Quad Logic Gate
for music synthesizers.
The Quad Logic Gate is a very simple module that can be built in one of five flavors : XOR, OR, AND, NOR and NAND. It is a simple way to gain additional control of gate and trigger pulses within a system. It can also be used for some simple signal multiplying. The XOR is well known as a square wave "ring modulator", though interesting effects can also be generated using the other configurations. Each gate has an in-built LED to indicate the status of the output of that gate.
With some chip types, it is also possible to have some of the gates presented on the panel as basic inverters. For XOR and NOR gates, tie the disused input of that gate to 0V. For NAND gates, tie the disused input of that gate to +15V. It is not possible to create inverters with AND and OR gates.
A little on how it works:
Each input is configured to detect a voltage over approx. 2 volts. When this voltage is reached, a logic HIGH is presented to the corresponding gate input. The output will react to what is present at the inputs, and the result, depending on the gate chip used, is buffered by the emitter follower, and used to drive the LED and output jack.
Truth tables for the different logic gates that can be used in this circuit.
Due to a manufacturing error, the overlay on one version of these PCBs has become cluttered. Print out the above file to assist with assembly.
There are four LED resistors, marked RL. The value of these is determined by the type of LED you have chosen. Try out your LED with various resistors before assembling the board. If you are using common LEDs, 1k to 2k2 would be appropriate. If you are using any of the super-bright modern LEDs for their unique colors, the resistors can be much higher, 10k to 33k for example, so that the LEDs don't dazzle you.
There are five 100n decoupling capacitors. These can be 1206 SMT or regular capacitors. The one on the logic chip must be mounted on the rear of the PCB regardless of the chosen type.
Wiring Diagram for CGS39
This is a guide only. Parts needed will vary with individual constructor's needs.
Article, art & design copyright 2001 by Ken Stone