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Posted (edited)
Hello everyone, I am a freshman, studying about PLC. I work with S7-400 system. Please help me how to read digital input signal directly from humidity sensor using PLC. Thank you so much. Best regard! Edited by bonnyng
Posted
Pls describe more information (type of humidity sensor) in order to people help you, for analog type you need ADC module. Anw, there are usually 3 pins of original humidity sensor: GND,5V and DATA, base on its data sheet, you will know how capacity and frequency of data of this sensor to handling the signals in your program
Posted
Hi mechatronic_hieu, Thank you so much for you reply. I want to read digital signal from SHT11 humidity sensor, SHT11 have 4 pins, include Vcc, GND, serial clock input and serial data bidirectional. I want to know exactly does PLC siemens S4 can read digital signal directly and what are the functions, blocks (LAD) need to transfer the signals. I want to read humidity so that I can use this data to control some output actions, maybe it is a comparison with a constant number to create a signal level. Best regards Bonnyng
Posted
By your description of the sensor do I suspect you can't use ordinary PLC I/O to read data from it. Found a data sheet of the SHT11 The SHT11 is intended for a supply voltage of 3,3Volt. It can only be read by using the I2C communication protocol. I'm afraid you don't need a PLC for this, you need a microcontroller. Data Sheet
Posted
Hi Bryll, Thank you so much for you attention. Using PLC with sensor humidity is one of my input parameter to control a system including many inputs. I've read PLC siemiens manual, they've shown that PLC siemens can work with both digital and analog input signal. I think I have to spend more time to figure out how to use it. Do you have any suggestion or should I start from what to due with this problems? Thanks again Best Regards.
Posted (edited)
The digital inputs for PLC's are very often 24VDC, at least it's the most common type I use. Analog inputs are quite often 0-10V, 0-5V, -10- +10V and 4-20mA or 0-20mA. Might have missed one or two analog input types. If you want to use the sensor you specified, you have to build a circuit for its supply voltage. You also need to have a circuit for the I2C communication bus (for the correct voltage levels), and of course implement the I2C communication protocol in the PLC. The I2C bus is developed by Philips Semiconductors, here's the specification. Don't think you can manage that with ordinary digital I/O's, since you need to both read and write data with the same "pin". I would look for a humidity transducer that will give an analog output and could be supplied with 24VDC if I where you. Do a Google search and you'll find several I think. Edit: Forgot to mention that the I2C bus is for communication between components on circuit boards, not with automation equipments. Edited by Bryll
Posted
Hi Bryll, Thank you so much. I'm trying to find an analog Humidity sensor component now. I hope I can figure out this problem. In the worst case, I might use I2C protocol combining with microprocessor, but I really don't think much about it. Thanks for your help again. Best Regards.

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