Cheap Relay Switch?

So I built this relay switch using parts from sparkfun:

The switch is lovely...but expensive. After the PCB board, caps, led, transformer, resistors, screw caps and the switch itself it came to about $10

I have a project that will require about twenty of these. Before I drop 200 bones does anyone know of a cheaper alternative.

I am using the arduino to switch twenty 1 amp loads at 5 volts.

Thanks!!!!

What are the loads? Different types of loads will tolerate/require different power switching techniques.

What are the loads?  Different types of loads will tolerate/require different power switching techniques.

The loads are twenty 10" strands of .01" flexinol muscle wire, each with a resistance of 5 ohms. Each wire will require 1amp at 5 volts.

Yeah - don't buy the parts from SparkFun.

Now that you got a model to work from (and hopefully a schematic that you know how to read), you can translate that to a PCB perfboard design, and mount all of them on one PCB:

1 x http://www.allelectronics.com/make-a-store/item/ECS-4/SOLDERABLE-PERF-BOARD-LINE-PATTERN/-/1.html

(might need 2)

20 x http://www.allelectronics.com/make-a-store/item/RLY-541/5VDC-SPST-N.O.-DIP-REED-RELAY/1.html

(this relay may or may not handle the voltage on the contacts; no reference to what that is, but the current rating for the contacts is in the range needed)

40 x http://www.allelectronics.com/make-a-store/category/530200/Resistors/1/4-Watt-Resistors/1.html

(couldn't tell what each was, but probably 220 ohm or 1K)

20 x http://www.allelectronics.com/make-a-store/item/1N4004-TR/1A-400V-RECTIFIER-SPECIAL-T-R-PKG/1.html

(you'll never run out!)

20 x http://www.allelectronics.com/make-a-store/item/LED-1/T-1-3/4-RED-LED/-/1.html

20 x http://www.allelectronics.com/make-a-store/item/2N3904/NPN-TO-92-TRANSISTOR/-/1.html

1 x http://www.allelectronics.com/make-a-store/item/SHS-40/SINGLE-ROW-HEADER-1-X-40/-/1.html

20 x http://www.allelectronics.com/make-a-store/item/TER-302/2-POSITION-PCB-TERMINAL-BLOCK/-/1.html

Unless I missed something, these should be all the parts you would need; you should be able to do this project for under $50.00.

4 Channel Relay Board ?

Why not just use an opto isolater w/triac output like this? We are sorry but you have encountered an error

It will handle the voltage and amperage you are using and all you need to interface with the Arduino is a resistor. (unless you really need the led, then 2 resistors and an led.) :smiley:

Cost would drop to about 50 cents per channel or about $10.00 total plus the cost of perfboard and connectors.

Why not just use an opto isolater w/triac output like this? We are sorry but you have encountered an error...

It will handle the voltage and amperage you are using and all you need to interface with the Arduino is a resistor. (unless you really need the led, then 2 resistors and an led.)

Cost would drop to about 50 cents per channel or about $10.00 total plus the cost of perfboard and connectors.

Photo-Worx:

  1. I don't think that part will handle the current needs - the spec sheet says it can only handle a 1A surge; continuous mode (which is what these ninitol wires will need) current on that device is likely to be less than the 1A needed.

  2. That device has a triac output, meaning it is meant to be used for AC voltages on the switching side; Justin didn't mention whether he was switching AC or DC; just 5V @ 1A...

Now, if you could find an opto-isolator with a MOSFET output, with a continuous current rating of at least 1A or higher (I would want at least 1.5A or more), then you might be able to get the cost down much further than by using relays.

However, that device you reference won't likely work for this application, at least based on what I can see from a quick skimming of the spec sheet.

$10 doesn't seem bad for a P&B relay and the other parts you listed.

Is your load AC or DC? If the load is DC why not just use FETs? A 50A FET
would be in $0.50-$1.00 range (probably less). I would put a zener clamp on
the gate and a current limiting resistor to protect the digital output.

Not sure why you need a transformer.

(* jcl *)


www: http://www.wiblocks.com
twitter: http://twitter.com/wiblocks
blog: http://luciani.org

4 Channel Relay Board

yea not too happy with them right now, saved like a whole 60 cents, been waiting on my order for 3 weeks now

I would be fine if they were slow and I saved a bunch (ie sure electronics), but they are slow and not all that impressive on price

Okay, a simple resistive load. And ideal candidate for switching with a transistor.

