Doing an continuity tester from scratch (never done one before) Any ideas, suggestions and or criticism on my schema?:
If you want to succeed how you you know without a goal or spec?
1) Cost , not to exceed
2) Effort ( by you and end-user )
3) Your purpose? make money?
4) Tester Purpose ?
trace simple passive copper connections ?
find sliver shorts or almost opens ? ~ 100 mOhm? <1 Ohm?
User selectable? Conductance visible with several LED scale ? < 1, <0.1 < 0.01
Battery life? type?
Your design is is basically a rough current sink and no precision reference with a comparator and output magnetic buzzer and LED.
It could be made much cheaper and simpler if that's all you want to do.
Or you can add a bit more and do more with it.
But first decide what you want to do with a simple spec.
This design uses a 13mA CC sink with 0.65V/47R.
DMM's use about 1mA but measure voltage and display Voltage in Diode test mode.
There is no right or wrong numbers, but some are more useful.
Some are useful for tracing shorts and measure down to mV drops with higher current.