Controlling 12V with 0.5V signal

Discussion in 'The Projects Forum' started by Georacer, Sep 20, 2010.

  1. Georacer

    Thread Starter Moderator

    Nov 25, 2009
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    I want to control a 12V, high current source (a few amps, propably under 10) with a controlling output that comes out from a device. The issue is that the amplitude of the controlling signal is only 0.5V, making it impossible to use a simple transistor assembly or a relay.
    I already read this thread: http://forum.allaboutcircuits.com/showthread.php?t=10473 but didn't get much out of it.

    tom66 suggested that I could use a comparator to get a higher voltage signal first. I found it clever.

    Has anyone else a different idea?
     
  2. tom66

    Senior Member

    May 9, 2009
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    How is this different from the last post?

    Anyway, one other way to do it would be to use a transistor configured as an amplifier. Maybe bias the signal so it just about switches on the transistor. The signal transistor could then control a power transistor or relay.
     
  3. retched

    AAC Fanatic!

    Dec 5, 2009
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    You could use the .5 volts as an input to a uC and when sensed, trigger whatever you wish.

    The line could be run into an ADC input on the uC. The uC could sleep until awaken by interrupt via the .5v signal.

    That would really just be another way to get 0-5v logic from the 0-.5v signal.

    Comparator would be a touch easier.
     
  4. Georacer

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    Read the previous thread, while it's still up.
     
  5. wayneh

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    I endorse the approach. I'm doing something very similar right now and it's working great. You can use the full 12v to power the comparator and to pull up the output to switch a MOSFET to handle the current you need. The only "tricky" part might be making sure the comparator switches states the way you expect, full on and full off, not oscillating in between.
     
  6. Georacer

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    What would be the reasons behind oscillation?
     
  7. tom66

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    Think about it: what if you get exactly 0.5V in? The comparator will switch at a half-way point, and depending on the relay this could cause what is known as relay chatter as the comparator rapidly changes between low and high, as the input signal is unlikely to be stable. This will cause the relay to wear out and the lights to flicker. The solution to this is to ensure that the signal always has enough of a margin, for example, if you want 0.5V to be your set point, compare it to 0.4V.
     
  8. Ghar

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  9. Kermit2

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    Has your 'friend' only looked at the output with a DC volt meter? I'm repeating what I said at your 'other' post, from which this quote was taken.

    I don't think this AUX port is putting out a steady .5 VDC. I think it is using a PWM or other Digital data signal. I went to the website and read all about the product and all the fancy 'add on' sensors, and devices that can be added to this alarm. The are all addressed digitally by the alarm unit. He needs to read it with a scope and SEE what is really on that line, or talk to an installer that has put one of these units in a car.
     
  10. Georacer

    Thread Starter Moderator

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    Kermit2, I understood what you said and will take it into consideration. If this is the case, the comparator will be useless.

    P.S. I laughed with the quoted "friend". It reminded me of some very funny situations of people too embarassed to ask help themselves. However, this is not the case. No, sir, I 'd never pay a wad of money to ensure the safety of a 15-year-old Corsa.
     
  11. Kermit2

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    I do tend to overuse those "quote" brackets.

    " :) "
     
  12. windoze killa

    AAC Fanatic!

    Feb 23, 2006
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    If the signal IS a true 0.5VDC output then Toms suggestion is the simplest and probably the cheapest.

    PS. I wouldn't pay a heap of money to protect a 15yo Corsa either. I WOULD spend a heap of money advertising the fact that it is unlocked and ready to steal. :D
     
  13. retched

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    "AWW CRAP!! Keys are stuck in the ignition again! AND THE DOOR WONT LOCK!

    Forget it...Im going to bed...." ;) ;)
     
  14. wayneh

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    A slow-moving signal very near the reference voltage will challenge the comparator. For instance I'm dealing with a thermostat situation where the transitions take minutes, not milliseconds. It's not hard to fix, but you just need to realize that the comparator isn't a perfect on-off switch. If your signal is truly either 0 or 0.5v, you won't have any trouble.
     
  15. tom66

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    Here is one circuit using a transistor biased to switch at around 0.5V.

    It's probably the lowest cost way of doing it. Q1 (input) should be a small signal NPN transistor such as 2N3904. Q2 (relay) should be a relatively high power transistor, capable of sinking at least 250mA; a 2N2222 would work well.

    Here is the code for Falstad circuit sim:

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  16. Georacer

    Thread Starter Moderator

    Nov 25, 2009
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    Thank you for your hard work, tom66!

    Just a question: Which is the green and which is the yellow signal, down at the graph?
     
  17. tom66

    Senior Member

    May 9, 2009
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    Green = Voltage
    Yellow = Current

    The scope traces probably don't mean much in this sim.
     
  18. Georacer

    Thread Starter Moderator

    Nov 25, 2009
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    Yes, but which voltage? On the input, or on the controlling coil of the relay?
     
  19. tom66

    Senior Member

    May 9, 2009
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    I look now at the traces and realise they are completely wrong.

    When the input is 0.5V or so, the relay coil is disengaged, and the normally closed contact is connected to the 12V input; this turns on the lights.

    When the input is less than 0.5V or so, the relay coil is engaged, disconnecting 12V from the normally closed contact and turning off the lights.
     
  20. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    If the .5 V sig. isn't pure DC, add a RC filter, say 10k & 10 μF, feeding a comparator with a bit of + feedback, 1/10 V or so as prev. suggested.
     
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