Replacing switch / resistance of a switch

tcncj · Apr 17, 2018

Not sure where to ask this question. But I hope someone can help me.

I have a control panel that has a 3 buttons.
They starting to fail (and now they did) and I want to replace them.
The 3 buttons have a shared + and ground wire
The controls know which button is pressed based on the resistance of each button.
I measured the resistance and it's indeed different for each button.

But I'm a complete electronic noob.
What I did was using my multimeter at 200k to measure the resistance once a switch is pressed.
For example, one button had 16.2 (at 200k setting at multimeter).
My new button had 0.7 at the 200 setting (higher setting at multimeter resulted in 0).
So my question is. How can I make the resistance the same as the original button?

Mark Rand · Apr 17, 2018

You need to solder some resistors between the individual switch contacts and the line (either side of the switch is fine). If you've got a local Maplin shop, they might have some left in the closing sale. Failing that CPC will sell you more than you need as a minimum quantity, but the price isn't much.

16.2 on the 200k or 20k setting would be 16.2k Ohms, you may find that the other values are about 4 and 8 k repectively, but the meter will tell you!

Whereabouts in the UK are you?

tcncj · Apr 17, 2018

Thanks for the quick reply Mark.
These are the readings for the original stuff

16.2 Ohm
6,9 Ohm
and 2.6 Ohm

How do I calculate which resistors I need?
On the new buttons I read 0.7 (at 200 setting, original switches are measured at 200K). Is this 0.7 Ohm?
If so? for example: to reach 2.6 Ohm I just need to add a resistor of 1.9 Ohm?

thermite · Apr 17, 2018

tcncj said:
Thanks for the quick reply Mark.
These are the readings for the original stuff

16.2 Ohm
6,9 Ohm
and 2.6 Ohm

How do I calculate which resistors I need?
On the new buttons I read 0.7 (at 200 setting, original switches are measured at 200K). Is this 0.7 Ohm?
If so? for example: to reach 2.6 Ohm I just need to add a resistor of 1.9 Ohm?

Just me, but.. for repair, not new construction, I'd get 3 same-same 20 Ohm (on-edit - 20K Ohm) trimpots plus a spare or three if there was room to mount them. About 36 US cents each, our "usual suspects'.

Rheostat-connect each ("slider" to one end), dial-in what the machine is happiest with now, be able to compensate for drift, years later,

And go attend to something else in greater need of attention.

Mind - on size/space and price, I have assumed very low "signal" current levels. Do check that.

Mark Rand · Apr 17, 2018

The measurements were done in the 200k scale. That means that the values are kiloOhms, not Ohms.

Go for 2.7k, 6.8k and 16k.

rons · Apr 17, 2018

tcncj said:
I measured the resistance and it's indeed different for each button.

But I'm a complete electronic noob.

Can you show a picture of those magical switches (to clear up confusion).

thermite · Apr 17, 2018

rons said:
Can you show a picture of those magical switches (to clear up confusion).

What confusion?

Switches themselves, generally identical, not weird or uncommon SKU, this sort of approach.

Resistors values differ. Buggers as make 'em never managed to agree a global standard for just the one Ohmic value. Go figure.

tcncj · Apr 17, 2018

Mark Rand said:
The measurements were done in the 200k scale. That means that the values are kiloOhms, not Ohms.

Go for 2.7k, 6.8k and 16k.

Ok
But the new switch is measured at 200 (not 200k)
So that means the new button is 0,7 Ohm
Old buttons are 2.7k, 6.8k and 16k
If I understand it correctly I need a resistor to bridge the gap between 0,7 Ohm and 2.7k, 6.8 and 16k
Can I use 2 or more resistors behind each other?

awander · Apr 17, 2018

The 0.7 ohm you are measuring is just the resistance of your test leads and the new switch. It is inconsequential. View it as "zero" ohms.

Just use the 2.7K, 6.8K, and 16K resistors, as noted above.

Yan Wo · Apr 17, 2018

This is the craziest control arrangement I have heard of.

thermite · Apr 17, 2018

Yan Wo said:
This is the craziest control arrangement I have heard of.

Yeah? Try a 1940's. '50's U.S. Army field-mobile Acetylene generating plant - where it is done with a maze of thin tubes carrying low-pressure compressed air and interfacing to magnetic sensors and actuators.

No rude electricity wanted, thanks! Annoys the sleep cycles of the notoriously psychotic Acetylene @ around 300 PSIG - enroute to a MUCH safer bed - in the cylinders.

"Sergeant? Why do you have the entire manifold system down that side of the plant shut off and plugged?"

"Sir, nearly all of these plants that have blown-up so far were charging cylinders off that side, and Air Products Engineers still haven't figured out why."

Damned good man, that Sergeant.

Mark Rand · Apr 17, 2018

Yan Wo said:
This is the craziest control arrangement I have heard of.

Not really. I've used similar arrangements with embedded controllers. If you have an A/D input available, it allows you to sense multiple combinations of switches with only two wires.

thermite · Apr 17, 2018

Mark Rand said:
Not really. I've used similar arrangements with embedded controllers. If you have an A/D input available, it allows you to sense multiple combinations of switches with only two wires.

Verily.

Even by the tail-end of the 1960's, "digital" still being a hot, hungry, noisy, and fragile world, I ha' thought half the Engineers I knew were ready to MARRY Burr-Brown, Analog Devices, or go for a Ménage à Trois!

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