Connect with us

Help with 555 device and interfacing with another device

Discussion in 'General Electronics Discussion' started by Boguita, Mar 31, 2016.

Scroll to continue with content
  1. Boguita

    Boguita

    12
    0
    Mar 31, 2016
    Hello all of you!

    I'm very new to the world of electronics, so i praise for your patience in helping me and making me understand what am i doing wrong in my project. Thanks in advance for your time.

    So here it goes:
    I have one 555 device that runs on 12v and i wish to "interface" it with another device (one hotspot) that i can only find 1.8v in two test pads.

    What i need sounds fairly simple: when i switch the hotspot on i want it to connect to the 555 so that it activates a relay, that will light up some led light while the hotspot is on.

    I've tried multiple things: 547npn transistor, connecting the base to the 1,8v with a resistor in the middle that in one simulation made it look like it would work, but it didn't. I don't know why.
    I also tried to mess up with the control voltage from the 555 so that the comparator would be triggered by 1.8v instead of 12v but i also didn't had success with it.

    The 555 device is from velleman and i have the schematics from the simulation i was running with the transistor trying to act as trigger. I will upload that image, but i am open to any solution that is in the reach of my (low) capabilities.

    Thank you
     

    Attached Files:

  2. davenn

    davenn Moderator

    13,645
    1,884
    Sep 5, 2009
    hi there
    welcome to EP :)

    you problems lies with the way you have wired up the LED and relay
    you are trying to power the relay via a 1k resistor and the LED ... that isn't going to work

    I don't see what the relay is supposed to be doing ?? at the moment, even if it was able to be activated
    it isn't switching anything as there is nothing connected to the switched contacts

    Remove the relay and the diode ( the diode is connected the wrong way around anyway)
    Leave the 1k resistor to the LED and connect the other side of the LED to the NPN transistor collector
    Change the 10k on the transistor base to around 4k7,


    Dave
     
  3. Boguita

    Boguita

    12
    0
    Mar 31, 2016
    Hey dave! thanks for your reply.

    Well i think that i didn't explain it like i should, or the images aren't clear on explaining my problem.

    The relay works as this is mounted. I didn't draw anything on the relay because i know how to connect my final led through it and at the time it wasn't relevant to my problem.
    The situation is to have the 1.8v triggering that relay through the 555 device. OR any other way that it can work! :)
    The led light on the schematics belongs to the 555 of the shelf device, not the led light i need to power.
     
  4. dorke

    dorke

    2,342
    665
    Jun 20, 2015
    What is the purpose of the 555 in your circuit?

    If what you need is to drive a relay and a LED-on constantly once there is 1.8V from the "hotspot".
    You can do that with a single NPN transistor without the 555.
     
  5. Boguita

    Boguita

    12
    0
    Mar 31, 2016
    I don't need the 555 or the relay, it just seemed safer since i would like to have both voltages isolated so that no harm is done to the hotspot.

    i have been thinking about that. (using the npn to power the external LED directly) But i am afraid of damaging the "hotspot" since i am not experienced with transistors.
    Also i don't know the current from the 1.8v point from the hotspot to determine the resistor i should use to power the base from the NPN.
    And should i put the led anode directly to +12v, (the led is on a switch, so i can't know the value of the resistor but it says it's already installed so that it works with 12v) and the cathode to the collector, emitter to 12v gnd?
    Would that work?

    thank you
     
  6. davenn

    davenn Moderator

    13,645
    1,884
    Sep 5, 2009

    The circuit was very clear as were the errors in the circuit

    You CANNOT run the relay via a 1k resistor and LED, The relay will never get enough current
    The diode across the relay, as I stated, is back to front

    so If you really need to power the relay, then you must remove that LED and 1k resistor
     
  7. davenn

    davenn Moderator

    13,645
    1,884
    Sep 5, 2009
    ohhh and I agree with Dorke's comments, there probably isn't any need for the 555 and its circuitry anyway
     
  8. Boguita

    Boguita

    12
    0
    Mar 31, 2016
    I don't know how to explain this to you but i'll just post the original diagram from the device. maybe i've messed up on copying the schematics to the simulator, i don't know.
     

