# Connecting a sensor to a PLC

I have some questions to ask about connecting a sensor to a PLC. I have decided to use Siemens LOGO for this application.   
  
1\. Are inputs usually in the form of electrical current? Or does it depends on the type of PLCs?maybe voltage?  
2\. What does it mean by analog value processing is in the 12V DC and 24V DC? Is it the input signal to the PLC?  
3\. How does the PLC recognizes that a input is a HIGH or a LOW? Is it something to do with the programming?  
4\. Does it matter if we choose the type of power supply for the PLC? It depends on the availability of the power supply,is it?  
  
Thank you!

> 1\. Are inputs usually in the form of electrical current? Or does it depends on the type of PLCs?maybe voltage? <  
  
The signal can be either, depending upon what your system will accept. A common signal is in mA, with a 4-20mA range.  
  
> 2\. What does it mean by analog value processing is in the 12V DC and 24V DC? Is it the input signal to the PLC? <  
  
Figure out what your system runs on (usually 24V, but can be 12V or even AC). Otherwise, you could get a seperate power supply for the sensor (but why?).  
  
> 3\. How does the PLC recognizes that a input is a HIGH or a LOW? Is it something to do with the programming? <  
  
That depends on the amount of current or voltage (let's use current, using the 4-20mA range) the PLC is receiving from the sensor. Reading the sensor's specs will give you the signal range (Temperature, or whatever), then you would scale  
that range with the 4-20mA range.  
  
> 4\. Does it matter if we choose the type of power supply for the PLC? It depends on the availability of the power supply,is it? <  
  
Of course, it needs to be the correct voltage and all that, and be able to handle the current draw of the sensor(s), etc.

> I have some questions to ask about connecting a sensor to a PLC. I have  
decided to use Siemens LOGO for this application.   
>   
Ok, first LOGO Siemens analogic inputs   
  
> 1\. The sensor, current 4-20 mA, the power supply circuit is the 12 a 24 V.  
  
Brigde Wheastone to sensor. Voltage the power supply the PLC is 24 V or 120 V  
or 220 V,   
  
> 2\. the analog value processing is in the 4-20 mA or 0 -10 V or mV. Is it the  
input signal to the PLC  
  
> 3\. Programing the PLC recognizes that a input is automatically  
  
Espero haber ayudado un poco,  
My english no is correct, reply my email, please

Hello, Steven;  
First off, a Siemens LOGO! is not a PLC, but a programmable relay. A PLC (S7-200, for example) can do a lot more than a relay; but for very small  
applications you can do sequencing with this.   
Now for some quick answers:  
1) all digital (as opposed to analog) inputs are given in voltage, either AC or DC.  
2) The value given here, 12V DC, 24 V DC, 110 V AC, are the high limit that the electronics will accept without being destroyed.  
3) All electronics establish high and low range values for the signals. On a 24 V DC input, a signal ranging from 18 to 24 V DC could be a logical 1, whereas a value of 0 to 8 V DC would be logical 0; a value from 8 to 18 V DC would not be recognized as valid in this example. Every input module is different, and manufacturers will give you these specs in their catalogs.  
4) Power supply input for your module does not affect the input signal values; you choose your controller to suit the available supply power.  
If you go to the "http://www.sea.siemens.com":http://www.sea.siemens.com page (Siemens USA) you will find on the  
bottom right a free self-study section; one of those trainings is called Basics of PLCs. I believe you will find more complete answers there.   
Hope this helps,  


The Siemens LOGO unit is usually 24 vdc inputs derived from the logo itself. I think 2 of the digital inputs may be configured as analog values (0-24 vdc ? perhaps!)  
The Logo power supply may be 240vac or 24vdc depending on the type you have.  
  
Good luck

Correct, in 12/24 V DC versions, 2 inputs can be used as analog input, 0-10 V. Also in the new modular range, you can add analog input modules.  
  
In 24 RC versions, the power supply is 24 V *AC* as well.  


ya, thanks everyone. I've got some idea about Logo now. Does it matter if I provide the right voltage for the logical 0s and 1s but not the  
current? As in our example (4-20mA). Actually, my sensor is made of 2 aluminium plates (maybe 3cm by 3cm) to measure the moisture content of  
soil. May not be accurate but still works. I know how to calculate the value for the voltage reading. Maybe calibrate it a bit and hopefully  
it will be fine. I used a transistor for the signal amplification. And this will be filtered by a RC filter into the PLC. Is that OK? I hope  
so.