How to make a water level meter with uln2803 first part

In this tutorial we will see how to make a water level meter circuit with the built-in uln2803.
The printed circuit is included, the operation of the same will be explained, as well as we will see the characteristics.
It will explain how to assemble the zonda, to measure the water level in a water tank.
This is the first video of two, in the second video, we will bring the measurement data to the internet, with some cloud service.

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The ULN2803 integrated circuit is used as a power interface for circuits with CMOS and TTL technology. This integrated has inside 8 NPN transistors in Darlington configuration with protective diodes, to be able to control motors, inductive loads,relays, and more. Each output can control up to 500 mA so you can control different types of loads. Outputs can be connected in parallel when higher loads need to be handled, however it is advisable not to exceed the total power of the circuit.

Features:

- 8 NPN Darlington transistors in common emitter
- 8 reverse voltage suppression diodes to handle inductive loads, with common cathode connections
- Outputs of 0.5A (500mA) max.
- Transistors can be connected in parallel for greater current capacity
- Output support voltage: -0.5V to 50V
- Max power: 1.47 W
- Input resistance to the base: 2.7k
- Input voltage: -0.5V to 30V
- 5V TTL and CMOS compatible inputs
- Encapsulated: DIP 18-pin

Applications:

Command of relays, lamps, or displays (LED or gas discharge), etc.
Logical buffers
Line drivers
Among other applications

Operation

To activate each output of this integrated circuit, the water must not be pure, in its contents it must have some salts, which help to conduct electricity, that when reaching each of the levels in the container, closes the electrical circuit of the ulN2803 inputs, which detect the high voltage level and turns on the LED corresponding to each output. At each output we can connect for example a relay, a buzzer and even the pump of an engine to refill a water tank, if necessary, up to a certain level and disconnect the water pump when reaching the desired limit.

The following figure shows the ULN2803 pinout. Comes with 8 input pins and 8 output pins.

The following figure shows the logical diagram of ULN2803. It is a visual representation and arrangement of how diodes are connected in the component.