View Full Version : Simple float switch relay PCB diagram in Postscript format...
MarkS
02/04/2004, 11:29 PM
This is the design I am building. It is very simple. It uses one float switch and one 5A PCB mount relay (http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&category%5Fname=CTLG%5F011%5F002%5F013%5F000&product%5Fid=275%2D249) from Radio Shack.
There is nothing complicated about this at all. The float switch registers low water, the relay switches on and the pump pumps water from the reservior into the sump. Two switches can be added with little modification.
You'll need Ghostscript and GSView to view the diagram. Both programs are available for free download here: Ghostscript and GSView (http://www.cs.wisc.edu/~ghost/) You'll need to change the file extention from .txt to .ps
The method of getting the diagram onto a PCB is easy, but complicated to describe. You'll need to print this onto a transparency and use a photosensitive PCB blank. I am assuming that those interested know how. Maybe someone else can fill in this part.
MarkS
02/04/2004, 11:38 PM
Here is what is should look like. Shown here about 2X normal size.
barebottoms
02/05/2004, 01:52 AM
Nice. Fairly simple board, should work great with toner transfer too. I can give a quick tutorial on home etching boards if people are interested.
reefkeeper168
02/05/2004, 10:15 AM
Originally posted by barebottoms
Nice. Fairly simple board, should work great with toner transfer too. I can give a quick tutorial on home etching boards if people are interested.
How do you transfer image from a pc drawing to a copper laminate plate?
barebottoms
02/05/2004, 11:06 AM
Originally posted by reefkeeper168
How do you transfer image from a pc drawing to a copper laminate plate?
A Clay based paper and an Iron.
MarkS
02/05/2004, 11:47 AM
Originally posted by barebottoms
A Clay based paper and an Iron.
That's one way.
What I've done before and am going to do here is print this onto a transparency, tape it to a photosensitive PCB in a dark room and expose it to light. The light reacts with the photosensitive coating and transfers the traces to the board. It is then etched.
It's a bit more complicated than that, but that's a general overview.
barebottoms
02/05/2004, 11:58 AM
I use photosensitive PCB too. It depends on the layout, if it has a lot of thin traces I generally use photosensitive. If its something simple where I can make nice think traces then I use transfer method because lead time on Photosensitive boards can take a while sometimes and cost so much more.
gbtower
02/05/2004, 12:59 PM
Originally posted by barebottoms
I can give a quick tutorial on home etching boards if people are interested.
Interested! Maybe another thread?
MarkS
02/05/2004, 01:24 PM
Originally posted by gbtower
Interested! Maybe another thread?
Agreed!
Here is an update.
I added space for two fuses, one resistor and an LED. You'll have to figure out the resistor value based on your choosen input voltage.
BeanAnimal
02/05/2004, 03:50 PM
MarK and others,
It would be a good idea to add a diode to the relay coil. Especialy of you are using a small PCB mount style relay. The diode will absorb any "spike" that the coil cause when it turns off. Without getting technical a 12 volt relay can cause a 1,000 to 1,500 volt transient spike when its field colapses. It is cooman and accepted practice to shunt this surge with a diode or MOV.
This spike causes breakdown of the relay coil, contacts and anything else in the circuit. It can be catastrophic, or simply degrading over time.
You really dont even have to redesign the PCB...just solder it between the coil legs. SOME Relays come with an aproproate diode already installed in the housing.
This is the same idea as decoupling capacitor that should be used on ALL ICS. You can sometimes get away without it...but why take the chance.
Bill
Very cool.
You need to get a copy of CircuitMaker.. I'm running CircuitMaker 2000 here, great application. Makes all of this simple.
They make iron on transfers that you print on a laser printer to etch. You print, iron, etc. No exposure required.. Photosensitive, blah! I had enough of that developing my own photos..
Depending on how many you need or want, there are prototyping shops (some in China that are dirt cheap), that will make a small run of boards for you relatively cheap. The best part is they are drilled already and can be dual sided.
jb
MarkS
02/05/2004, 05:08 PM
Originally posted by grim
They make iron on transfers that you print on a laser printer to etch. You print, iron, etc. No exposure required.. Photosensitive, blah! I had enough of that developing my own photos..
Two problems. First, I do not know where to get them (should be easy to fix). Second, I do not have a printer. I'll need to go to Kinko's. I am not sure if they'll let me use my own stock and am uncertain what this special paper might do to the printer.
barebottoms
02/05/2004, 05:20 PM
You do not need the special transfer paper. I've had way better luck with a good clay based paper than the "special" transfer paper.
On simple layouts, regular paper works great, you might have to touch up some traces now and then with a Sharpie.
Kinkos will let you use your own stock, at least the ones I've been to.
MarkS
02/05/2004, 05:30 PM
BeanAnimal, like this?
BeanAnimal
02/05/2004, 09:36 PM
Yes the diode should be parrallel to the coil. The diode should be situated so that current can flow from the ground side of the coil back to the positive side of the coul.
In other words the "arrow" of the schematic diode should point TO THE POSITIVE SIDE OF THE COIL!
If you put the diode in backwards, you have simply created a direct short to ground!!!!!
SO the STRIPED side of the actual diode will be connected to the positive side of the coil.
Simply put, a diode is a one way valve...it allows current to travel in only one direction! When the spike occurs, the diode help to suppress it.
