This circuit was obtained from Motorola documentation from 1969. The setting applies to other operating systems, including lower supply voltages. The zener determines the output signal.
Today we have more compact solutions. This is from a Radio Electronics magazine from October 1990....
This circuit is from an 80's documentation, making use of PUTs that are not currently common...
This circuit multiplies the input voltage by 3 and can work with up to 12 V input. The current is a few...