5vdc reed relay - 2.6A power supply need to step it down to 20mA

Discussion in 'The Projects Forum' started by jmeredith, Nov 18, 2009.

  1. jmeredith

    Thread Starter New Member

    Nov 18, 2009
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    Hi guys, I am working on a project and I need to use a 5vdc relay (RadShack#275-0232) to close a circuit, but my power supply is 5vdc/2.6A

    How do i step down the 2.6A to 20mA?

    resistors? pot? trying to keep it simple

    Thanks!

    -Jason
     
  2. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
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    The relay will only draw what it needs, it can be connected directly to the 5V
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    Hello Jason,
    Like blueroomelectronics wrote, if the relays' coil is rated for 20mA@5v, then it has a resistance of 250 Ohms (R=E/I, or Resistance = Voltage/Current) so if you place 5v across the coil, the maximum current that will flow through the coil is 20mA.
     
  4. jmeredith

    Thread Starter New Member

    Nov 18, 2009
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    ahh ok... I remember as a youngster I burned up many things... didn't want to see a puff of smoke...

    I am using a PIR sensor and the signal voltage is 3.2vdc, is there any way to bump that up to 5vdc so I can make the relay actuate?

    I am so excited I built my first opto-electric circuit lastnight... I forgot how fun this stuff is! Glad to have found such a great resource here at AllAboutCircuits

    -Jason
     
  5. jmeredith

    Thread Starter New Member

    Nov 18, 2009
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    the whole circuit I am trying to work on here is 5dvc power supply -> PIR -> PIR(signal) -> 5vdc reed relay ->closed contacts (when motion present)

    the power supply feeds the PIR, but when motion is detected the PIRsignal is only 3.2vdc this isnt enough power to close the contacts in the relay

    hope that gives a better over picture :)

    -Jason
     
  6. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
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    You need a transistor to drive the relay
     
  7. artcatcher

    New Member

    Nov 20, 2009
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    Hi I am working on a similar PIR Relay project and its my first electronics project so if anybody could go to the trouble of posting pics please it would be great
     
  8. jmeredith

    Thread Starter New Member

    Nov 18, 2009
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    will a standard n2222 work?
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    A 2N2222/PN2222 can likely be made to work.

    If your PIR outputs 3.2v, then the easiest way to configure the circuit would be to have the transistors' collector sinking current from the relays' coil.

    Emitter is connected to ground.
    Base is connected to the PIR output via a resistor.
    If the relay coil only requires 20mA as you mentioned before, then the transistor will require 1/10 of that current, or 2mA.

    A rough calculation is:
    (Vsupply - Vbe) / (Ic/10)
    where Vsupply is the output of your PIR sensor, which is 3.2v.
    Vbe is the voltage measured between the base and emitter when current is being supplied to the base. We'll use 0.7v for this.
    Ic is the desired collector current; stated as 20mA

    So:
    (3.2v - 0.7v) / (20mA / 10)
    2.5 / 0.002 = 1250 Ohms
    1250 Ohms is not a standard value of resistance.
    Here is a link to a table of standard resistance values:
    http://www.logwell.com/tech/components/resistor_values.html
    Bookmark it; it may prove useful later.

    E24 (5% tolerance) values are most commonly available to hobbyists.
    In this case, you will need to select the next lower value.
    120 x 10 = 1.2k Ohms is the closest standard E24 value that you can use.
    It would also be OK to use a 1.1k or 1k Ohm resistor, if you have those available.

    2.5v/1k Ohms = 2.5mA; x 10 = 25mA collector current for operation as a saturated switch.

    You will also need to use a diode connected in reverse across the coil of the diode. If you don't, the reverse-EMF from stopping the current through the coil when the transistor turns off may destroy the transistor.

    The cathode (the end with the ring or stripe) goes towards the +V supply.

    See the attached for a simulation of the circuit.
     
    Last edited: Nov 20, 2009
  10. jmeredith

    Thread Starter New Member

    Nov 18, 2009
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    wow! thanks Wookie, this is extremely helpful... Artcatcher, I will take some photos when its all wrapped up.

