Basic circuit design with 2 different voltages

Discussion in 'Digital Circuit Design' started by CLiFoS, Jul 1, 2018.

  1. CLiFoS

    Thread Starter New Member

    Jul 1, 2018
    3
    0
    Hello everyone,

    As you may understand from my question, I'm a complete starter/curious on electronics and am trying to create my own electric door opener for my backyard gate.

    The idea is to have an RF remote control and I'm using Adafruit 1095 and 1096 (5VDC) to trigger the 12VDC electric strike so I added a relay G5Q-1A-DC5 and a voltage regulator MC79L05ACP for the receiver 5VDC.

    I think it's something like the attached image.

    Can you guys confirm the wiring is correct and if I'm not missing anything?
    I'm not sure if the 12VDC and 5VDC can have the same common ground and the same with the relay.

    Thanks in advance for helping :)

    PHOTO_20180701_190734.jpg
     
  2. danadak

    Distinguished Member

    Mar 10, 2018
    2,891
    626
    Is the electric strike power simple 1/2 wave unfiltered unregulated supply ?

    upload_2018-7-1_14-55-29.png


    If it is add more C at input of VR to get the ripple out.
     
  3. crutschow

    Expert

    Mar 14, 2008
    22,004
    6,364
    Yes all the circuits can (and must) have a common ground.

    Your relay appears to be backwards. The remote should control the relay coil and the contacts control the strike.

    Both the strike and the relay coil should have a diode (1N4148 or similar) across them (cathode to plus) to suppress the inductive transient.

    What's the strike coil current or its resistance?

    You mention two different types of remote receivers.
    Can the one you are using handle the 40mA relay coil current?
    You will need to add a transistor driver otherwise, such as a 2n7000/2.
     
  4. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
    7,583
    1,845
    The 79L05 is for regulating -5V. You need the 78L05 for +5V.
     
  5. CLiFoS

    Thread Starter New Member

    Jul 1, 2018
    3
    0
    Thank you for your replies.

    The seller didn't provide much info about the electric striker but I managed to determine that it draws about 0.95A and peaks at about 3A for a few milliseconds.

    I did draw the relay backwards! Thanks for letting me know :)

    "You mention two different types of remote receivers."
    It's only 1 receiver and 1 Key FOB. You can see the products in the link.

    "Both the strike and the relay coil should have a diode (1N4148 or similar) across them (cathode to plus) to suppress the inductive transient."
    Thanks for the tip. I will add the diode :)

    "Can the one you are using handle the 40mA relay coil current?"
    You ask a very good question. I asked in the Adafruit forum and "The spec sheet for the chip does not mention a current limit. But 10 or 20mA seems to be about the practical limit".
    I was originally considering using an Arduino Pro mini that will do the 40mA but forgot to check this when I changed my approach. Thanks for reminding me of this.
    I can't find a "2n7000/2". Did you mean 2n7000 like this?
    Just had a crash course on transistor drivers and if I understand correctly, it would go between the receiver and the relay where G is receiver pin, S connects to ground and D to relay, correct?

    Just one more thing, I think I made a mistake and ordered a 3S Li-Po and a 3S BMS charger but I think I should have went for 4S!
    3S may work when the battery is fully charged but I won't be able to use most of the power in the battery for the strike, isn't it?
    They don't say what's the minimum voltage for the strike and I'll have to test when I get the battery. If I change to a 4S, would the maximum 16.8V be acceptable for this circuit or woulr I have to add anything else?

    Thank you very much for your help :)
     
  6. CLiFoS

    Thread Starter New Member

    Jul 1, 2018
    3
    0
    Thanks
     
  7. crutschow

    Expert

    Mar 14, 2008
    22,004
    6,364
    You showed two links in your first post (Adafruit 1095 and 1096).
    Are those both the same device?
    Yes.
    Yes, again.
    Depends upon the minimum voltage the strike requires.
    The voltage of a LiPo battery does not drop much as it discharges, so if it works with the battery fully charged, it will likely work until the battery is near total discharge.
     
  8. danadak

    Distinguished Member

    Mar 10, 2018
    2,891
    626
    For striker/relay drive circuit should look like (for using MOSFET) -

    upload_2018-7-2_5-16-31.png

    Diode protects MOSFET during turnoff of inductive load (relay/striker).

    1K ohm limits peak gate charge current for a UP output.

    10K ohm to keep MOSFET off when UP output is tristated which can
    allow leakage to turn on MOSFET.

    If not using UP you can eliminate the 1K ohm. Make sure you do not exceed
    the Vgsmax ratings of the MOSFET you choose.

    Regards, Dana. upload_2018-7-2_5-16-31.png
     
  9. olphart

    Active Member

    Sep 22, 2012
    78
    18
    I'll suggest measuring the striker solenoid for both inrush and hold currents.
    Size a resistor for wattage and value to hold striker after actuation.
    Size a capacitor (several KuF+) to reliably feed inrush current pulse.
    This will significantly reduce battery draw after initial activation and striker coil heat.
    Advantage increases proportional to striker on time.
    The RC time will limit how frequently inrush on activation will work.
    Resistor is between + and relay input, cap is from there to ground.

    I had to do this on a vehicle lift safety latch. It was on for up to 90 sec.
    Using direct drive power wasn't an issue, but the coil went stupid hot.
     
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