TV Lift - Limit Switches & Controller Relays

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ecannell
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TV Lift - Limit Switches & Controller Relays

Post by ecannell » Sat Mar 20, 2004 9:13 am

I intend to build a bed which into the bed foot will be a LCD TV lift. Needs a reversible motor, limits switches, and some kind of remote switch. TV will be, say, 30 lbs or so. I will use cable to "lift" the TV up to the viewing position.<p>I can spend $700 to $3700 for a commercial solution that frankly must include a whopping amount of gross margin $$. Like most readers in this forum, home grown will be much cheaper.<p>Has anyone built a TV lift of any kind?<p>For the limit switch, I would like to use the magnetic sensing type. Anyone know of a vendor?<p>Finally, I need a controller/relay/whatever that controls the motor. It uses inputs from the remote (up/down) and the limit switches (stop going down, stop going up). Any ideas where such a device can be bought?<p>Thanks in advance!<p>-eric

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Chris Smith
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Re: TV Lift - Limit Switches & Controller Relays

Post by Chris Smith » Sat Mar 20, 2004 6:52 pm

If you really want to save some bucks, ball bearing "instrument/drawer/rack mount" slides by "Jonathan" are available for less than 50 bucks a pair. Their limit side ways is in excess of three times what you need, and your going vertical.<p> A simple DPDT relay and a couple of “Cherry” brand lever switches can be the control part of your system. The motor part, is simple.

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dacflyer
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Re: TV Lift - Limit Switches & Controller Relays

Post by dacflyer » Sat Mar 20, 2004 7:18 pm

see my post "having a file not found day."<p>i had same question the otherweek.. and this is what was replied to me and it was a easy circuit.<p>posted by...Russ Kincaid <p>The circuit is at: http://www.geocities.com/russlk/limitckt.jpg
You will have to copy and paste the URL in the address bar, geocities does not allow links. <p>as for the remote part... i hacked one of radio shacks...remote controled A/B switch..used for selecting cable.. the units base just controls a relay inside.. you can use this to control your relay A is unenergised , B is energised.
make sure you use a relay that is designed for continous duty.. so A would = tv down / and B could = tv up.. if your lucky you might find you a motorised jack like used in one of them chair lifts.. and also if you like to use ball bearing rails as vertical stablizers.. Lowes Hardware store has some awsome european drawer sliders that handle 100+lbs.. brand name.. Knape & Vogt
i just bought some of these and they extend enough so that you get full use of drawer..
most drawer slides still leave 3-4" inside the cabinet,, anyway..hope this helps..good luck<p>BTW i am using this circuit to control a roll top type cover on my entertainment center.. when tv's
not in use..i can cover it up. and it just looks like anyother cabinet :D Phuuuu :p

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Re: TV Lift - Limit Switches & Controller Relays

Post by ecannell » Sat Mar 20, 2004 11:05 pm

Appreciate the advice.<p>In looking at the circuit diagram, everything looks pretty straight forward; however, it is not clear how the motor can operate again once a limit switch is activitated. Once a limit switch is opened (and the motor stops), how does it begin to go in the other other direction, since the limit switch is still (presumably) open and thus no circuit for power to flow through?<p>I am guessing that I don't udnerstand how the limit switch works. Your insight would be appreciated :) <p>-eric

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Re: TV Lift - Limit Switches & Controller Relays

Post by ecannell » Sun Mar 21, 2004 8:34 am

Never good form to reply to your post :) , but in reviewing the Cherry limit switches, I came across one with both NC and NO terminals. <p>Presumably, I could wire one of the poles (of each of the 2 switches) to ground in order to make a complete circuit to make the motor drive in the other direction.<p>-eric

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Re: TV Lift - Limit Switches & Controller Relays

