Attiny85 based dual lead acid battery charger

Superbender has a constant problem with his camper bus batteries. Normally camper is left over the winter and so batteries dies without proper care. In camper there are two batteries – main starting and auxiliary. So he decided to build a dual battery charger to keep them alive over the winter season.


He already had a 15V 5A power supply which seemed to be just right. Since batteries will be connected to charger over all winter time, charging current doesn’t play a significant role. Charging circuit is pretty simple. Control is based on Attiny85 microcontroller which controls both charging channels. Two ADC channels are used to read voltages of both batteries Resistor dividers scale voltage down to microcontroller level. Charging circuit is controlled by transistors that are optoisolated from microcontroller. Battery charger works in charging and monitoring mode. It compares voltages of both batteries to upper 14V and lover 12V thresholds. When lower limit is reached – charging of this battery begins, when it’s been charged to 14V, then charging is stopped. During long time, normally there will be one battery charged overtime.

Being smarter with minimal AVR development board

Sometimes simplest ideas work best. If you like prototyping a lot, then you know, that building complete circuits on a breadboard can be pain. If your projects include microcontroller like AVR this part is always the same – micro and programming header. It may be OK if part is through hole, but if it’s in SOIC package. Baoshi found a simple solution to this routine.


He used common 28 pin SOIC/SSOP to DIP adapter and soldered microcontroller here. Then on remaining pads he soldered ISP header on top. Of course he had to bend legs to make it fit on surface. Since there is another SMT footprint on bottom, here he could electrically connect programming header to corresponding programming pins. This way without significant investment, he made robust module, that can be plugged to breadboard and used right away. As someone suggested, there are still place for decoupling cap and probably other support circuits. Great idea and time saver.

Outstanding steampunkish VFD wristwatch

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.

Redesigned bike light controller gives exactly what you need

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.