Keep your plants watered with this watering system

If you like growing plants at home, then you know that they are restricted in space and climate. There are some things that you can do and there are that you can’t. You cant control the temperature and light much especially in areas where you live or work. It can be put near the window and temperature is mostly a room temperature. But there is one parameter that can and must be kept in mind – moisture. It is vital to almost any plant to keep soil at some moisture level and this is what you cannot ensure properly unless you keep checking it constantly. So why bother when this work can be done by putting a simple circuit.


Frank Donald built a really simple setup that works on Atmega16 based circuit. Microcontroller here is to read soil moisture sensor that comes as cheap module which includes comparator. So it gives high signal when soil is dry and low when wet. But to make it a bit smarter, he is reading analog signal from sensor, so microcontroller could read this signal and according to its level control a relay to water pump. By setting some thresholds he made simple hysteresis to prevent relay from chattering.

In other hand similar result can be achieved much simpler – you can connect sensor digital output directly to relay driving transistor and have similar result.

Water softener level detector

In places where water contains higher concentration of dissolved minerals is considered to be hard ware. It does not affect human health, but teapots, washing machines and dishwashers suffer from calcification. To avoid this problem, there are water softeners used where special salt is used to soften water. Of course this salt also dissolves and needs to be refilled time to time. David always forget to refill the tank because there is no visual indicator of it. So what you don’t see – you don’t care. To avoid future conflicts with his wife, he decided to add visual indicator to water softener.


On front panel he mounted LED bar graph where salt level is displayed. Also he added photocell to detect room light. It simply detect if someone is in the room or passes by by dropping a shadow. Other time bar graph is turned off. This hopefully prolongs life of LEDs and saves a bit of electricity. Another part is measuring salt level. Since salt is hard material, he used Sharp GP2D12 Infrared Distance sensor. It is able to measure distance from salt from 10cm to 80cm.

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.