Diode Transistor Logic

DTL AND gate, triggering LED

I learned today about Diode Transistor Logic, which was the predecessor to Transistor-Transistor Logic (TTL), which was replaced by CMOS. This goes back to the 60’s, but was the technology that put men on the moon. With DTL, the logic gate is a set of diodes, with a transistor providing the amplification. The circuit above and below is an AND gate:

DTL AND gate, triggering LED, schematic

The basic idea, as I understand it: if either input is 0V (logic low) then current runs through the diode, pulling down the voltage on the base of the transistor. But if both inputs are 5V (logic high) then the diodes do not conduct, and the base of the transistor remains at +5V, turning on the transistor and the LED.

This is a poor quality video, but I needed something to show it works. In the video, INPUT1 is a 1 Hz square wave signal, and INPUT2 is an 8 Hz square wave signal. When brought together in an AND gate, the combined signals cause the LED to blink 4 times for the first half of each second.

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