This circuit provides a voltage of 5 V with a maximum current of 100 mA for the TTL logic supply. The circuit is from an old publication from the 70's admitting inputs from 7 to 8 V for one of the inputs and from 10 to 40 V for the other. Resistors R1 and R2 determine the accuracy of the output voltage. The 50 ohms resistor can be 47 ohms.
