PopEye – a safety glass addon

It is not a pretty sight when a drill bit rotating at 20,000 rpm gets caught on a work-piece. Even a light foam core model then seems like a deadly projectile, especially if it’s headed straight for your eye. This happened to me a few months back. What makes it worse is that I had my safety glasses right on my forehead! I had pulled them up to examine the part for a minute and forgot to put them back on. I want to make sure that this doesn’t happen again. Hence, PopEye.

PopEye is a safety glass add-on that detects when you take your glasses off and cuts power to the tool you’re using. It seems like a relatively straightforward idea, and for the most part, it is. One can use a Bluetooth low energy beacon placed on the glasses. The beacon is switched on whenever the glasses are put on. An accompanying power relay will go between the mains supply and the power tool. It will listen for the beacon packets and will switch off the power if it can’t detect the beacon. This entire setup can be completed quickly over a weekend. I explored IR and other wireless protocols as well, but the simplicity and extensibility of BLE make it the best choice. So, the major challenge is to detect when the glasses have been put on correctly.

3M_safety_glasses
3M 91252-80024 glasses.

Before proceeding much further, it is important to narrow down a bit and focus on a particular type of safety glasses. Safety glasses come in at least half a dozen different configurations. I chose to focus on over prescription glasses type, with an elastic band instead of stems. The elastic band will ensure direct contact with the face, making detection easier. I chose to go with 3M 91252-80024 glasses(image below). It is a cheap and effective choice. Although I will guide the design to have high dimensional tolerance so that it will work with any similar glasses.

Safety glasses on forehead
Strong contact with the forehead marked in red.

Back to the problem at hand, detecting when the glasses have been put on. When I put on the glasses, right away I noticed a few points of detection. The forehead/brow, arch and cheekbone regions make a tight direct contact with the skin when the glasses are put on correctly. I would imagine, based on facial anatomy, the arch and cheekbone contacts can be interchanged for support at the bottom, but the forehead must necessarily make good contact with the glasses. More importantly, if I move the glasses up to my forehead, there isn’t a strong contact. So, we have a long (~10cm) strip which makes good contact only when the glasses are put on correctly.

To detect contact in this region, a light touch micro-switch easily comes to mind. It can be placed on any part of the glasses which come in contact with the skin only when the glasses have been put on correctly. But it can get false positives, e.g., if the glasses are packed inside a bag.

Using skin conductivity is the next best idea. Two conductive pads can be placed on the top of the glasses such that they contact the forehead when the glasses are put on properly (see image above). If the glasses are not put on properly, the circuit will be open. A microcontroller can then read the resistance between the pads and only switch on the BLE beacon when the resistance is in a sensible range, say 1K to 1M ohms. This will prevent accidental switching on if the glasses are packed in a box, even if placed along with metal tools -thus shorting the pads. It is unclear if a microcontroller is needed, a clever analog switch should be able to achieve this while saving cost and -more importantly- power.

After doing some experiments with this, I found that this method works most of the time. Of the six people I tested on, only one person had a facial structure that prevented the pads from making contact with the skin. (Image above.)

NFC can be used to quickly pair safety glasses to a tool. So, in a shared workshop, people can sign punch in and punch out tools by simply waving their safety glasses close to the power terminal for the tool.

The next steps for development are as follows:

  • Test different pad configurations: length and separation of pads, material: conductive cloth, pogo pins, copper tape, spring contacts..
  • Explore analog circuits for switching power to the BLE beacon.
  • Procure and test BLE and beacons, and RFID/NFC tags.
  • Design PCBs to fit the brow of the glasses.

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