I am designing a test jig (within a 6U rack) that mates with 6 removable trays. Each tray can hold 16 sensors which can also be removed. The system will be intended to function in the following way:

  1. Technician will populate all trays with sensors,
  2. Trays are entered into rack and all sensors are tested,
  3. Trays are removed from rack and sensors that failed are removed first and binned. The rest are passed and bagged.

My issue is production have requested for indicators next to each of the sensors on every tray that indicate a pass or fail.

When the boards are disconnected from the rack they lose power and although LEDs and external battery would be the first solution....they have asked for alternative solutions. Is there such a component as an indicator relay or something along those lines? If anyone has a good solution for this issue then please let me know.

Software GUI is currently ruled out also as they believe looking at a screen and then at a PCB could lead to human error...

  • \$\begingroup\$ Do the indicators need to be permanent or just long enough for the racks to be powered down, removed from the racks and sorted? \$\endgroup\$
    – nvuono
    Commented Aug 9, 2018 at 14:59
  • 2
    \$\begingroup\$ Doesn't need to be permanent but 'long enough' would have to be minimum an hour. \$\endgroup\$ Commented Aug 9, 2018 at 15:10
  • 1
    \$\begingroup\$ Supercapacitor + LCD display? \$\endgroup\$ Commented Aug 9, 2018 at 15:34
  • 1
    \$\begingroup\$ You could also go mechanical, put a spring that a solenood releases. This ejdcts the faulty unit and raises a flag. Then when you load it resets the spring and you can launch it again if needed. \$\endgroup\$
    – joojaa
    Commented Aug 9, 2018 at 16:28
  • 5
    \$\begingroup\$ An overvolted tantalum cap will leave a visible mark \$\endgroup\$
    – PlasmaHH
    Commented Aug 9, 2018 at 16:53

6 Answers 6


There are non-volatile electromagnetic indicators that can be used.

enter image description here

They are a little oddball though. They were originally developed by the Canadian unit of Ferranti-Packard, but the patent has long since run out.

They work like a bistable relay (electromagnetic) and retain their state with power off. In the old days they were used in quantity for airline status displays in airports.

Personally I would consider fitting the trays with batteries (or applying power later) and using LEDs. You could store the information in a non-volatile memory such as a small microcontroller with on-board EEPROM or FRAM if the power has to be interrupted.

  • 3
    \$\begingroup\$ So flip dots, basically? \$\endgroup\$ Commented Aug 9, 2018 at 14:33
  • \$\begingroup\$ @ChrisStratton That seems to be a current name for them. Ferranti seems to have called them "Flip disc". \$\endgroup\$ Commented Aug 9, 2018 at 14:43
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    \$\begingroup\$ @bizarre: Spehro won't mind me saying this but accepting an answer so quickly will discourage others from posting potentially novel or radically different solutions which might be much better or at least give you different insights. You can unaccept and re-accept if his answer is still the best. \$\endgroup\$
    – Transistor
    Commented Aug 9, 2018 at 14:53
  • 1
    \$\begingroup\$ @Transistor Thanks for the advice (sorry I am new to this..). I thought initially flip disc/dots would be the ideal solution, but I am still searching for a solution with a smaller pcb footprint. Thanks also to Spehro for his answer. \$\endgroup\$ Commented Aug 9, 2018 at 15:12
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    \$\begingroup\$ @nvuono - it sounds like the "trays" the OP is building are a recirculating testing tool, not the shipped product. So the same flip-dots would be used over and over. \$\endgroup\$ Commented Aug 9, 2018 at 15:30

The flip dots mentioned in other answers would probably be more suitable if you can still find them, but another possibility is an E-Ink display: they're available in forms with individual dots, rather than the usual dot-matrix or 7-segment form. E Ink Corporation SCD722002 That's a completely non-volatile display, that can be directly driven by 7 pins of a 5V microcontroller, no additional circuitry required. (I'm not sure if 3.3V would be enough to get usable contrast. These displays want 15V for full performance, but the main issue with using only 5V is that it takes about a second to change state.)


I was running on a similar train of thought as Spehro.

enter image description here

Figure 1. Magnetic latching indicators - flip-disk and flip-dot displays. Source: eldisrl.com.

These are similar to the elements that make up seven-segment displays on some petrol pumps.

enter image description here

Figure 2. These paraticular units have a two-wire coil and flip state is determined by polarity reversal. Source: Datasheet.

enter image description here

Figure 3. A manually reset and electrically unlatched indicator was used on the old manual switchboards. Source: Telephone switchboard, Wikipedia.

They were also used for call-bells but I can't find an image of anything modern. Would you 3-D print your own with a little solenoid release mechanism?

  • \$\begingroup\$ That indicator in the top picture looks slightly like HAL 2000 \$\endgroup\$
    – Voltage Spike
    Commented Aug 9, 2018 at 15:45
  • \$\begingroup\$ @laptop2d 9000, probably. \$\endgroup\$
    – Mast
    Commented Aug 10, 2018 at 8:34

I managed to find these Miniature bite indicators that come in a variety of sizes, colours and voltage ranges, so thought I'd share these with everyone who is interested. Thanks for the help in pointing me in the right direction! http://www.flamecorp.com/PDF/L3-Electrodynamics/L3%20Ball%20&%20Flag%20Indicators.pdf


Producing a battery + LED setup or even adding a permanent flipdot to each sensor may not be suitable for your needs but I would recommend adding a couple pins to attach a test indicator that could be placed quickly on each board before testing and then removed and reused before the next batch is inserted for testing.

In your case this could either be a flipdot + driving circuitry or a battery with some LEDs and indicator logic to keep your sensor footprint to a minumum.

We have some interface cards with a setup like shown below:

Test Module Example


You might want to have a look at electronic paper (e-paper) displays. The displays come in all kinds of sizes and shapes, and they retain the set images for atleast weeks some even months when power off.

We use them in a system where we have limited power, so we set the image and then cut the power to the display. The image is still visable, and when we need to update the display we power it on again.

More info can be found on https://en.wikipedia.org/wiki/Electronic_paper

biggest drawback is you can not see them in the dark, and the pixels are only two toned, (whith many different combinations)


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