Esp8266 ADC airflow sensor - how to reduce voltage?

ESPHome
1

Hi,

I’m replacing a failing Alusta house ventilation system controller with a esphome based controller and sensors.

Yesterday evening I took apart the airflow sensor that comes with this system. It turns out is quit a simple sensor: 12V DC input and the (analog) output based on airflow is 0.5 to 4.5V DC.

I have measured the output voltage and indeed it goes up if the airflow is increased by a higher FAN speed. lowest is 0.5 (no airflow), highest is 4.5V (max speed).

How can I precisely reduce the voltage from 0.5-4.5V to 0-1V or 0-3,3V so the ADC of my esp8266 can read the full range of values?

I am not that familiar with electronics, so I need some expert help :wink:

Michel

2

I think you’re after a voltage divider.

https://learn.sparkfun.com/tutorials/voltage-dividers/all

3

Thanks,

I understand, but I was looking for some advice on what resisters to use exactly.

4

Screenshot_20200307-023556
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The size isn’t hugely important except to say that you don’t want to go too small and allow a high current flow.
The ratio is what’s important for a voltage divider, and according to the above calc you want a ratio of 3.5 : 1 to size your resitiors.

5

Noting that it will be difficult to find a resistor with a value of 35 (it’s not in the e24 series) you could use:

5.6 and 1.6 as both are in the e24 series of resistors and have a ratio of 3.5.

Or any multiple of ten of both (within reason, as noted by @jivesinger above).

560R and 160R would be a bit small and allow too much current.

56K and 16K would probably work.

560K and 160K is getting into the too small current region.

I’d say 5K6 (5.6 Kilo Ohm if you have not seen that notation before) and 1K6 (1.6 Kilo Ohm) is pretty much spot on.

6

Thanks. Sadly I’m sot able to get the 1.6 nor the 3.5Kohm from the local supplier.

image
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I ordered some from Aliexpress now.

7

You don’t want 3.5K Ohm. You want 1.6 and 5.6.

Also tell your local supplier to get with the times. Stocking the e12 series is so 1980.

8

I understand, it’s eighter 1K + 3.5K or 1.6 + 5.6K. I ordered 1.6 + 5.6 and now I just need to sit back and wait for 6 weeks :wink:

Thanks!

Michel

9

Or you could just use a trim pot such as this
s-l300

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