I hope that someone can help me find a suitable relay for my purposes.

I need to be able to briefly (don't know exactly how long at present, but I expect less than 1 second) short 2 wires together under control by a PC.  I unfortunately don't know what signal levels these 2 wires are, and I don't have access the equipment these wires are connected  to, so cannot investigate myself.

I've been using an mbed for another PC interface piece of work, so as I already know that I can interface my software with the mbed, and get it to do the sorts of things I want, I thought it would be best to stick with an mbed board.  I've written an app on the mbed which allows a PC to send a command to it and it raises a DigitalOut pin high for a fixed period.  I'll connect a relay to the DigitalOut pin and that'll short the necessary wires together.  Nice and straightforward.

The problem I have is that I've not yet found any suitable relays to be driven by the mbed.  I only seem to find ones with are 5v or higher, and have quite high current requirements.  Can anyone recommend a relay, available in the UK, which can be directly driven by the 3.3v / 40mA output of a DigitalOut pin on an mbed?

As I said, I don't know the voltages of these wires, but I suspect it's a signal (I did ask, as I wondered whether the mbed to could raise/lower one of the wires, without having a two wire solution, but didn't get a helpful response), as a momentary short it's used to trigger an operation in a piece of equipment.

Any help would be appreciated.

Thanks

http://www.phidgets.com/products.php?category=9&product_id=3051 is a nice module already on a PCB

 http://www.active-robots.com/products/phidgets/index.shtml is the UK dist

I think you also need to run a 5V supply to the board using the three wire black connector. It's possible that you might need a bit more 5V current for it than is on the mbed 5V supply if you switch more than one relay at a time.

They also have a solid state relay module - faster more reliable long term but less current.

Hi,..

I really doubt you'll find a 40ma/3.3v relay.

I could find this.. but it draws 100ma, even though it's TINY.

http://uk.farnell.com/coto-technology/9852-03-00/relay-reed-spdt-smd-3-3vdc/dp/1081692

plus, a relay switching circuit is a transistor, a resistor and a relay. (oh, don't forget the diode, I always forget the diode). If you have some power available from somewhere, just use that to switch the relay by adding a transistor and resistor to the relay and using the mbed digitalOut. This pic is 12v, but it doesn't have to be. This is ALL that breakout board is. Just spec a small 5v one if you have 5v around. (or if you are always using a USB).

Do you have power available from anywhere other than the mbed? Remember you only get VU (5v) when you are plugged into a usb port, are you always planning on being plugged into a USB port ?

These are odd replies, but it's an odd question.

I'd find out why you want to "short" the wires, it *may* be just a big mosfet/transistor you need. They do some stonking great power ones now, if its not really a "short".

You might not even need a power one.

btw, if you don't know what's in them, how do you know the whole thing won't go "bang" ? (or more possibly "melt") or what to spec ? If you use a relay, how will you be able to check the contact ratings will be ok, if you don't know the levels in the wires ? 

I think its easy enough as above, but personally I'd really want to find out what I was shorting.

Sorry for the really brief reply, need to dash.

The mbed will be connected via USB (it's communicating to the host PC via serial over the USB connection), so can rely on 5v from USB being present.

I've tried finding out what happens when the wires are shorted, voltages, current etc, but didn't get any helpful replies.

There is a bit of equipment which has a key override. Turn the key briefly and it'll trigger one operation of the equipment.  The key switch shorts the wires together.  I need to do the same operation with the mbed.  All I could get out was that I needed to short the wires to get the same effect.

Hope that helps,

Thanks

David Styles wrote:

Hi,..

I really doubt you'll find a 40ma/3.3v relay.

I could find this.. but it draws 100ma, even though it's TINY.

http://uk.farnell.com/coto-technology/9852-03-00/relay-reed-spdt-smd-3-3vdc/dp/1081692

plus, a relay switching circuit is a transistor, a resistor and a relay. (oh, don't forget the diode, I always forget the diode). If you have some power available from somewhere, just use that to switch the relay by adding a transistor and resistor to the relay and using the mbed digitalOut. This pic is 12v, but it doesn't have to be. This is ALL that breakout board is. Just spec a small 5v one if you have 5v around. (or if you are always using a USB).

Do you have power available from anywhere other than the mbed? Remember you only get VU (5v) when you are plugged into a usb port, are you always planning on being plugged into a USB port ?

These are odd replies, but it's an odd question.

I'd find out why you want to "short" the wires, it *may* be just a big mosfet/transistor you need. They do some stonking great power ones now, if its not really a "short".

You might not even need a power one.

btw, if you don't know what's in them, how do you know the whole thing won't go "bang" ? (or more possibly "melt") or what to spec ? If you use a relay, how will you be able to check the contact ratings will be ok, if you don't know the levels in the wires ?

I think its easy enough as above, but personally I'd really want to find out what I was shorting.

 

That's exactly the circuit I'm using to enable/disable 50 ohm termination in my project. The resistor is 1k.

Some reed relays are designed to be driven directly from logic, as are almost all SSRs. But it's easier to just use that circuit along with a normal relay unless you are constrained for space or power.

The Phidgets relay boards from the earlier post have the driver circuits on it.

Thanks for the help, I've managed to get something which should work using the diagram above and 5v from the 5v USB out pin.