DHT22 temp/humidity sensor has wrong readings with relay

Hi there, im new on this forum and want say hello to everyone, at begin want to apologise for my english, i cant write technical language, but i will try to describe my issue best i can, and i have hope, that somebody help to solve it.

My first project is a humidity/temp control board, based on arduino uno clone, it's a board with ch340 and on centre of board are two push buttons marked as KEY L and KEY H, but lets we back to problem, my set working perfect just with dht22 and three temporary led's conected to pins four, seven and eight which are responsible for controling fan, mist maker, and heater. Problem begin when i connect relay switch, a sensor start go crazy, when i set desired humidity at 10 percent, a relay is ON which is wrong, normal humidity is around fifty percent, so it should be off, and swithing OFF when i change on ninety percent, and temp sensor also giving wrong readings.

Any ideas, what is a reason of that behaviors? Thanks for reading

Ps. can't use six, seven, eight and nine on my laptop :)

Welcome in Arduino land,

The reason can be that the relay is drawing too much power, causing the humidity sensor to be underpowered resulting in wrong readings.

Can you post a schema how you connected the relay? It should have a power source of its own and a diode etc.

Check this - http://www.electroschematics.com/8975/arduino-control-relay/ -

Wow, thanks budy for quick reply, this board frying my brain for last two evenings :), i plug + and gnd from arduino board straight on breadboard bus, and dht22 and relay draw power from this bus, led’s use only gnd bus, and anode with resistors are plug straight to pins, four, seven, and eight. Board is powered from netbook usb, i belive it may supply curent only around half amper, and i have no clue how much curent need relay. I will post pics bit later, need drop wife to town. Thanks again

Have a look at this tutorial.

Hi there, thanks for any advice, still strugle with relay switch, a one idea why it do not switching on and off properly, is insuficient curent from digital pins, even powered from separate power supply (5-7v, 80mA) don't give expected reaction. Arduino gnd and relay power supply was connected together, only (5-7v+) was pluged to Relay VCC. I also check readings from dht22 using serial monitor, all things seems to be fine, i have some pics with wiring,for me look fine, but i will wait for your opinion.


The digitals pins of an Arduino CAN NOT switch a relay. They can provide 10-20 mA not the 100's needed for a relay. An arduino can drive a power transistor like in the proposed tutorial. Have you read it?

What are the specifications of the relay? datasheet?

Sorry, your link makes no sense to me.

what u don't understand in link?..... few pics with wiring with led's, and datasheet for relay, if doesn't work, just tell me, using cloud first time, do i need to buy extra shield for relays for pin output curent step up? Yes i did look into tutorial, i dont have curently transistors to make it.

Ps. tried relay just with power supply, without arduino, is it normal that IN1 and IN2 work only when short with GND? i think it should switch on with 5v+ at IN1 or IN2 and not with gnd..... wtf?

I can't get a good picture of the wiring because I do not want to spent 30 minutes to reverse engineer the wiring.