For various reasons you’ll learn as you build your electronics knowledge, the easiest way to control such loads is to connect the positive side to the power supply, and switch the ground side using a transistor.

Transistors aren’t ideal switches like relays: they all have a resistance that’s somewhat higher than mechanical contacts. Bipolar transistors are cheap, but the amount of energy they waste becomes significant when you’re switching larger loads. MOSFETs tend to be more efficient, but also more expensive and sometimes more difficult to use .

What you probably want for this project is a “logic-level MOSFET”, which has the efficiency of a MOSFET, but is easier to interface to 5V logic. You can find examples in the Playground area for hooking them up to Arduinos and loads.

If you want to use relays, you can use google to search for "parallel port relay board" for some kits that would bring your cost down from the sparkfun cost. If you want to use MOSFETs for a less expensive solution, get some MTP3055VL MOSFETS (made for use with 5v logic level on the gate), several 74HC259 latching multiplexing chips to individually operate the MOSFETs, and a hand full of resistors and such. You probably can do your project fairly cheaply.

4 Channel Relay Board ?

There are some things Futurlec has a good saving on, others not so good. I tend to use them for ordering for the parts bin, not for a project that I want "right now" ;D

It usually takes around 3 to 5 wks for delivery, but the shipping is cheap!!

Another thing - they store your credit card info. I don't like that, so I pay via paypal.

Ken H>

get some MTP3055VL MOSFETS (made for use with 5v logic level on the gate), several 74HC259 latching multiplexing chips to individually operate the MOSFETs

Is it possible to control twenty MOSFETs individually from the arduino digital out pins rather than using a latching chip? (arduino mega)

Ok, so I had some of these TIP120s laying around and though I’d give them a shot. They seem to work great. Any reason this schematic might be problematic? (I am using a computer power supply with common ground to the arduino)

The TIP120 may work but not that well. With a collector current of 1A
you will probably have a voltage drop in the 0.75V-1V range. You
will be dissipating 1W of power in the transistor body. The 1V
drop means your muscle wire will only be run at 800mA (5V-4V)/5Ohms.
You are probably close to the power limit of this device unless
you add a heatsink.

A FET is a much better choice. A logic-level FET is your best choice.
tt should not be difficult to get a FET with a 100mOhms on Resistance.
This reduces you drop to 100mV and power dissipation to 100mW.

If you do decide to use the TIP120 you need a resistor in series with the
base to limit the base drive. Also your schematic shows the TIP120
as a FET.

(* jcl *)


www: http://www.wiblocks.com
twitter: http://twitter.com/wiblocks
blog: http://luciani.org

A FET is a much better choice. A logic-level FET is your best choice.
tt should not be difficult to get a FET with a 100mOhms on Resistance.
This reduces you drop to 100mV and power dissipation to 100mW.

Ok I jumped onto Digikey to search for what you are describing. I applied a filter to only show FETs rated at 100 mOhms. The search returned an overwhelming amount of FETs, one of which I have below. Is this what you are referring to?

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=IRF9Z34NLPBF-ND

Thanks!

Also, here is what Zoomkat suggested. Looks like its rated at 180 mOhms

My pick (if we only look at Digikey) would be the Fairchild FDP8878.
It has a specified maximum Rds(on) of 19mOhms at a Vgs of 4.5V.
(I am assuming you are running a 5V Arduino). It is also $0.88(1) See http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=FDP8878-ND

Comparing the FDP8878 to the IRF9Z34 --

  1. The Rds(on) is specified at logic levels. The IR device was specified
    at -10V (negative because it is a P channel). If you look at the Rds(on) graph
    you will find a higher Rds(on) spec at Vgs=-5V.
  2. The device is an N channel rather than a P-channel. Typically P
    devices are more expensive than N devices for the same performance.
  3. I believe that the TO-220 package has a lower thermal resistance
    than the TO-262.

(* jcl *)

Ok thanks, I'll give it a shot.

I was looking at the spec sheet for the FDP8878-ND MOSFET. It says it has a logic level input but has a gate charge of 23nC @ 10V.

Will this work with the arduino's 5v output? If not, I am considering this one here. It is a couple cents more but has a gate charge of 23nC @ 4.5V.

Any suggestions?

Thanks!!!