Provide an Arduino code for this circuit diagram
provide picture in a good resolution
is this a test for us?
What is that big pink U shaped thing?
U shaped thing is not functional, it is either not connected or shorted
The "pink thing" seems to have a label that looks like 'heater', but it is blurred and covered by the cursor.
We are not a code writing service. We will help you with advice on how to code or even correct mistakes.
Also How do you expect anyone to write code when you don't tell us exactly what you want the code to do?
You might want to look at this How to get the best out of this forum before you proceed any further.
We only know what you tell us, and without knowing what you want, we don't stand a chance. Even if some one had already written code to post to you.
Tropical fish tank water temperature controller? The indicated temps are however a bit high 
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The LCD display does not display a temperature on the screen and the fan keeps on running. Please help me what is the problem
You need to post your code in code tags
The LOW output voltage may not be low enough to shut off current flow.
Try a 1K ohm resistor from the Arduino to the base of each transistor, and another 1K resistor from each base to GND. Voltage divider. Works for me.
crossposting
Which is crossposted? The display or the transistors?
The LCD display does not display a temperature on the screen and the fan keeps on running. Please help me what is the problem
@sbahlerh_andy, do not cross-post. Threads merge.
You need to have a variable resistor connected to pin 3 of the LCD module to adjust the contrast.
See this tutorial:
https://lastminuteengineers.com/arduino-1602-character-lcd-tutorial/
for more information.
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