The idea came to reality while I was planning for a long vacation :-). This single board computer was built to control aquarium appliances like air pump, filter, heater, etc. The heart of this computer is AT89C2051 chip. This project is a derivative of an idea by Wichit Sirichote from Thailand. The original idea was redesigned to fit the need of my fishes :-). The output is driven by ULN2003A chip. The board has a RS232 interface for downloading the programs. MAX232 is used to do the TTL<->RS232 level conversion.
The unit has three relays connected to ULN2003. More relays can be added as needed. There can be a maximum of 7 relays connected to ULN2003.