Posts: 7
Joined: Sun Jan 25, 2015 1:13 pm

Design needed.

Tue Mar 31, 2015 2:23 am

I'm currently working on a college project based on IEEE paper raspberry pi as a wireless sensing node. I'm using a dht11 sensor for measuring temperature and humidity. According to the data sheet I have used an 4.7 Kohm resistor with the dht11. And because it is failed to work replaced it with a 10 Kohm resistor suggested by a thread here. I am currently obtained a partial output. The real problem is my project evaluation committee members are insisting on the design details of resistor used. I've told them its according to the data sheet. But the want me to show the selection by using equations. By so far I can only learn that it us used as a pull up resistor. Some say it as a current limitter. Can any one help me with the design and walk me through the design of the resistor used with dht11?

User avatar
Posts: 1820
Joined: Mon Mar 23, 2015 7:39 am
Location: Rochester, MN
Contact: Website

Re: Design needed.

Tue Mar 31, 2015 7:32 am

Pullup resistors are typically NOT designed, as it were, because they usually are not critical. They are chosen based on the characteristics of the device driving them, but usually with wide tolerance for success. A weak pullup would be in the range of 100K. A medium strong pullup would be in the range of 10k to 15k. A very strong pullup would be in the range of 3k to 4.7k or maybe as low as 470 ohms.

This sensor sends a digital signal (with critical timing) by grounding (going low) . The pullup assures the 5v is held high when the sensor it not going LOW; because the output from the sensor is probably an open collector... has no HIGH of its own. The sensor grounds against the pullup resistor. If the resistor is too weak the HIGH will not be maintained, and if its too strong (too small) too much current will flow through the sensor's open collector. You need to know the current limit for the sensor's collector (its forward voltage and forward current) then you can use ohms law to calculate the value of the pullup so that the forward current of the open collector is not exceeded.

What is silly about this is that you don't really need to calculate anything... a pullup resistor of 4.7k to 10k will work; maybe even as high as 15k. I would use a 10k resistor as the pullup resistor because that is what is recommended... nobody really goes through the science of calculating the value 'critically' because it isn't really critical. Sometimes, from sensor to sensor, you have to experiment a bit.

Just saying.

<sorry if not too helpful>

Return to “Automation, sensing and robotics”