    Currently I am using a Sensatronics U16 (http://www.sensatronics.com/index.php/industrial-monitors/model-u16.html)

    I am working toward a more intelligent home :) so far I have:

    -a photo-resistor to detect DAY/NIGHT
    -attic temp probe
    -bedroom temp probe
    -hottub temp probe
    -PIR (9v battery powered with LED->photo-resistor for signal leg)
    -PIR (5v power supply with LED -> photo-resistor for signal leg)
    -Magnetic Switch to detect Overhead Garage door (open/closed)
    -Magnetic switch to detect Garage Man-door (open closed)
    -photo-resistor to detect if refrigerator light is on(ie. door left open)


    other project for the house
    -Moisture sensor to alert us if the cats need water (had to toss something in that the wife would like :))

    -AC Current sensing for appliance ( I would love to know if the Coffee pot was left "ON")
    Any thoughts on how to accomplish this would be greatly appreciated!

    -AC Current Sensing for Whole House (dont think this is possible with the U16 unless there is a debug mode that lets you see actual resistance values... might have to be another microcontroller)

    -Water usage for whole house (want to have some kind of Flow Meter that can detect how much water is being used.

    I have seen a few flood protection systems out there, and some Amp meter systems that are serial based but I want something that will have xml data out so I can trend it online.

    there are endless possibilities you can do to make your house "smarter" I am going to look into stepper motors to raise/lower our bedroom blinds. and am working on a I/O port on an Internet camera to be able to remotely open or close the overhead garage door.

    keep on making!

    -Jason
     
  11. BMorse

    Senior Member

    Sep 26, 2009
    2,675
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    If you are into home automation, you should check out my B.U.M.S. (Bathroom Utilities Management System)... I designed it to manage my kids use of their utilities in the bathroom, with it I can control everything in their bathroom including the shower, faucet, exhaust fan, etc. Each user has to use an RFID tag to activate the shower functions and the system has presets for each user, the shower temp is automatically adjusted per user. I can also access the system via internet through its built in HTTP web server..... I am currently modifying the system to be used in more than one place in the house, I am implementing X-10 protocols also to where I can control lighting and other appliances also.....
    http://www.morse-code.com/id155.htm


    [​IMG]
    My .02
     
  12. jmeredith

    Thread Starter New Member

    Nov 18, 2009
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    Wow Brent thats awesome! I want to start playing with stepper motors, where is a good place to start with PC controlled stepper motors?

    did you machine the belt cogs yourself?

    Must be nice to have access to a machine shop! thats great :)

    -Jason
     
  13. jmeredith

    Thread Starter New Member

    Nov 18, 2009
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    Hey guys can anyone help me with a water detector included is the wiring diagram, the only thing is I am not sure I have R2 hooked up correctly.

    Is R2 supposed to be a potentiometer? I couldn't find any 1M pots, so to test I just put in a regular 1M Ohm resistor

    other than that everything else is wired up per diagram... I am not getting any closure on the relay with the sensor probes in water...

    what am I doing wrong? I do have a couple of 100k pots, maybe I can switch out?

    thanks,

    -Jason
     
  14. SgtWookie

    Expert

    Jul 17, 2007
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    This really needs it's own thread, as you are "hijacking" your own topic.

    Please start a new thread, and call it something like "High water level detection"; just copy/paste your original text to the new thread.
     
  15. BMorse

    Senior Member

    Sep 26, 2009
    2,675
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    "PC controlled" stepper motors still have a stepper motor driver board in between the PC and motor.... I built my own using the UDN2998W, and a PIC16F84A (there are tons of circuits out on the web, there is a current thread right now on a stepper motor controller right here on AAC).... most CNC type controllers would work, it is just a matter of pulsing the Step input on the board for however many steps you want the motor to take, and set the DIR to High or Low depending if you want the motor to go left or right......There was another thread around here where I posted the Parallel port connections to a stepper motor controller....

    No, the cogs were an off the shelf Item..... the only thing I actually made for the motorized valves were the mounting brackets, everything else was an off the shelf item..... (makes it easy for DIY home automation folks to make their own without exotic machinery ;))

    I do have access to a machine shop with a really awesome CNC Plasma cutter!:D But I tend to custom make my own "tools" when needed, and I still love doing things the old fashioned way, with sweat, tears and sometimes, a lot of blood:eek:.......
     
  16. jmeredith

    Thread Starter New Member

    Nov 18, 2009
    23
    0
    I guess I should have called this thread Jason's House Projects, then I wouldn't hijack myself, and I could keep all the threads in one handy place... oh well... new thread created :)

    thanks,

    -Jason
     
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