Post by rshayes » Sun Mar 21, 2004 6:00 pm

If the motor is a DC motor, the direction is probably reversed by reversing the polarity of the power to the motor. In this case, the limit switches can be placed in series with the motor and diodes can be used to allow only one direction of operation when the limit switch is open. This will allow the motor to be backed off the limit switch and prevent it being driven further into the limit switch.<p>It is a good idea to mechanically arrange the limit switch such that it will not be damaged if the motor fails to stop. This can be done by using a cam or a flexible actuator to operate the limit switch.<p>Limit switches usually use a series connection of normally closed contacts. Switches are more likely to fail open (often due to dirt or oxidation on the contacts) and this makes the system shut down in the event of an open switch failure. Contacts can weld closed, but this usually happens only when the contact is operated well beyond its ratings.

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Re: TV Lift - Limit Switches & Controller Relays

Post by rshayes » Sun Mar 21, 2004 6:22 pm

You will probably need two relays, one to control power (off or on) and one to control direction (up or down).<p>It is tempting to wire the relays such that operating one moves the motor up and operating the other moves the motor down. Don't do it this way. Operating both relays at once shorts the power source and either welds or vaporizes the relay contacts. Changing direction can cause this condition if the pull-in time of the relays is shorter than the drop-out time.

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Re: TV Lift - Limit Switches & Controller Relays

Post by amuron » Sun Mar 21, 2004 8:28 pm

Rather than a micro switch, you could use a reed switch and a magnet.<p>http://www.hamlin.com/index.aspx?tabindex=3&tabid=7<p>You could also use a hall effect sensor.<p>Ron

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dacflyer
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Re: TV Lift - Limit Switches & Controller Relays

Post by dacflyer » Mon Mar 22, 2004 8:42 pm

you only need 1 DPDT relay...and 2 snap switches..lever types... i use the NC sides ..once you switch the relays position the motor reverses and rest at next limit switch.
see my other post in the forum...there is a diagram there..and its a link in the 1st post i posted here too...

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Chris Smith
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Re: TV Lift - Limit Switches & Controller Relays

Post by Chris Smith » Mon Mar 22, 2004 9:42 pm

The DPDT relay works to provide you with two choices. up/down. The limit switches, cancel one choice at the end of each stroke, this leaves the relay, set to return only, and it does.

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dacflyer
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Re: TV Lift - Limit Switches & Controller Relays

Post by dacflyer » Wed Mar 31, 2004 4:09 pm

i like limit switches better than reed switches..limit switches can be set more exact..

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Edd
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Re: TV Lift - Limit Switches & Controller Relays

Post by Edd » Wed Mar 31, 2004 9:29 pm

however, if you are caught in a situation of having to use reed switches ,in this situation ,where you would have to be using a reed switch with its fixed bias magnet in order to make a normally closed switch element.(Unless you opt to reverse the action by having a conventional normally opened reed switch element feed a relay where the action is reversed by its contacts) You can attain an optimizing of positional sensitivity by mounting the reed elements longitudinal axis at 90 degrees to the lifts line of travel.
If the reed was mounted in the same axis as the approaching bar magnet, it would progressively produce an increasing flux field as it first encroaches upon the closest reed magnets pole and then the distant one, on up to the point of such magnetic neutralization that the reed unlatches.
With the units rotated 90 degrees…..to the lifts line of travel…..the approaching magnets flux field would be equally encompassing both polar elements of the reed elements biasing magnet up until magnetic field neutralization occurs up to the point where reed latch out occurs.
The latter mounting would have required less overall lateral mechanical movement in acquiring latch out, so a more precise positioning with its utilization.
73’s de Edd

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Chris Smith
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Re: TV Lift - Limit Switches & Controller Relays

Post by Chris Smith » Wed Mar 31, 2004 9:40 pm

Yes, the cherry switches, and the relay both have the two position, NO and NC,... so that when one is off, the other allows the power to continue to the opposite direction or logic.<p>[ March 31, 2004: Message edited by: Chris Smith ]</p>

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