AND with diodes

An AND operation is implemented with diodes. At the beginning of computer technology switching states have been done with diodes on DTL-way (Diode-Transistor-Logic) and not with transistors only. The AND and OR operations can be implemented easily. If both switches are open, the current flows through the diodes in forward direction, the voltage drop is not high enough to let the LED light up. In this circuit it does not matter which button is closed. As long as the diode remains open, the voltage drop is maintained. Only when both switches are closed, both cathodes of both diodes are connected to +9V, and become non conductive and the red LED will light up via the 10kΩ resistor.