I want to build a continuity tester that's fast, tiny, and can attach to a board I'm reversing so I don't need full multimeter when reversing boards away from a desk. The hardware files for this can be found on GitHub at sarahemm/continuity_tester.
- 2018-03-30: Assembled Rev002, seems to work quite well other than that it beeps/lights when there is no continuity, and stops when there is. Started working on an ECO to fix this (swap -/+ on the comparator) but broke a pin off U1 while doing so. Will need to order a replacement.
- 2018-02-15: PCBs for Rev002 arrived, ordered parts to assemble one.
- 2018-01-25: Added protection diode to schematic/board, checklisted everything, generated CAM files, and sent to OSH Park for manufacturing. Rev002 tagged in GitHub project.
- 2018-01-23: Spent the evening in SPICE figuring out what is required, added a Zener diode to BOM which will protect the input comparator from being connected to up to a 100uF capacitor charged to 12V, which should be enough. Need to add this to schematic/board still.
- 2018-01-17: Mostly finished PCB layout for Rev002.
- 2018-01-16: Started working on Rev002, to fix all known issues.
- 2018-01-11: Received PCBs and parts, assembled and started testing.
- 2017-12-13: Completed PCB design, sent to OSH Park for manufacturing.
- 2017-12-11: Started putting idea together after spending lots of hours reversing for the DMS project. BOM and schematic done, PCB layout in progress.
- Fast, latching to catch quick pulses as probe is dragged along multiple pins.
- Visual/audio switchable options
- Lasts a long time on a CR2032 or similar
- Clips or magnets to a board I'm working on to save having another thing to put somewhere.
- No programmable logic, entirely simple analog/digital logic
- I almost never have projects that don't involve firmware, I'd like this to be one of them.
- Makes it easier for people to replicate without needing programming hardware/software
Holes for jacks are way too small (addressed in ECO A)- Fixed in Rev002
Footprint for trimmers fits, but is way way oversized- Fixed in Rev002
LED footprints are backwards (anode/cathode swapped, addressed in ECO B)- Fixed in Rev002
Beeper/LEDs respond "analogly" due to the capacitor on the TRIGGER line- Fixed in Rev002
Will add another 555 in the next revision and add a real timer for the latching functionality
Second 555 rather than a 556 so we can have one on always for the sensing and turn the second on as-needed for the beeper, saving battery power
Current draw idle is a bit higher than I'd like, should try to optimize this further- Optimized in Rev002
Switching to a comparator that draws <1uA instead of 165uA will help somewhat
Optimizing resistor dividers in front of the comparator should help a lot
Duty cycle control isn't really useful, should reevaluate need for it in Rev002- Eliminated in Rev002
Comparator input offset is 7mV and sensing voltage is 7mV, should increase the sensing voltage somewhat to make this reliable- Fixed in Rev002
Switch sticks out a fair ways, should either find one with a shorter actuator or move it slightly further onto the board so it's better protected from bumps- Fixed in Rev002
- Parts are too close to the ICs, makes it hard to assemble. Even an extra mm would help.
- Mechanical holes for SW1 are too small.
- Indicates lack of continuity instead of continuity (stops when continuity is detected).
- A - Trim legs on both jacks slightly to enable them to fit through the holes.
- B - Mount LEDs upside down to work around the reversed footprint.
- A - Trim 2 mechanical legs on SW1 slightly to enable them to fit through the holes.
- Rev001 w/ ECO A+B
- Idle - 465uA
- Beeper - 1.5mA
- LEDs - 3mA