Relay VS Push Button: blind test (how to tell the difference)

Question

I have a classic push button that closes the circuit when pressed down. This push button was replaced by a simple relay, the relay is activated when 3V goes through its trigger points, and the circuit is closed. The result is the same, the circuit is being closed.

Now, if I was on the other side of a wall and someone was to replace the push button with the relay (case 2), how can I tell the difference from my side? I do not see anything, so I don't know when the other person pushes the button or when he triggers the 3V for the relay. If I have access to an oscilloscope would I be able to see a different when the circuit is closed in case 1 VS case 2? Is there any simpler test?

See my beautiful drawing:

Is there some application here, is it homework, or is this just a thought experiment? — Samuel, Mar 14 '17 at 01:54

score 5 · Accepted Answer · answered Mar 14 '17 at 01:54

5

Contacts bounce, and pushbuttons usually do this more than relay contacts and in an unpredictable way. If you happen to know the specs of the relay, or at least, the type (e.g. reed relay) you can make an educated guess by looking at a scope print of that bounce.

answered Mar 14 '17 at 01:54

Janka

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Or you could characterize the contact resistance of both, likely it would be different. (I.e. measure current through and drop across the contacts of each.) – John D Mar 14 '17 at 01:56
The riddle should be solved with a scope, so it's the bounce they want to see. Given this is educational. – Janka Mar 14 '17 at 02:00
Interesting idea. Pretty sure there would be an observable difference. But not convinced you would tell which was which though if you were always behind the wall and had no vision/access to the switch and relay to make comparisons to test against. If its random switches and relays, either one could have the longer bounce depending on the contact type. Also, you have to factor in the human finger issue... What happens if I click the switch slowly... vs.. "whack it on.." – Trevor_G Mar 14 '17 at 05:55
Contact resistance would solve the human factor, but again you need to know what you are looking for in terms of specific relay and switch, and if you are switching much current the R value will change over time. – Trevor_G Mar 14 '17 at 06:04

score 3 · Answer 2 · answered Mar 14 '17 at 11:03

I compared the contact bounce for both cases. And I am now able to tell the difference!

The relay always shows a last bounce that seems to be always at the same period from previous bounce (in the case of that particular relay: ~460 μs).
The push button in the other hand shows random bounces for each push which seems to be the "factor in the human finger issue" as suggested by @Trevor.

Cool experience, thanks for your help guys!

score 0 · Answer 3 · answered Mar 14 '17 at 01:57

Nothing would be different in either case. An O-Scope (overkill) or a simple meter can be used to detect a change in potential in your circuit on the right side of the wall, the same will hold true regardless of how the circuit is completed. Open circuit, there is a difference in potential between the two wires, closed circuit, no difference in potential.

Ah, after reading the other response, I think I misinterpreted the question. I like the contact bounce idea. It's unlikely that the contact resistance is going to be consistently different enough unless maybe you had a lot of empirical data to compare to.

Relay VS Push Button: blind test (how to tell the difference)

3 Answers3