.