SteveSpencer
Posts: 351
Joined: Thu Mar 28, 2013 9:19 am
Location: Nottingham, UK

Replacing pushbutton switches

Thu Apr 06, 2017 9:56 am

This seems such a simple question, that I am almost too embarrassed to ask it. However, the stupid question is the one you didn't ask, and should have, so here goes :oops: .

I have a device, which can be raised or lowered by means of two buttons (Up, and unsurprisingly, Down). These are connected to the "business end" by a simple 4 core cable terminated by RJ11 connectors. The buttons are pushbutton switches, as determined by inserting an unterminated cable and connecting two of the cores together.
I did not design or build the device, I merely want to adapt it to my needs ;)

I would like to be able to control the device from a distance. There are not really "safety" concerns, since I have determined that on reaching the top and bottom of the range, power to the motor is cut automatically. (No, it isn't an electric window...)

So, my first thought was that I could use two mechanical relays to replace the switches, by connecting to the common and NO connections of the relay.

My second was to use something like an opto-coupler (either two channel or two devices) to achieve the same thing, but smaller and quieter.
Something like the ILD615 type would do, I thought, assuming the voltage and current in the circuit are appropriate.

However, in the datasheets I see that the VCEO and VECO max ratings are significantly different. This means I have to get the connection of the opto-coupler the right way around. As I don't have a schematic for the device, I need to "buzz out" the four pins.

Can someone give me advice on identifying which way around the cables are? I can "buzz out" the switch end to determine which pair is which (easy part, look for "infinite" resistance, then "no" resistance), but determining which of a pair should be collector and emitter escapes me.

Luckily, the software for this is simple enough, and I do know enough to put resistors inline on the LED side of the opto.

Thanks in advance
Steve S
No, I can't think of anything funny that won't offend someone if they want it to...

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Burngate
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Re: Replacing pushbutton switches

Thu Apr 06, 2017 10:52 am

The most obvious answer is get a meter. Failing that, a resistor-LED would tell you which is positive and which is negative - assuming it's not AC.
VCEO and VECO max ratings are significantly different
That's true of most bipolar transistors, and is down to the doping of the three layers - the base-emitter junction isn't meant to be reverse-biased, whereas the collector-base junction is.

Before I connected anything, I'd want to know the open-circuit volts and the closed-circuit current coming down the wire. That's where a meter is most useful

SteveSpencer
Posts: 351
Joined: Thu Mar 28, 2013 9:19 am
Location: Nottingham, UK

Re: Replacing pushbutton switches

Thu Apr 06, 2017 11:15 am

[fx: slaps forehead in Homer Simpson style]
Of course, I should have thought of that myself. In any case, I need to measure the voltage to make sure the opto is the right size, which will tell me the polarity, and then measuring the current after that to do the same...

It's amazing that the human brain can sometimes overlook the "b******g obvious". (In my defence, y'r 'onour, I am not always that dim...)

Thanks
Steve S
No, I can't think of anything funny that won't offend someone if they want it to...

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