I made the minimum system of MCS51 microcontroller at this project. You can find the example of MC51's minimum system schematic on the internet, there's a lot of it. But at this project i use my creativity to route and wire the minimum system board on my own. The components needed for this project included :
- AT89S51 IC
- LM7805 regulator
- Crystal 22 MHz
- 2x Capacitor 22pF
- LED
- Pushbuttons
- Pinheaders
- DC female jack
You can take a look at the picture below for the example schematic. I got this picture available online from here. But for this project i made some modification for the schematic that you can see at the PCB folder.
You can upload program to this board using USBASP programmer which can be purchased at local electronic stores. I've also created the port to upload program by extending the MOSI, MISO, and SCK pin. To test whether this board working or not, i upload a simple program to blink the LED in port 0. The file is main.asm that available in this repository.
I made the detector of water level using MCS51 as the microcontroller. I used 2 sensors for this project, one at the bottom of the tank and another one at the top of the tank. You can see the clear picture below 
Sensor i used in this project is very simple. I made two nearly-connected conductor surface at dot matrix PCB. Each of this surface was extended with cable and connected to my comparator board. The design of this board is available at PCB folder of water_level_sensor project. Then the sensor was connected to comparator, i used LM324 as the comparator.
The idea is quite simple. If there is no water touches the sensor, the resistance of the sensor is very high because it's open circuit. Because of this resistance value, the comparator will generate low logic. And vice versa, the comparator will generate high logic if there is water touches the sensor. The comparator board is equipped with LED as an indicator.
I move a servo using MCS51 with assembly language in this project. It may look simple, but generating pulse with this primitive language is not as easy as it said. The type of servo i used is tower pro sg90. This servo works between 0-2ms with period of 20ms. 
By setting the pulse 1.5ms, the servo moves to position "0". Pulse 1ms sets the servo to position "90" and pulse 2 sets the servo to position "-90".
This project include arduino nano as a pulse generator. Arduino nano will generate pulse with a frequency that depends on analog read value from potentiometer. This pulse will be read by the mcs51 board that connected to the LCD. This LCD then display the value of pulse. The code for mcs51 board is available in the assembly code folder and arduino code is available in arduino code folder of pulse_read project. The LCD type used in this project is Dot Matrix Liquid Crystal Display (LCD) HD44780.