Wristwatch projects are coll for many reasons. First of all how they are built – components need to be squeezed to make watch wearable. Another thing is display. What different types of displays can be used. LCDs and LEDs seems to be getting boring, unless they are used in some weird way. Other things also are important like overall look, power supply , features and controls. Johnengineer has been working on steampunk wristwatch for couple months and he came up with something awesome.
The unique idea was to use IVL2-7/5 VFD display. They are relatively small to fit on a watch and pulls out relatively small amount of current. It also requires low grid voltage (~12V). Display is transparent, so background PCB is visible and this can be considered a cool design feature. Clock is ticking around Atmega88 microcontroller along with RTC DS3231. Display is driven by dedicated MAX6920 12-bit shift register. Since watch is powered from single AA battery, there are couple boost converters – one for generating 5V to power electronics and another 13.5V to power VFD grid. While designing circuit there are future options included like phototransistor, barometric/temperature sensor and accelerometer. The construction of watch definitely falls in steampunk category as there are brass and leather decorative elements.
James is passionate with mountain biking. When dark comes proper lighting is required. For this he purchased a bike light controller which didn’t prove itself with its lack of button debounce, annoying strobe in super bright mode. Without thinking further he started his own controller project where he could select modes with single push button that would be properly debounced. He went with several lighting modes including 100% constant, 50%, 15% and 15% constant with a 2Hz, 100ms 100% flash strobe. Such strobe with steady lighting seems to work well when catching drivers attention.
For electronics part he’s chosen Attiny25 microcontroller which is small enough to fit in to existing enclosure with other components. He used 78L33 voltage regulator to get DC power supply for circuit from battery.
If you power your electronic device from battery, you probably want to have feature that tells when battery is running low. For rechargeable batteries it is crucial to know when voltage is nearing to threshold when lower drop would damage battery itself. Say LiPo batteries cannot be discharged lower than 3.0V. Josh have went through this problem by analyzing low battery indication options. He says that small circuit built around MCP65R46 can be good choice where voltage is simply compared to reference inside chip, but problem is that in that case you need additional components. Also such circuits draw a bit of current to work.
So he thought that in microcontroller based circuits you can get away without using any circuit and still have same functionality if not better. The fact is that AVR microcontroller is equipped with internal 1.1V bandgap voltage reference. The thing is that this voltage can be sampled with ADC against VCC or VREF voltage. So he suggest take VCC voltage as reference and measure bandgap voltage as unknown voltage. Then by applying simple formula calculate actual VCC. Once you have this voltage level, it is up to you how you indicate this – with LED, output on LCD. If you want to be precise, you should know that internal reference voltage can vary from chip to chip. So you need to test it against known VCC and then use calibration to get close values to truth.
When going down into Ethernet or any other signal, you find that there is nothing more than ones and zeros. So practically any programmable device theoretically can send any information in any format. The only limit is RAM space, speed and signal conditioning. Cnlohr was able to squeeze Ethernet functionality in to small Attiny85 microcontroller. He was able to do so without external circuits – twisted pair was directly attached to microcontroller pins. This is really dangerous way to do, because Ethernet signal this way is forced to be tied to mcu ground. Ifthere is a DC offset in the line, it can cause damage. Normally there is a magnetics involved in signal line.
Anyway this is great hack showing that anything is possible with minimal component count and small processing power. Attiny is clocked at 20Mhz and uses one twisted pair for network connection. IT is recognized as 10MBit Ethernet switch that can send hard-coded packets once one of couple buttons is pressed. To get 10MBit from 20MHz micro thee is an assembly language involved. As it is said, this project is more for playing but strictly not for production as it is dangerous and not reliable.