Help Needed Wiring a Low-Volt switch for 110v circuit

[rody084]

Hello there,

I am working on a project in which I would like to place some "rope lighting" on a mechanic's creeper and rig it so that when the head-rest is in the up position the lights turn on. Its not so much a functionality project, but more of an experiment and learning experience in electronics. What I need to find out how to do is the following:

I want to switch it using the body of the creeper as the two leads which operate the switch. (there is a part of the headrest mechanism which touches another part ONLY when in the up position and there is no continuity elsewhere).

So my question is, is there a way that I can use a low-voltage relay/switch to do this? (i obiously do not want 110v of electricity running through the body of the creeper) I want to see if I can have a low-voltage flow through the body of the creeper which is harmless and that low-volt circuit/switch/relay activates or deactivates the 110v circuit.

I would greatly appreciate anyone's help with this! I am really interested in electronics and looking to learn!

Thank you in advanced for your comments and suggestions!

Rody

 

[Integral]

To me, it does not seem like a good idea to use the frame of the creeper as your main conductor. I would use a micro switch to sense the upright position. You could use a low voltage (12 or 24V dc ) to the micro switch then use that to control a relay, simple would be a standard contact relay, fancy would be to use a SSR (Solid State Relay). The trouble with the SSR would be cost. This would mean that you would need a DC power supply on the creeper, that could present some space issues, but then you will be plugged in some how any way. Maybe use a wall wart to get the DC to the creeper... Just throwing out some ideas.

Good luck.

 

[Danger]

I agree with Integral about the trigger mechanism. While it's a clever idea at heart, using a switch would be far more practical. Although you didn't specify what kind of creeper it is, any that I've seen have been painted. You'd have to scrape that off to get decent conductivity. Also, it stands a good chance of becoming contaminated with grease, oil, etc. which could mess things up. For that reason, I would further recommend that you use a sealed switch as opposed to something like a mini-lever or pin type. My personal choice would be a magnetic reed switch such as the type used for doors and windows in an alarm system. A mercury tilt switch could also be used, but it will probably close sporadically just from the creeper being moved around and sloshing the mercury.

 

[Averagesupernova]

Integral and Danger, I don't think you realize the beatings that these creepers take. Mercury switch is a bad idea. If I'm correctly imagining where these metal parts go together to complete the circuit, the paint is already long gone. It's most likely a wear point.

 

[Integral]

The main problem with using the frame of the creeper as a conductor, is the likely hood of the user unintensionally closing the circiut with his body.

Why would you suppose that Danger and I have never seen a creeper?

I imagine Danger as having spend a fair portion of his life on one.

 

[Danger]

I imagine Danger as having spend a fair portion of his life on one.

Not as much as I would have liked, but yeah... I've got the requisite hours. Unfortunately, though, most of the time it was just a chunk of cardboard. (Just try getting a creeper under a Camaro on a gravel driveway without jackstands. )

 

[Integral]

Not as much as I would have liked, but yeah... I've got the requisite hours. Unfortunately, though, most of the time it was just a chunk of cardboard. (Just try getting a creeper under a Camaro on a gravel driveway without jackstands. )

Hummm sounds like the same brand of "creeper" I use. The same goes for my Probe... I can barely get my head under it. One side on the curb helps a lot!

 

[Danger]

No curb in my driveway, but the tracks are dug down almost a foot in some areas. Straddling one was almost like having a grease-pit.

 

[Averagesupernova]

The main problem with using the frame of the creeper as a conductor, is the likely hood of the user unintensionally closing the circiut with his body.

With low voltage that should be no different than handling 9 volt battery.

Why would you suppose that Danger and I have never seen a creeper?

I didn't say you had never seen one. Don't put words in my mouth. But, obviously you must not have a lot of real world experience with them otherwise you'd know the kind of beating they take and that a mercury switch will not stand up.

I imagine Danger as having spend a fair portion of his life on one.

How serious can either of you be about working under vehicles if you don't have the clearance to use a real creeper? Obviously you haven't invested in jack stands or ramps.

 

[Danger]

I never really needed them, so why bother? The only times that I've had to be under a chassis rather than just under the hood were to change starters or U-joints, or rebuild the clutch linkage. If you check my member photo, you'll see that I don't need a lot of clearance. Besides, as I said, creepers don't roll properly on gravel so it's just a pain in the ass to try using one. Cardboard slides nicely. My time on a real creeper was put in helping friends in their own shops.

