Introduction: How to Make a Water Level Meter With Uln2803

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Step 1:

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

Step 2:

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


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


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

Step 3:


To activate each output of this integrated circuit, the water must not be pure, in its content it must have some salts, which help conduct electricity, which when reaching each of the levels in the container, closes the electrical circuit of the ULN2803 inputs, which detect the high voltage level and light the corresponding LED for each output. At each outlet we can connect, for example, a relay, a buzzer and even a motor pump to refill a water tank, if necessary, up to a certain level and disconnect the water pump when the desired limit is reached. The following figure shows the ULN2803 pinout. It comes with 8 input pins and 8 output pins.

Step 4:

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

Step 5:

The following figure shows the schematic diagram of each Darlington pair.

Step 6: ​Materials

8 1K resistors

Step 7:

8 10K resistors

Step 8:

Step 9:

Male pins

Step 10:

An uln2803apg integrated circuit

Step 11:

One 18-pin socket

Step 12:

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