Quick electronics question

[Neale]

Are you concerned about the long-term effect of switching 20mA on the switch contacts in the probe? This is the kind of thing that I might have worried about until I had an interesting conversation with a friend who had spent many years working with industrial control systems. His comment was that using 24V was not just useful for noise rejection but the slightly higher switching currents also helped with long-term contact life and reliability. Switch too low a current for too long and the eventual oxide build-up could lead to poor contact but the higher currents (but much too low for arcing to occur) helped break through any potential oxide. Not sure if it's relevant here but I thought I'd lob it in anyway!

I spend many an hour musing on touch probes, generally based around the published designs for three-contact types, but wonder about mechanical switching reliability, where can I easily source gold wires/contacts, etc. In fact, anything to stop me actually getting on with the project...

[cropwell]

where can I easily source gold wires/contacts, etc. In fact, anything to stop me actually getting on with the project...

https://www.proopsbrothers.com/gold-...1103-595-p.asp

[Neale]

https://www.proopsbrothers.com/gold-...1103-595-p.asp

Whose side are you on, Rob? Another good excuse up the swannee...

[Robin Hewitt]

I am obviously not understanding.
Why not 5V through the resistor through the switch to ground?
The switch carries 5V minus the voltage drop across the LED.
Or less another 0.8V drop across the transistor if kept.

[Kitwn]

Are you concerned about the long-term effect of switching 20mA on the switch contacts in the probe? This is the kind of thing that I might have worried about until I had an interesting conversation with a friend who had spent many years working with industrial control systems. His comment was that using 24V was not just useful for noise rejection but the slightly higher switching currents also helped with long-term contact life and reliability. Switch too low a current for too long and the eventual oxide build-up could lead to poor contact but the higher currents (but much too low for arcing to occur) helped break through any potential oxide. Not sure if it's relevant here but I thought I'd lob it in anyway!

In the old days, this was referred to as having enough 'wetting current' through the contacts. AC was thought to be more reliable than DC though the introduction of these new-fangled transistor thingies into previously all-relay control systems made the use of DC inevitable.