    Attached Files:

  9. dorke

    dorke

    2,342
    665
    Jun 20, 2015
    It is very simple:
    What is the current of the 12V LED you need to drive?
    The circuit should look like this:
    It will be good for 20ma LED or lower currents,with that 1k base resistor.
    What NPN transistors do you have?
    1.8v led drive.JPG
     
  10. Boguita

    Boguita

    12
    0
    Mar 31, 2016
    Will test it right away. I don't have spec sheet for the 12v led. Will be back to you shortly.
     
  11. Colin Mitchell

    Colin Mitchell

    1,417
    312
    Aug 31, 2014
    You need a 330R resistor in series with the LED.
     
  12. davenn

    davenn Moderator

    13,645
    1,884
    Sep 5, 2009
    yes, you erred in your simulator drawing ;)

    that's all cool
    now, do you actually need the delay timing that the 555 cct is giving ?
    if so, use that original cct you have just shown
    if not, then use dorkes' direct transistor drive method


    Dave
     
  13. Boguita

    Boguita

    12
    0
    Mar 31, 2016
    i'm testing Dorkes' transistor drive but this seems rather erratic. it does light up but seems that my hand is controlling the base from the transistor as it lights up when i touch it....is it possible? :p
     
  14. dorke

    dorke

    2,342
    665
    Jun 20, 2015
    What transistor are you using?
    It is possible,what happens when you are not touching?
    Is the Led on and off with the "hotspot" on and off?
     
  15. Boguita

    Boguita

    12
    0
    Mar 31, 2016
    457 npn. when i touch the base its dim but lits up. but the hotspot is not controlling it. :(
     
  16. Boguita

    Boguita

    12
    0
    Mar 31, 2016
    Sorry , 547 i've mistyped it before
     
  17. dorke

    dorke

    2,342
    665
    Jun 20, 2015
    "but the hotspot is not controlling it."
    what do you mean by that is the LED on or off?

    Can you measure the 1.8V point to ground with the "hotspot" on and off?
     
  18. Boguita

    Boguita

    12
    0
    Mar 31, 2016
    My friends i've managed to do it!!!
    I don't know exactly why but i've found a way:

    If you check the photo attached, there is a ground lead that's soldered. That's ground for the hotspot.
    Right next to it is the original 1,8v pad that i was using and failing to light up the led.
    In my hand is the transistor base lead that i'm shorting to another 1,8v pad that i found now. It's hidden behind the hotspot's display.
    In that point the thing works like it should. On when the hotspot is on, and off when it powers down. i don't know the difference between the two of them because the voltage behaviour is the same, i guess that it has to do with current?

    Either way i've found a way that it works, thanks to you guys!
    Now i don't know how will i solder a cable to the pad that works without damaging anything but i'll try.
    I guess i'm owing you some beers!
    IMG_0764.JPG
     
  19. Boguita

    Boguita

    12
    0
    Mar 31, 2016
    And i'm back..... :(
    I have a little quirk. The hotspot devices when powers down starts charging right away. Meaning that the led won't go off because the 1,8v point stays on when charging occurs. So i wanted to make it charge only when its on.
    I tried to replicate the transistor setup but instead of having the cathode from the led light conected to the collector i have the negative lead that comes from the charging device (wich is being fed from constant +12v) and on the emitter i have ground that is both going to the negative from the car battery and hotspot's ground. This should be working, right? But it turns on as soon as +12v is present instead of waiting for the transistor to have signal on the base.
    Why is this happening? Shouldn't it work like with the led light? Or should i change the resistor on the base so it blocks drainage?
    I don't know the power consumption for the controller, and i am not sure how to calculate the resistor value if it has to be changed.

    Thank you again
     
    Last edited: Apr 1, 2016
  20. dorke

    dorke

    2,342
    665
    Jun 20, 2015
    Please show a drawing of what you did...
    A picture is worth a thousand words;)
    Can you measure voltages with a DVM?
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-