Please Help me Understand these relay Circuits

Discussion in 'General Electronics Chat' started by mattd860, Jan 9, 2016.

  1. mattd860

    Thread Starter Member

    Jan 9, 2016
    57
    2
    Hi All - I need some assistance with understanding some of the circuits shown in the first two pages of the attached document (please ignore page 3 &4). Particularly, I'm having difficulty understanding how two relays operate - the Automatic Wiper Relay on page one and the Windshield Wiper Intermittent Relay on page two. Both of these relays seem to operate in a way that I'm completely unfamiliar with.

    Let's focus on the Windshield Wiper Intermittent Relay on page two... I don't understand how it is triggered because the trigger doesn't appear to be energizing a coil and then going to ground like I'm used to with typical relays. In this diagram, The GRN/BLK wire as battery voltage (12v) supplied to 100% of the time on one side of the coil but that voltage is carried through right to the other side onto the GRN/RED wire which also has voltage 100% of the time. So how does it get triggered? When the relay is triggered, the voltage in the GRN/RED wire drop to zero for a second and then returns to battery voltage. Is there a coil or something inside the Rain Sensor that grounds the circuit?

    I also don't understand the BLU/WHT circuit that goes between the Windshield Wiper Intermittent Relay and the Windshield Wiper Motor (and also also connects to the Rain Sensor) seen on page 1. When the relay is triggered, The WHT wire connects to the BLK wire (going to ground???) which leaves the BLU/WHT wire connected between the Wiper Motor and Rain Sensor. The WHT wire has continuous voltage and so does the BLU/WHT wire until the relay is activated. When the relay activates, the voltage in the BLU/WHT wire drops to zero and returns to battery voltage then the wipers make a full sweep.

    Can someone please make heads or tales of this so I can better understand how the wiper motor is activated via the rain sensor?? If someone can simply describe how this type of relay works I would appreciate it. Thanks!!!!
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,156
    3,063
    Yup. The voltage on the GRN/RED is there because it's coming through the coil itself and is essentially "open" on the other side. Then the sensor (or more likely the computer?) activates the coil by connecting the RED to ground. That's very typical in automotive, to control things by connecting them to ground, leaving them connected all the time to +12V.

    Too sleepy to look at the other circuit, but I'll predict it behaves very similarly.
     
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  3. mattd860

    Thread Starter Member

    Jan 9, 2016
    57
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    Thanks Wayne!! Your explanation was perfect so I now fully understand how this operates.

    I now need to come up with a programmable I/O board and I am slightly familiar with Arduino, but let me ask you - is there any way to program the Arduino board (or any other type) to read ohms of resistance as an input? I want to use GRN/YEL wire on page two of the attachment as an input, and based on that input (1ohm, 6ohms, 10ohms, etc) I can program the appropriate output. For example, if input is 10 ohms, output will pulse on for 5 seconds and off for 5 seconds.

    In the past I've only ever worked with voltage input & voltage output and never used resistance as an input.
     
  4. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    No, the Arduino won't read ohms. However, it will read voltage. If you know the current, you calculate the ohms. This is often done with a pot wired as a voltage divider, giving a known current.
     
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  5. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    The way you read a resistance is to convert it to a voltage.

    The schematic shows you have access to one lead of a pot, the other end is grounded. Perfect. You drive the end you have and read the voltage, and frm this you infer the resistance.

    There are a few ways to drive this depending on how accurate you need make the measurement. A resistor is easiest but the readings are not linear (you have a classic voltage divider).

    A constant current source is linear but may allow the voltage to go too high or not high enough. Too high for example when the external resistor is not there, too low if the current source needs too much voltage to start. Both can be accounted for as part of the normal design process.
     
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  6. mattd860

    Thread Starter Member

    Jan 9, 2016
    57
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    I'm a little confused about how to read voltage from the lead of the existing pot (GRN/YEL) wire. Typically there is at least three wires on a pot, fixed voltage, ground, and the third wire is the result after the voltage hits resistance. In my situation, I only have two wires. I can send, for example, 5v to the GRN/YEL wire that goes to the pot which would be reduced as the voltage travels through the resistor to ground, but then what goes back to the analog input on the arduino board (or any other system I decide to use)??

    Can the GRN/YEL wire double as both the supply voltage for the pot and the analog input?
     
  7. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    It sure can, like so:

    [​IMG]
     
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  8. mattd860

    Thread Starter Member

    Jan 9, 2016
    57
    2
    Thanks Ernie!! That is great news! My little project is becoming much much easier. I'm so glad I joined this forum.

    I noticed that you added a resistor to the 5v wire (circled in red) - what type/size resister do you suggest?
     
  9. mattd860

    Thread Starter Member

    Jan 9, 2016
    57
    2
    Ernie - Can you please clarify the need for the resistor in the diagram you made and what type of resistor you recommend? Thanks!
     
  10. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    You can have any values you want, all your doing is creating a voltage divider across the variable resistor, so adjust the resistors to give you a voltage from 0 to 5v for the A/D input, a starting point would be a fixed resistor of 10k, and a variable of 100k.
     
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