The circuit shown in Figure 1 is a NOR (Non-OU) logic port in which an LED illuminates when either input is at the low level. The main feature of this circuit is its compatibility with TTL logic which allows it to be connected to the output of microprocessors, microcontrollers and other devices that use this logic.
This is an interesting solution for making a TTL indicator system without the need for special integrated circuits. The LED can be of any color, but given the lower drive voltage, the red ones are best suited for this solution.
The printed circuit board for mounting this indicator is shown in figure 2.
Material list
Q1 - BC548 - NPN general purpose transistor
D1, D2 - 1N4148 - general purpose diodes
LED - common red LED
R1 - 3.9 k ohm x 1/8 W - resistor
R2 - 120 ohm x 1/8 W - resistor
Several:
Printed circuit board, wires, solder, etc.
