We have a Pager that works, but no pager service is available. People have moved on to phones.
Amateur radio receivers
- Incoming SMS
- Outgoing Landline
These devices consist of four independent voltage comparators that are designed to operate from a single power supply over a wide range of voltages. Operation from dual supplies also is possible, as long as the difference between the two supplies is 2 V to 36 V, and VCC is at least 1.5 V more positive than the input common-mode voltage. Quad Comparator LM339
An application from an old TI datasheet showing the versatility of this comparator. For me it was a workhorse and like LM324 was used in many products.
Current drain is independent of the supply voltage. The outputs can be connected to other open-collector outputs to achieve wired-AND relationships.
Differential to TTL convertor using LM339