 

[rody084]

thank you all for your comments and I myself am also the cardboard scrap type when it comes to working on my car... i need something that can quickly be moved and play it off since cops in my area give you citations for working on your car when parked in the sidewalk and i don't have a garage.

anyway, the mercury switch sounded like a great idea but that is probably true that rocking it could make it go off and i sure wouldn't want that thing to break open anywhere near my skin...

being a complete novice to electronics, I don't know what a micro switch is... can anyone give me a link or explain this to me? :)

ive seen the magnetic sensors used on doors and windows before... how do these work? i imagine the circuit isn't completed by the magnets... the magnets must triger something like a relay would.

my idea of using low voltage switching is exactly for the reason Integral pointed out... i don't want 110v of electricity using my body to close the circuit... but i figured 9-12v should be no problem at all. I was actually thinking of scraping some of the paint off for conductivity...but I am liking this magnet idea and I am curious about the micro switch.

how would i wire the magnet to this? I would probably put one magnet on the stationary frame and the other magnet on the support for the headrest which moves up and down so that when its closed the magnets are next to each other and when its up the magnets are separated...

thanks again to everyone for your input!

 

[berkeman]

I googled magnetic reed switch, and got lots of hits. The magnet basically pulls a ferrous reed switch armature toward it, which closes the reed switch circuit. You see them used a lot in door alarms, for example.

http://www.google.com/search?hl=en&lr=&q=magnetic+reed+switch

 

[Danger]

Rody, it's as Berkeman explained. The switch package is essentially just a relay with the parts separated. Rather than an electromagnet closing the contacts, a permanent magnet is brought into physical proximity and pulls the moveable reed into contact with the other one. Just epoxy the components into place after a few taped-in-place tests to make sure that the alignment is right. The parts are sealed, so you won't have to worry about contamination. Remember to solder the connections, and use shrink tubing rather than electrical tape to insulate them. It provides better mechanical strength to the joint, and won't crystallize over time.

 

[Averagesupernova]

I would mount the reed switch, relay and small low power transformer in a plastic case. As Danger said, epoxy the reed switch into place. Place it on a wall of the case so a magnet can come close to it (through the wall of the case) when the headrest comes up.

 

[rody084]

this magnetic reed switch sounds great! i think I am going to try this out...but what I am still having an issue with is how to switch the 110v source... from what I saw most reed switches are 20v max... and i really would like to stay away from 110v switching if at all possible.. so I am wondering if anyone can tell me a way to do a low-voltage reed switch for a 110v circuit (in this case my rope lights)

thank you!

 

[berkeman]

One way to do it is to use the low-voltage reed switch circuit to energize a 110V relay circuit. So you'll need the LV magnetic reed relay, the HV relay, and a transformer to convert the 110Vac down to the 24Vac for the magnetic reed relay. Be careful wiring it all up!

BTW, wouldn't it be easier to just use low-voltage lighting on your creeper? Aren't you going to be running over the power cord all the time? You could just use a 12V solid lead acid battery on the creeper, and plug it in for recharging when you aren't using it.

 

[Danger]

Just use a simple SPST NO relay with a 6-12VDC primary coil and a 120VAC @ 2 amps or so rating on the secondary contacts. Run the primary circuit from a battery (or low-voltage transformer; might as well, since you'll have 120VAC on board anyhow) through the reed switch to the primary side of the relay. Mains power then gets switched through the secondary contacts to the lights. No need for a master switch, since you can just unplug the thing to disarm it.
As a side-note, though, you could save a lot of hassle by just using 12VDC lights in ther first place.

 

[berkeman]

BTW to the OP, if you do use 120Vac, be sure you know what you are doing, and make it safe. You want to design and build it like you were going to submit it to UL for safety approval, even if you don't. Like, definitely you will need a fuse in series with your power cord, and since it is for use working around the car in potentially wet conditions, you will need the power feed to be GFCI protected. All AC connections must be dressed correctly, and no contact with people should be possible. If you haven't worked with AC mains power much in the past, I'd recommend using the 12V lights as Danger and I are suggesting. It's just too easy to get hurt working with AC mains.

 

[Danger]

Hey, Berkeman, I didn't realize that there was a second page to this thing already, so I didn't see your post #16 until after I submitted mine. I guess it's true that 2 great... naw, that can't be it.

 

[rody084]

im going to take up your advice and just skip this project and do a new one with 12v lights...110v is just too much risk especially for a complete novice like myself. i think I am going to look into something called LED-Flex which are pretty cool and are low-voltage andim going to give the magnetic reed switch a try.

thanks to everyone for your input!

 

[Danger]

You're welcome. Post a pic when you're finished.

