How would you replace these buttons with relays?

Community PCB Design
1

Hello all.

I have a commercial device I’m trying to modify. It has these little surface mount momentary switches (see attached picture). I want to remove the switches and replace them with relays to be controlled from a microcontroller on a different board (with a ULN2003 Darlington Array in between).

How do you solve this problem? I found some relays that sort of fit the footprint (G6L-1F), but then I have to solder wires to the little SMT tabs on the other side of the relay for its control. Or, I’ve though of soldering on a surface mount to socket adapter, but I haven’t found ones that match the footprint yet. I’ve also thought of drilling through the SMT pad and making it into a through hole (would that even work?)

All those solutions have drawbacks. Is there a best practice for doing this? If not, what have you done that worked in the past?

The switch dimensions are 6mm x 6mm, and the legs are 4.5mm apart on sides and 10mm apart width wise to the outside, if that helps.

Thanks,

Brian

2

What’s the voltage at the 4 pins of the switch? Is the line switching to VCC or VSS/GND? Have you probed it with your multimeter?

3

I would have thought transistors or MOSFETs would be a better option. You might even be able to omit these and drive the signals directly with the microcontroller. It really depends on what the voltage levels on the switches are.

4

If you need isolation and the load is DC, I’ve had good success with Toshiba’s [TLP222A photorelay.](Electronic Components and Parts Search | DigiKey Electronics)

5

The switches use some kind of matrix arrangement, which I have not decoded. For the sake of getting something done, that’s why I’ve decided on relays. Version 2 might let me figure out a smarter way but for now 1-for-1 replacement is the way to go.

I’m more asking about the mechanics of it, how do you go from a four-tab SMD arangement to something more interfacable?

Thanks,

Brian

Edit: Oops, sorry, realize I didn’t answer any of the questions.

thebecwar: Of the 4, the bottom 2 are the ones that touch a trace. Left is Vdd (4.5-5V, as I said there is a matrix arrangement) and the right pin is either GND or like 1V.

MichaelN: See above for why I have to go with relays for now.

propjohn: Thanks, I’ll check that out and see if there is a surface mount version.

6

They look like B3S SMT SPNC push to make switches.

http://uk.farnell.com/omron-electronic- … /dp/177807

http://www.farnell.com/datasheets/490609.pdf

Where as shown the left pair of pins are 1 contact and the right pair are the other contact.

If I am correct then you only need single pole relays and can simply wire the contacts across them.

7

You said that there are only 2 pins with a trace, and most likely those buttons are similar to other types of momentary push buttons, so you should only need a single pole relay (4 pin).

[EDIT: Whoops… looks like I should have read the last line of mattylad’s post. Here I thought I had a useful new suggestion…]

8

mattylad:
If I am correct then you only need single pole relays and can simply wire the contacts across them.

Right, that’s correct. But how? Is there any way to do this without wiring itty bitty wires to the relay SMD tabs?

9

kd5crs:
Right, that’s correct. But how? Is there any way to do this without wiring itty bitty wires to the relay SMD tabs?

Is there space on the reverse side?

Other than little wires - perhaps if you get the right pitch relays you can drill holes and fit it through the board, bend and solder the

pins onto the switch pads.

Or get a SMT relay that also has the same pitch.

Although this means you will still need itty bitty wires going to whatever powers the relay.

A mod is a mod and they never look pretty until they are incorporated into the design.

10

Mike from Sparkfun technical support had a good suggestion: using cut-edge vias like those seen on the sides of this board: http://www.sparkfun.com/products/8497

That way I could make a “break-in board” to interface my own circuit with the surface mount pads. Or I guess you could call this a break-out board, just upside down.

Has anyone done that in Eagle? If you’ve had a board made with those, are there any tricks I should know? Right now I’ve just got a through-hole pad half-on and half-off the board edge. Is that all that’s required?

Thanks.

Brian

11

kd5crs:
Mike from Sparkfun technical support had a good suggestion: using cut-edge vias like those seen on the sides of this board: http://www.sparkfun.com/products/8497

…

Has anyone done that in Eagle? If you’ve had a board made with those, are there any tricks I should know? Right now I’ve just got a through-hole pad half-on and half-off the board edge. Is that all that’s required?

I believe that's referred to as "castellations". You'll need to work with your PCB manufacturer to determine how they want to see these represented in the Gerber / Eagle files. I don't think BatchPCB can do this type of thing.
12

This is a case of trying to modify an existing board - not design a new one.

So that sort of edge via/castellated edging etc will not be possible.

PErhaps adding the relays to a smaller PCB and wiring that up then affixing it would work?

13

Well, with the castellation idea, I’d just desolder the buttons and then solder on a castellated breakout board with the relay and other connections on it. The existing board would not need to be modified beyond removing the buttons, I’d just use the existing button pads.

Brian