Introduction: Face Touch Alarm
Touching our face is one of the most common ways that we infect ourselves with viruses like Covid-19. An academic study in 2015 (https://www.ncbi.nlm.nih.gov/pubmed/25637115) found that we touch our faces an average of 23 times per hour. I decided to design a low cost, low power device that would alert you every time you are about to touch your face. This rough prototype could be refined very easily and although you are unlikely to want to wear this all day, it might be a good way to train you to reduce face touching and therefore reduce the spread of the virus.
Most forms of motion sensing use accelerometers or image processing. These are relatively expensive, require continuous power and therefore also a relatively large battery. I wanted to make a device that only consumes power when the behaviour triggers it, and that could be made at home for less than $10.
The device has three parts. A necklace and two small elastic bands on each wrist. It uses the principle that a magnet moving near a coil of wire generates an electrical current in the wire. When the hand moves towards the face, the magnet at the wrist generates a tiny voltage across the coil. This is amplified and if it is higher than certain threshold it switches on a small buzzer.
- 100 - 200 metres of solenoid wire. Most wire is too thick. Solenoid wire is insulated with a very fine coat of varnish so that you can make lots of turns in the coil while still keeping it relatively small and light. I used 34 AWG - which is about 0.15mm diameter
- Cable ties or sellotape
- A single supply low power op-amp. It needs to be able to operate at 3V. I used a Microchip MCP601.
- 2 resistors (1M, 2K)
- 2K trimmer resistor
- A 3 - 5 V piezo buzzer
- Any basic npn transistor (I used a 2N3904)
- Some veroboard
- CR2032 (or any 3V coin cell battery)
- 2 small powerful magnets
- 2 thick rubber bands or some compression support material (like compression socks)
Step 1: Wind the Coil
The coil needs to be one continuous piece of wire so it unfortunately it can't be hooked and unhooked like a necklace. Therefore its important that the coil diameter is big enough for you to get it over your head. I wound mine around a circular former (a wastepaper basket) with a diameter of about 23 cm (9 inches). The more turns the better. I lost count of how many I made but by testing the electrical resistance at the end I think I ended up with around 150 turns.
Take the coil from the former gently, and secure the coil with cable ties or tape. It is important not to break any of the delicate solenoid wire as it will be almost impossible to repair. When you have the coil secured, find the two ends of the wire, and remove the varnish from the last cm (last half inch) of each end. I did this by melting the varnish with a soldering iron (see the video attached).
These ends can be soldered delicately on to your detector circuit board. For my prototype I soldered the ends on to a small piece of separate veroboard with a socket header, so that I could use experiment and use jumper cables to connect it to different circuit designs.
Step 2: Build the Detector Circuit
The schematic and final circuit are shown above.
I use an op amp in a non-inverting configuration to amplify the very small voltage generated across the coil. The gain of this amplifier is the ratio of resistances of R1 and R2. It needs to be high enough to detect the magnet when it is moving about 10cm from the edge of the coil relatively slowly (about 20-30cm/s) but if you make it too sensitive then it can become unstable and the buzzer will sound continuously. Since the optimal number will depend on the actual coil you build and the magnet you use I recommend you build the circuit with a variable resistor which can be set to any value up to 2K. In my prototype I found that a value of approximately 1.5K worked well.
Since the coil will also pick up stray radio waves of various frequencies I included a capacitor across R1. This acts like a low pass filter. At any frequencies higher than a few hertz the reactance of this capacitor is much less than the value of R1 and so the amplification drops away.
Since the gain is so high, the output of the op amp will really only be "on" (3V) or "off" (0V). Initially since the MCP601 can output 20mA I thought that it might be able to drive a piezo buzzer directly (these require only a few mA to work). However I found that the op amp struggled to drive it directly, probably due to the capacitance of the buzzer. I solved this by feeding the output of the output through a resistor to an npn transistor which acts like a switch. R3 is chosen to make sure that the transistor is fully on when the output from the Op amp is 3V. To minimise power consumption ideally this should be as high as you can make it and still ensure that the transistor is on. I have chosen 5K to ensure that this circuit should work with almost any popular npn transistor.
The final thing you need is a battery. I was able to run my prototype successfully with a 3V coin cell battery - but it was even more sensitive and effective at slightly higher voltage and so if you can find a small li-poly battery (3.7V) I'd recommend using that.
Step 3: Make the Wrist Bands
If a magnet is worn close to each hand, the action of raising the hand towards the face will trigger the buzzer. I decided to create two wrist bands with elastic support sock material and used these to keep two small magnets at my wrist. You could also experiment with a magnetic ring on one finger of each hand.
The induced current flows in one direction around the coil when the magnet enters the region of the coil and in the opposite direction when it leaves. Because the prototype circuit is intentionally simple, only one direction of current will trigger the buzzer. So it will buzz either when the hand approaches the necklace or when it moves away. Obviously we want it to buzzer on the way to the face and we can change the polarity of the generated current by flipping the magnet. So experiment with with which way around makes the buzzer sound when the hand approaches the face and mark the magnet so that you remember to wear it the right way around.
Step 4: Test
The size of the induced current is related to how quickly the magnetic field changes near the coil. So its easier to pick up fast movements near the coil than slow movements far away from it. With a bit of trial and error I was able to get it to work reliably when I moved the magnet at about 30cm/s (1 ft/s) at a distance of 15 cm (6 inches). A bit more tuning would improve this by a factor of two or three.
Its all a bit crude at the moment since the prototype uses "through hole" components but all the electronics could be easily shrunk using surface mount components and the limiting size would just be the battery.