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Simple switch trigger?

Discussion in 'General Electronics Discussion' started by fishercounter, Nov 29, 2013.

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  1. fishercounter

    fishercounter

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    Nov 29, 2013
    I'm trying to add a switch that controls two seat warmers at once. I know I need two diodes to isolate the new circuit so the old switches don't trigger themselves.

    Anyways here is the problem I'm having.

    I hooked up a lead to one switch, and when I touch the lead to the chassis ground it's just like I'm pressing the switch, so perfect. BUT when I add on the diode (yes I know they have a direction) it no longer works, what could the diode be doing to cause it not to work?

    Do I need to add a capacitor? the diode is interfering somehow, I just don't understand what it's doing. I know the voltage drops from 4.7 to 4.4 and the amps drop from 0.9 to 0.8 when the diode is inline.

    I thought a diode simply allowed current to flow through, but I think the voltage drop is causing the circuit not to identify "my switch", how do I compensate for the diode? capacitor? my electronics knowledge is limited. Thanks.
     
  2. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Can you draw a schematic of your original circuit with your modifications included?

    Chris
     
  3. fishercounter

    fishercounter

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    Nov 29, 2013
    that's the best I can do, you can see where I tapped into the wire right before the momentary switch. The way the switch works that controls the seat warmer is, press and release the two lights light up showing low and high ON, press and release again turns off the high, press and release again turns off both high and low warmers.


    So with the wire I have going right before the switch, if I touch that wire to ground I can mimic the switch no problem by tapping it to ground over and over. Works great. But when I stick the diode at the end of that wire....no go. I know the direction of the diode matters and I tried several diodes. But it's just 1ma, I think it's the voltage drop from the diode? does that make any sense?

    thanks!
     

    Attached Files:

  4. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    I was bored so I drew it the way you described it. These heaters may draw substantial current so your Diodes have to be rated > than this.

    Chris

    [​IMG]
     

    Attached Files:

  5. fishercounter

    fishercounter

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    Nov 29, 2013
    I understand your circuit, it's what I originally wanted to do. Tie into one switch and have the two diodes there to block them off from the original circuit.

    From the circuit diagram I uploaded I thought the main current for the warmers were controlled by a relay, and the little momentary switch told something in the "MCU" to control those relays. The wires going to the switch on the car are pretty thin also, I don't think they can carry enough current for a seat warmer.


    If the diode is causing the original circuit design to not get enough voltage or amps, what can I do? assuming the diode is big enough to handle the amps and voltage. Is there anyway to compensate for the voltage drop and amperage drop I see with the multimeter when the diode is in line? I'm almost at the point of going with a relay switch, but I kind of want to understand what is really going on.
     
  6. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    Somehow you posted your schematic before I posted mine. I want look your print over before I opine any further.

    Wait! Never mind. Your heaters are NOT switched ON to ground as you described! From what I see in your print the heater elements are relay switched from the high side, not the low side. My schematic was not designed for high side switching. Rethinking time!

    Chris
     
  7. fishercounter

    fishercounter

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    Nov 29, 2013
    I went with the a relay idea insteal but I didn't have any big relays lying around so I used some micro 5v relays. I stepped down the 12v voltage to 5v and tested everything on a bread board. It all worked great, put it in the car...relays worked fine...aux trigger was working great. Tied them into the momentary heater switch....worked once...both heaters came on. Then nothing, I couldn't even ground trigger the relays.


    So....I guess micro relays were a bad idea? even if they were rated like 30A and 200V? How could that tiny switch that only has 5v on it and from what I could tell was only using 1ma blow my micro relays?

    I guess I'll order a bigger relay.
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    How were you "stepping down the 12V to 5V", and how did you test it?
     
  9. fishercounter

    fishercounter

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    Nov 29, 2013
    7805 voltage regulator. I had a spare 12v transformer that was 1a rated and hooked it up to a bread board. I used a multimeter and it was stepping down the voltage fine. The relays were triggering no problem.

    Its really confusing, I'm going to pull the thing out tomorrow and test it again to see if its fried. The micro relays should have been enough for the 1 ma current I measured. Maybe I'm not understanding something. The circuit looked simple enough to tap into....I just wanted to have the remote control trigger the seats on after I remote started it.

    Oh and the relays used 80ma and the aux port can provide 200ma on the car. It was working fine until I hooked it up to the actual switch. I was triggering the relays with the remote no problem.
     
    Last edited: Nov 30, 2013
  10. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    It took a while for my old eyes to focus on your circuit but I see now that the control switching for the heaters are momentary and do in fact switch to the ground rail. I will be quite surprised that the 700mV voltage drop from the diode junction would cause issues. Please post your diode part number.

    Chris
     
  11. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    I'm hitting the sack but I wanted to post this before I do so. It's a 2 transistor method to switch your MCU lines low. R1 & R2 are your existing pullup resistors. S1 & S2 are your existing push button switches.

    This should eliminate the diode drop that you're concerned about. When Q1, Q2 are turned on their Collector voltage should be lower than 200mV. That said, I still think the original diode isolation method should work. Are you sure your diode isn't a Zener?

    Chris


    [​IMG]
     

    Attached Files:

    Last edited: Nov 30, 2013
  12. fishercounter

    fishercounter

    6
    0
    Nov 29, 2013
    It wasnt a zener diode just a regular 1n4001 or 1n4007 there were a bunch I tried ....wouldn't work with any.

    So my voltage regulator shorted out somehow. I took it out and put a 100ohm resistor...and the relays worked fine.

    So everything is working. The thing I switched that made a difference was using the ground the switch was using. So the relay was mimicking everything exactly like the switch.


    But why would it matter which ground was used??? Isnt every ground the same?

    And why would the diode idea not work....I dont understand that.

    I think your last circuit would have worked too if I used the ground the switch used.

    Should I also add a fuse into my circuit? If the relay switches use 90ma.....should I put in a 1amp fuse? Does that prevent a short if the relays stop working? Is that the purpose?
     
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