 

[rody084]

i have a new question on this...

if i want to wire a box with a magnetic reed switch, andi want the lights to turn on when the box is OPENED (magnet is apart from reed switch) then do I need a "Normally Closed" reed switch?

 

[berkeman]

i have a new question on this...

if i want to wire a box with a magnetic reed switch, andi want the lights to turn on when the box is OPENED (magnet is apart from reed switch) then do I need a "Normally Closed" reed switch?

No, I would think you would want a normally-open switch, but check the specs to be sure. For a switch or pushbutton, the "normal" state is generally the non-actuated state. It requires some action like pushing the button to put it in the non-normal state. I'm not exactly sure how that would apply to a magnetic reed switch, though. Do you have a datasheet or other specs on the magnetic reed switches you are looking at?

 

[rody084]

Thanks for the info. I ended up talking to DigiKey. My transcript is below if anyone is interested:

info: Your request is being processed. A representative will be with you momentarily.
info: Your Digi-Key representative for this Live-Help session is Quinn Ext 2289.
Quinn Ext 2289: Welcome to Digi-Key Live-Help. How may I assist you?
Rody: Hi, i was just speking to Andy and my browser crashed, could you transfer me back to him please?
Quinn Ext 2289: Yes
info: Please hold a moment while I transfer your chat.
info: Your Digi-Key representative for this Live-Help session is Andy ext. 1851.
Andy ext. 1851: Welcome to Digi-Key Live-Help. How may I assist you?
Andy ext. 1851: Andy ext. 1851: You would want normally closed. This way when you close the box, the magnet opens the contacts, and disconnects the light
Andy ext. 1851: Andy ext. 1851: This way the when the contacts are closed (when the box is open) the contacts are closed which complete the circuit
Andy ext. 1851:
Rody: hi andy, i was just speaking to you about the LED circuit
Andy ext. 1851: yep
Andy ext. 1851: You would want normally closed. This way when you close the box, the magnet opens the contacts, and disconnects the light
Andy ext. 1851: This way the when the contacts are closed (when the box is open) the contacts are closed which complete the circuit
Rody: okay great! so which one is normally closed on your site?
Andy ext. 1851: These spst types (magnetic reed switches) will usually be normally open. SPDT types may have a normally open and closed style
Rody: so do you have an NC on your site that you know of?
Andy ext. 1851: http://dkc3.digikey.com/PDF/T062/1525.pdf
Andy ext. 1851: this page takes you to some of our reed switches
Andy ext. 1851: The Meder ones towards the bottom center have a normally closed option. 374-1030-nd
Rody: oh yea i see it. that one would work with my idea?
Rody: are these the miniature glass ones?
Rody: (thank you for all yourhelp!)
Andy ext. 1851: The switching current is .25amps and the body is around 1" long
Rody: that sounds good to me
Andy ext. 1851: http://rocky.digikey.com/WebLib/Meder%20Electronics/Web%20Info/MK3.pdf
Rody: okay, so that takes care of that...now LEDs...
Rody: i found this on your site: 160-1499-ND which is red like i want...and inexpensive. now i need some help figuring out the reisistors for this
Rody: agai, i want to wire 20 LEDs in parallel on a 9v battery.
Rody: from your site i see that its a 2v LED but i can't find the mA on it
Andy ext. 1851: It is 50mA max
Rody: so if i want to run this circuit in parallel...with each LED at 2v and 40mA of power...then do i need a 175 Ohm resistor per LED?
Rody: running off a 9v battery
Andy ext. 1851: one moment
Rody: thanks!
Andy ext. 1851: To get the ohm value needed, you take supply voltage (9v) minus forward voltage of led (2.0v). So that is 7v. The divide that by forward current (50mA). This would leave you with a value of 140. But it is good to go up to the next highest value to compensate for tolerance
Andy ext. 1851: Basically a 175 ohm would work
Andy ext. 1851: one moment
Rody: okay, cool. and that would be one 175 ohm at each LED if wired parallel. do you think that will eat up the battery too quickly? since i think those batteries have 200mAh...and i think i am drawing around 1000mAh, right? thanks!
Andy ext. 1851: one moment
Andy ext. 1851: You may actually only need 1 resistor, double checking
Rody: for parallel or for series?
Andy ext. 1851: I found a good website. They recommend still using a resistor for each led. This way if one led goes bad, then the rest of the leds will not go bad. Basically it would allow for higher current at each led.
Andy ext. 1851: Here is that website:
Andy ext. 1851: http://www.mindspring.com/~jeffpo/ledlite.htm
Rody: i am concnered about the current draw too... what do you think of what this diagram gave me? http://www.tdelaney.com/Rody/wiring.JPG
Rody: what my reasoning was is that if i have 20LEDs at 50mA each wired directly in parallel then i would be drawing 1000mA. the 9v batteries have about 200mAh capacity...so that would leave me with about 12 minutes of power...but that other diagram shows that the circuit would draw only 200mA. so i would have an hour of power... does that make sense?
Rody: (thank you again for all of your help with this!)
