DHT22 over cat 5

[Derrick Adkins]

Hello, does anyone know what can be done about crazy readings from dht22 over cat 5 at about 75 feet? My first reading is always OK after I connect the sensor to my pi. Every reading after that is several degrees off and the humidity is reading at about 3300%. I have read everything I can but can't seem to get it to work. '

I have added 1k resistor with no luck

I have added 10k resistors on one, then both sides with no luck

I have added a 104 (.1uf) capacitor with no luck.

I put the signal wire with the ground in the same twist on my cable with no luck.

I added a second ground on pin three on the sensor with no luck.

I am afraid to switch to 5v because I read that the gpio on the pi can only handle 3.3v

This is driving me CRAZY!

[Gadjet Nut]

The 75 feet has its own resistance so I think your idea of trying a 1k resistor is on the right track. Try other resistors between 1 and 10 k. A more scientific way to do is to use a multimeter to measure the resistance of the cable. Then subtract the cable resistance from 10k to determine the resistor to use. If the cable resistance is greater than 10k then reduce the length of the cable. You can also use the multimeter to measure the voltage to ensure the sensor is getting 3.3V.

Another option is to go wireless. Have you seen our new WIFI Temp and humidity sensor? :)

[Dave E.]

I don't know what can be done to solve the problem but offer these thoughts:

cat 5 is a cable with twisted pairs over which differential signals are transmitted with good drivers.

The DS18B20 or dht22 sensors use 1 wire technology where data from the sensor has

to be sampled by the Raspberry Pi at certain bit intervals. Ideally the data would have well defined

logic high and logic low voltage levels but the real life issue of cable capacitance and the pullup resistor

can degrade the signal, especially with a 75 foot cable. I would doubt that the dht22 has a signal driver

suitable for driving a cable of that length. The DS18B20 offers CRC to check data integrity.

I would mount the dht22 on a Rpi cobbler board and get it working. If possible use an oscilloscope

to view the data. Then splice in some cat 5 cable and observe the signal. A lot of work.