Foot switch sensor

Discussion in 'The Projects Forum' started by DonFL, Jan 6, 2007.

  1. DonFL

    DonFL Thread Starter New Member

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    Not sure exactly how to title this one, but hopefully I can explain.

    My workshop has a dust collection system, and to allow remote foot control of the dust system blower I have 5 SPST foot switches in parallel across the switch contacts for the blower.

    My problem is, one switch on and off is fine...but if another switch gets turned on for any reason, there is no way to tell which ones are on or off...you can spend some time trying to figure out which switch or switches are open or closed.

    So, I am trying to come up with a way to indicate which switches are closed. The first, and probably more expensive way is to get foot switches with DPST contacts, and just use the second set as part of a set of indicator lights. Very simple...just an extra pair of wires from each switch, a set of indicator lights, and some power. But..right now..I haven't been able to find a DPST version of the switch (though, I'm still looking..)..

    But..I was thinking of doing it by either sensing current flow thru branches of the switch lines, or something of that nature. There might need to be a little logic there to figure out which switches are on by the current flow, or lack of, but it was a thought. Not sure though, the best way to sense current and use that as a yes or no kind of signal. Having the switches in parallel makes this kind of tricky. There is 120V across the switch until one is turned on, and they you have about 5 amps flowing in that branch. Of course, as more are turned on, you have current flow in each particular "branch"...(I'll be glad to attach a drawing if I've managed to totally confuse everyone...)..

    Anyway..this is my first visit here, and it looks like there are a lot of good idea people...so I'm all ears for some good ideas.
  2. beenthere

    beenthere Retired Moderator

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    Hi,

    If you don't mind a bit of rewiring, it's pretty easy. I have a situation in my basement where I need to control a light from 4 locations. I used 4 12 volt pushbutton switches and a latching relay to switch current to the lamp.

    If you go to a central relay, you get to take the high voltage off the footswitches. The need to indicate which switch is hot goes away, as the relay is the only element carrying current. It only costs a realy and a 12 volt transformer.
  3. mrmeval

    mrmeval Distinguished Member

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    Each foot switch can control it's own relay that controls both the blower and a lamp. The lamp can be driven by the line voltage if it's compatible, if not you can drive it from a compatible AC source since the relay contacts are isolated. The relay coil voltage can be the same as the line voltage and directly driven by the foot switch, you won't need an extra transformer then. Pick a relay with two normally open contact pairs, you will probably want a contactor rated at the blower motor current plus 25%. I do like the idea of using low voltage for the control signals, it saves wear on the foot pedels, it's saver and you can use the lower voltage for small bulbs.

    You can design a low voltage control system that will light the right lamp and also activate the relay. Benefit is there would be low voltage on the foot pedals, you could have a master reset button you can press on the way out of the room and it would be substantially cheaper than the cost of four relays.

    You don't have to use lamps, LEDs with the proper current limiting will do. You also don't have to use a mechanical relay, just pick the right solid state relay.

    Another one is to use a current monitor circuit and steal a small amount of the blowers power to drive an LED or to just monitor current flow and have the LED light from it's own power. I'm not too familiar with that.
  4. DonFL

    DonFL Thread Starter New Member

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    I was considering something like that, but, it still doesn't tell you which switch or switches is closed...unless I'm really missing something. My idea was a panel with a lamp for each switch...lit means it is closed, dark means it is open.

    I might be missing something about what you are saying..can you elaborate?
  5. DonFL

    DonFL Thread Starter New Member

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    As much as the idea of multiple relays was something I was trying to avoid, this is the one way that I've been thinking. Figuring...a 12V system to close the relays, and two sets of contacts, as you mention. Based on what I have found so far, I think this will still be cheaper than than new footswitches, even if I find ones (which I still have not..) with DPDT or DPST contacts...I'm probably at a minimum of 100 bucks for the footswitches, if they exist, maybe more. I can do relays and a solid power supply/AC adapter for less.



    I'm very comfortable with mechanical relays, but not at all experiences with solid state relays..any resources/tutorials you can point me towards?


    That was sort of plan B, but it gets tricky when you think about where the current flows, and doesn't flow, depending on which switch is closed. Ultimately, I'm beginning to believe the most economical choice is multiple relays, be they solid state or mechanical.