Andy ext. 1851: Here is a great link for parallel hookups. You may run into a problem because of how many leds you have in circuit. Basically if you have 4leds hooked up in parallel it will drain less off the battery than 20 leds would. Which means battery is constantly being drained and therefore would not last too long.
Andy ext. 1851: http://www.theledlight.com/ledcircuits.html
Andy ext. 1851: This website descibes that using a wall transformer, you would not have to worry about drain, but with a battery it is more of a problem. On here they show only using 1 resistor as well. This way the voltage is constant, but the current is distributed evenly through each led
Andy ext. 1851: this may mean that you would require a higher amp/hour battery, or a larger source
Rody: but the other guy was recommending indiv resistors for reliability, right?
Rody: what type of reisitor is best...ceramic or carbon?
Andy ext. 1851: correct. The problem with using individual resistors for each led with one power source is the drain will be a lot more on the battery
Rody: ah i see.
Andy ext. 1851: Each resistor is fine for this type of application, as long as the resistor has a high enough wattage
Andy ext. 1851: Carbon film are common
Rody: great thank you!
Rody: okay, so i am ordering these parts... how long does it take to ship out?
Andy ext. 1851: No problem
Andy ext. 1851: I would have to get you to our sales department for that information
Rody: oh... do i need to place these on a circuit board? how do i get all this stuff connected up without anyting buning? lol
Andy ext. 1851: I can either transfer you there, or you could call in at 1-800-344-4539
Andy ext. 1851: A circuit board would be a good idea.
Rody: i am very new to this...which one do you recommend for my applicatoin? these leds will cover an area about 6"x6" and will be in an enclosure
Andy ext. 1851: http://dkc3.digikey.com/PDF/T062/1627.pdf
Andy ext. 1851: this is a good page to look at. It is page 1627 in our catalog.
Andy ext. 1851: Here you will see that we have many different sizes to choose from
Rody: yea i do see that... i definately need your advice on this! hehe
Rody: i don't know what type i need
Rody: i want to lay out the 20LEDs in the shape of a heart
Andy ext. 1851: It looks like our closest size would be 6.3 x 9.19. Something like part number v1226-nd
Rody: can these be cut?
Andy ext. 1851: I have heard of people cutting these yes
Rody: whoa...expensive...hehe. is there a less expensive alternative?
Andy ext. 1851: Everything is either to small or larger in size (and more expensive)
Rody: hmm... well maybe i'll skip the board for now. the only other thing i think i am missing is the 9v battery connector
Andy ext. 1851: a 9v battery holder?
Andy ext. 1851: a pc mount type or a holder with leads coming off it?
Rody: the thing that clips onto the battery leads and has two cables coming out of it
Andy ext. 1851: 72k-nd
Rody: could i use the 473-1004-ND to set everything up? i am concerned about the heat and fire and sheetrock buildings
Andy ext. 1851: These do not have the pre-drilled holes
Rody: i can make the holes...i just got concerned with heat and flamability
Andy ext. 1851: ok
Rody: okay, so i have the batt strap, board, LEDs, reisistors and the mag reed switch...anything else you can think of that i need?
Andy ext. 1851: These seems like everything
Rody: great alright thank you very much for your help and time again!
Andy ext. 1851: No problem, thank you and have a nice day
Andy ext. 1851: Thank you for visiting. Please feel free to contact us at anytime.
Rody: could you transfer me to sales please?
Rody: thanks again!
Andy ext. 1851: Sure, one moment
info: Please hold a moment while I transfer your chat.
info: Your Digi-Key representative for this Live-Help session is Judy ext. 1264.
Judy ext. 1264: Welcome to Digi-Key Live-Help. How may I assist you?
Rody: hello,
Rody: i have a question about shipping charges/ETAs
Judy ext. 1264: Okay
Rody: i have several items in my cart and i am wondering how long you usually take to ship things out
Judy ext. 1264: We can usually ship the order same day if the parts are in stock.
Rody: the site isn't showing me the shipping charges...when can i see that?
Judy ext. 1264: The shipping charges will not show until the order is processed as the charges depend on the weight of the package and the shipping method that is chosen.
Rody: is tere a way you cna give me an estimate?
Judy ext. 1264: If we have your email address listed once the order is processed you will receive and email confirmation that states the cost of the parts and the shipping charges.
Rody: 1 40 160-1499-ND LED RED 5MM ALINGAP 632NM 2 2 374-1030-ND SENSOR MAGNETIC CYLINDER REED 3 10 27QBK-ND RES 27 OHM 1/4W 5% CARBON FILM 4 20 150QBK-ND RES 150 OHM 1/4W 5% CARBON FILM 5 5 10QBK-ND RES 10 OHM 1/4W 5% CARBON FILM 6 2 72K-ND STRAP BATTERY FIBRE BASE 9V 7"LD 7 1 473-1004-ND
Rody: here...i can separate the items with stars... 1 40 160-1499-ND LED RED 5MM ALINGAP 632NM ***** 2 2 374-1030-ND SENSOR MAGNETIC CYLINDER REED ***** 3 10 27QBK-ND RES 27 OHM 1/4W 5% CARBON FILM ***** 4 20 150QBK-ND RES 150 OHM 1/4W 5% CARBON FILM ***** 5 5 10QBK-ND RES 10 OHM 1/4W 5% CARBON FILM ***** 6 2 72K-ND STRAP BATTERY FIBRE BASE 9V 7"LD ***** 7 1 473-1004-ND
Judy ext. 1264: I will not be able to add all the parts into get you an estimate that way.
Rody: ok, no problem, i got it
Rody: thanks for your help!
Judy ext. 1264: Your welcome