Bill
MarkS
02/05/2004, 11:03 PM
So basically, place it in reverse of what you would think is correct? What rating should this diode have or is it not important?
MarkS
02/05/2004, 11:12 PM
This??
MarkS
02/06/2004, 12:53 AM
I just changed the pads on the relay to circles and made them a little bigger. This should help anyone trying to build this at home. Drilling tiny holes accurately is tough enough.
I also converted it to a PDF file for those who do not need or want Ghostscript/GSView.
Chamkeeper
02/06/2004, 01:07 AM
I dont understand the point of making a circuit board for this circuit. Three of these could be built in point to point on a generic board in the amount of time it takes to make one custom board.... Just my $0.02
MarkS
02/06/2004, 01:31 AM
Originally posted by Chamkeeper
I dont understand the point of making a circuit board for this circuit. Three of these could be built in point to point on a generic board in the amount of time it takes to make one custom board.... Just my $0.02
Purely for the fun of it. ;) I've already built this on a generic board.
BeanAnimal
02/06/2004, 02:55 AM
The diode question is sorta like the LED resistor question... but with a few more wrinkles.
Time to do some learnin!
When A relay de-energizes the COIL causes a pulse of electricity to shoot down the coil leads. In laymens terms, as the coils magnetic field colapses it releases its stored energy back into the coil.
This happens very quickly and the relay snaps back open very quickly. Easy on the load contacts, hard on the coil, power supply, switches and anything else connected to the coil power source. Totaly destructive to anything digital or delicate.
Placing a single diode across the coil helps to suppress this spike. The bad part is that it slows the action of the relay down (up to about 12 times!). This can cause more wear on the LOAD contacts. Heres why...
PUMP Motors or Solenoids are coils in general terms. This means they are also inductive loads and when the power is removed they create a spike also. This spike causes arcing on the relay contacts. Getting complex huh!
In any case...
A better solution is to use 2 diodes (I know you hate complications) with their anodes facing each other. OR a MOV instead of the 2 diodes.
Like this:
+ ---->|----|<---- -
The diode on the right is the reverse biased RECTIFIER diode (the one eluded to in the orignal post) The diode on the left is a FORWARD BIASED ZENER Diode.
Forward Biased and reverse bias simply describe the direction the postive side of the diode faces in comparison to the postive and negative side of the circuit.
Sizing the diodes:
For 12v applications using an average relay...
FOr example the McMaster Car Part #69585K45
This is a great choice of relays, as it is a very low "hold" current of 75mA (cheap to operate and power) and is rated at switching a 10 AMP Load.
For a 12V 75mA relay a 1N4148 RECTIFIER diode will most likely do the trick. (will work for relays that consume up to about 200mA)
We have nothing digital connected to this circuit, so allowing the SPIKE to reach a voltage of 30 or more volts before the "protection" kicks in is no problem....SO The more spike that is allowed, the less the DIODE has to absorb and the quicker the relay snaps! SO lets pick a ZENER DIODE.
IF used, the ZENER DIODE could be rated at about 36 Volts...A 1N4753A should do the trick.
Sorry if this is a pain in the ***! I am sure tons of folks are using float switches and relays without diodes. What can I say. People also don't wear seatbelts or unplug the circular saw without changing the blade!
IN all honesty, you can prolly get away without the diodes on a 12v relay. The only part I can really see failing due to the SPIKE, is the REED switch inside the FLOAT.
The reason your float switch failed was bacause you most likely melted the relay coil and created a direct short at the switch.
Your lookin at about 8 bucks for the relay (or use the one you designed the board for, i am sure it is similar) and a buck each at worst for the 2 diodes.
The float switches can be had for about $6 each on ebay.
Regarding the circuit board process. For our purposes here the toner transfer method will work..but the photmethod produces far superior results. Use either, but if you plan on tinkering with a lot of projects...invest a few bucks in the photo method.
BUy a dremal and a set of micro bits from ebay. Get the dremel "Drill Press" atachment, and your ready to make your own PCBs
Bill
MarkS
02/06/2004, 12:38 PM
Thanks for the info!
Originally posted by BeanAnimal
BUy a dremal and a set of micro bits from ebay. Get the dremel "Drill Press" atachment, and your ready to make your own PCBs
Already have the Dremel and drill press attachment. I just need to get the bits. The electronics store I am going to buy the board from sells them.
MarkS
02/08/2004, 12:23 PM
This is the final revision. Thanks for the help! I really appreciate it!
BTW, it prints out really nice with a lot of detail. I saved the PDF at 720 DPI. This means that I could do hair thickness traces without problem. I'm going to have fun with this.
When I get ready to develop and etch the board, I am going to take step by step pics and post them here. Even if no one wants to build this, they'll have a guide for their own projects.
BeanAnimal
02/08/2004, 03:20 PM
You are gonna bread board the thing first right?
I know for sure that I have designed thangs that catch on fire!
I may have even been electrocuted a few times and cuased localized grid power failures :P
Bean
MarkS
02/08/2004, 11:49 PM
Originally posted by BeanAnimal
You are gonna bread board the thing first right?
I know for sure that I have designed thangs that catch on fire!
I may have even been electrocuted a few times and cuased localized grid power failures :P
Bean
I've already done it.
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