Continuity Tester

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.

Status

• 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.

Features

• 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

Rev001 Issues

• 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

Rev002 Issues

• 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).

Rev001 ECOs

• 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.

Rev002 ECOs

• A - Trim 2 mechanical legs on SW1 slightly to enable them to fit through the holes.

Current Draw

• Rev001 w/ ECO A+B
  • Idle - 465uA
  • Beeper - 1.5mA
  • LEDs - 3mA