 

[berkeman]

Wow. BTW, when he said 50mA, that was the max. current, not where you would typically run them. 50mA is a *lot* of current for a regular LED. I'd guess you can run them at 10-15mA and be plenty bright, but you can experiment when you get them.

Also, you won't typically connect LEDs in series, even though that would save power in your application. The problem is that unless the LEDs are well-matched, they will not be the same brighness necessarily when being run at the same exact current. Using individual resistors and connecting the LEDs in parallel allows them to stabilize at slightly different currents, giving you much better brightness matching.

 

[NoTime]

Also, you won't typically connect LEDs in series, even though that would save power in your application. The problem is that unless the LEDs are well-matched, they will not be the same brighness necessarily when being run at the same exact current.

Unless there is a significant difference in the led internal resistance(not likely) to match the difference in conversion efficiency. You are going to get the same brightness variation series or parallel.
What you are talking about is what happens when you use a single droping resistor to feed multiple parallel leds.

 

[berkeman]

Unless there is a significant difference in the led internal resistance(not likely) to match the difference in conversion efficiency. You are going to get the same brightness variation series or parallel.
What you are talking about is what happens when you use a single droping resistor to feed multiple parallel leds.

Interesting...I wonder if that's what happened. I just remember that a long time ago I tried to save on something (I could have sworn it was putting them in series, though) with LEDs, and one turned on and the other didn't. Guess I'll have to go run a quick experiment...when I have time


EDIT -- To the OP, it might make more sense to use 4xAA or 4xAAA batteries, since you wouldn't have to waste so much of the voltage. That might be too bulky for your intended app, though. You could always just do a buck down from the 9V battery to about 3-4V instead.

 

[NoTime]

Interesting...I wonder if that's what happened. I just remember that a long time ago I tried to save on something (I could have sworn it was putting them in series, though) with LEDs, and one turned on and the other didn't. Guess I'll have to go run a quick experiment...when I have time

You can work this out just by doing the math

LEDs are current mode devices. You need a change in current to get a change in brightness. Note that a small change in Von can make a big difference with a single drop resistor. Different Von means that the color is different as well. A blue led will have a Von of around 3.6v.

EDIT -- To the OP, it might make more sense to use 4xAA or 4xAAA batteries, since you wouldn't have to waste so much of the voltage. That might be too bulky for your intended app, though. You could always just do a buck down from the 9V battery to about 3-4V instead.

If you are going to go through the trouble of designing active circuitry look into pulse mode.
Big increase in apparent brightness. Big decrease in power consumed.
Conceptually less complex than buck.
Just a 555 and a switch device.

 

[berkeman]

You can work this out just by doing the math

Yeah, I'll go back and figure out what had me spooked on the series connection. Could be I was working with jellybean LEDs mixed in a labstock drawer or something. No time at the moment to try it though (hey, that's funny!) -- we're in the middle of DVT on first Si. Busy times. (hey, that's funny too!)