    Thanks for all the input..this site is incredible...wish I would have known about it long ago..
  6. beenthere

    beenthere Retired Moderator

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    Hi,

    The idea with the one impulse relay means that the footswitches are momentary contact. The relay is the only active element, so a switch indicator is not really valid.
  7. hgmjr

    hgmjr Moderator Staff Member

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    Greetings DonFL,

    If you can handle the cost at around $12 US per motor being monitored, I think I have located just the ticket for your current monitoring requirements.

    Here is the link to the self contained current monitoring plus led indicator module. It is a current sense transformer this is ready to install with no special power or extra circuitry required. You can buy them from Digi-Key as they are presently in stock.

    If sensitivity to the magnitude of the current being monitored is a problem, keep in mind that you can wrap the conductor being monitored a couple of times through the opening in the current sense transformer to gain a little more sensitivity.

    hgmjr
  8. DonFL

    DonFL Thread Starter New Member

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    (deleted....)
  9. DonFL

    DonFL Thread Starter New Member

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    It's just one motor, but 5 switches, so I'd need to monitor current, if I count correctly, in 5 places.

    The price is comparable to doing it with relays as mentioned, the only thing that gets tricky is the logic of doing it this way. If you think about this as 5 switches in parallel, and, say, switch 5 is furthest away, and switch 1 is closest, depending on which switches are turned on and off, you don't have a clean indication of what is going on.

    For instance...say I turn the blower on with foot switch 5, so the current sensor LED for that switch turns on. If you turn on any switch closer to the blower, it will turn the switch 5 sensor LED off, and turn the closer one on. Then, when you turn the closer one off, the switch 5 sensor turns back on. Essentially, you are faced with turning them off in order "near to far" until you get all LEDs extinguished.

    This is, though, definitely one option I am going to look at more closely.

    As you can see, this is one project I thought would be incredibly simple, and while not technically complicated, it does make one scratch their head a bit (and least, I was scratching mine..:)...)...
  10. hgmjr

    hgmjr Moderator Staff Member

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    DonFL,

    This is the wiring of your blower motor as I had invisioned it. I imagined 5 switches in parallel.

    The ovals are the 5 current sensors. I imagined them mounted physically close to the switch.

    Did I completely miss the boat?

    hgmjr

    Attached Files:

  11. DonFL

    DonFL Thread Starter New Member

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    Not at all, except there is a distance factor. Each of the switches is a different physical distance from the blower. So current will only flow thru the closest branch to the blower when two switches are closed (shortest path)...so an indicator that was on will go out when a closer switch is closed.

    Might be easier for me to draw a diagram, but can't do it right now. Let me know if it would help clarify my point.
  12. hgmjr

    hgmjr Moderator Staff Member

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    I can appreciate your point concerning the differences in resistance on each of the 5 switch legs.

    There are a few things that you can do to address this imbalance.

    First, I think the choice of the most sensitive detector would give the best chance of detecting the current in the longer leg (higher resistance) when a shorter leg (lower resistance) is switched in parallel with it.

    Second, I would suggest that the longer legs would benefit from a couple of turns through the hole in the current transformer coil to obtain an increase in sensitivity.

    Third, You can always add some length or wire to the shorter legs in order to make all 5 legs fairly close to the same length. You can coil the excess near the breaker box from which each of the 5 switches radiate. This may not be practical if the difference in length is too great.

    These are the tricks that I can think of right off.

    hgmjr
  13. DonFL

    DonFL Thread Starter New Member

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    I don't think there is going to be a resistance factor. These are switches, so the closest closed switch to the fan blower will carry 100% of the current, even if others beyond that closes switch are closed.

    I've actually tested to make sure my thinking is correct here. You see current flow drop to 0 on a leg further from the blower motor when a nearer one is closed. That makes logical/electrical sense. If I had something other than switches, so that it was not a 0 resistance/infinite resistance situation, then I think what you are saying, could work.

    My leaning at this point is to bite the bullet and go with multiple relays...but I may just pick up one of those current sensors just for fun..

    Thanks for all the insight and ideas.
  14. hgmjr

    hgmjr Moderator Staff Member

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    Whatever solution you decide to pursue, I am sure that all of us here will be interested in how things turn out for you.

    Don't hesitiate to post any questions in the meantime as you proceed toward completion of your project.

    hgmjr
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