I have a bowling machine control, a partial modified schematic below. I only have access to the control at the interface pins, J#'s.
I do not have access to the full wave +24vdc. No J# on the interface pins.

I would like to monitor the status of switch SW4, regardless of the state of the push button. (The push button represents a lot of circuitry. )
I would like to do the monitoring of SW4 with 5Volt logic.

A simplified how it works: a momentary pulse to PR2, sealing PR2 ON thru SW5. This also turns ON PR1. PR1 starts bowling pins moving down a conveyor with a mechanical counter 0-9. As the first pin passes the mechanical counter SW5 state is switched maintaining PR1 and unsealing PR2. SW4, a home switch, will open as the first pin is loaded into the table, and close when all pins have been loaded. Counter SW5 is back on zero, and returns to the state shown. Keep in mind this is late 50's tech and uses a stepper relay. If there is interest I will post the full schematic.

 

My first impulse is to put a low voltage high frequency signal on the two pins and detect presence or absence of it capacitively coupled. It will not interact with the DC signal.

 

Alternatively, you might just be able to put 5V from an isolated supply across the switch through a biggish resistor to get a couple of mA when the switch is closed.

 

How about using a neon opto-coupler, mock one up as shown http://lednique.com/opto-isolators-2/opto-isolators/neon-optocoupler/
Across the monitor points.
the neon current when conducting is extremely low.

 

How about using a neon opto-coupler, mock one up as shown http://lednique.com/opto-isolators-2/opto-isolators/neon-optocoupler/
Across the monitor points.
the neon current when conducting is extremely low.

That might work since the device is all electromechanical. On the other hand, the same thing could be done with an LED optocoupler and the appropriate resistor. I do like the idea though.

 

Another circuit idea using a mosfet.

 

Another circuit idea using a mosfet.
View attachment 233060

I think this would require the 5 volt system to be floating on the 24v. Otherwise when sw4 is closed the coil PR2 would just turn on. BTW I have little mosfet experience.

 

I think this would require the 5 volt system to be floating on the 24v. Otherwise when sw4 is closed the coil PR2 would just turn on. BTW I have little mosfet experience.

No because diodes D1 and D2 isolate the 5 volt supply therefore relay PR2 can only activate when using the push button.
Mosfet no different then a standard transistor.

 

Here's a version using an optocoupler that completely isolates the 5 volt supply.
Will need a connection to the negative side of the 24 volt supply.
There is a constant 5 ma through PR2 but should not activate relay.

 

The neon optoisolator with an appropriate isolated AC source and a suitable current limiting resistor. I suggest neon because it is far more forgiving of assorted higher voltage spikes, and with the statement that "the pushbutton represents a lot of additional circuits we have no clue about assorted spikes. Neon device last a very long time, many years if not overly abused.

 

What i was thinking, but I don't know if you have access to it. Put a resistor across the pushbutton that can supply say 5 mA. Not enough to pull in the relay. Now put an OPTOMOS relay which needs about 1 mA across the contact to be monitored. The "coil" is a LED that only needs 1 mA.

 

1. Is the 24 Vdc filtered at all, or is it plain full-wave rectified AC all the way down to zero-crossing points?

2. From the schematic it looks lik there is 0 V across the switch when closed. When it is open, what is there? With a battery-powered DVM, what do you get in both AC and DC modes? A scope shot would be great if you have a floating or battery-powered scope.

With this, then some form of post #6 or #9, powered by a USB 5 V wall wart, should get you a clear signal. I think opamps first, but the optocoupler idea should work, assuming that 1950'2 t4ech can spare a few mA leaking across the open contacts. 50 mA seems high to me for the LED current; plus, that might be enough to cause something to move. I'd go with an opto with 100% CTR and drop the LED current to something under 5 mA.

ak

 

@AnalogKid I'm thinking something like this: https://www.mouser.com/datasheet/2/240/CPC1018N-1546057.pdf

It only takes 1 mA to operate.

BUT, you need a resistor across the pushbutton that provides >1 mA and provides less than the amount of current for the relay to activate. You need to account for the relay resistance as well. You would need access to ground.

 

50 mA seems high to me for the LED current;

With reference to post #9.
That's only if the "push button" closed while SW4 is open which shouldn't happen according to the operation in post #1.
The actual current, factoring in the resistance of the relay coil, would be less.

 

Given that the TS understands how the circuit actually works, and given that the portion in that line represented by the push button is more complex, what is the problem with the neon opto-isolator mentioned by Max in post #4, and with additional comments in post #10?
The TS has already mentioned that much of the circuit is not accessible, and from the portion that we can see, there is a fair amount of inductive spikes from those power relay coils. And the circuits with digital logic will need regulated isolated supplies. So the neon scheme can use either a separate transformer to provide an isolated 100 volts or so at as much as 3mA, or a capacitively coupled voltage doubler off the transformer secondary to provide about 100 volts DC for the neon scheme.

 

And the circuits with digital logic will need regulated isolated supplies.

That goes without saying as the TS mentioned: " I would like to do the monitoring of SW4 with 5Volt logic."
No problem using a neon optocoupler other then it would require a 100 volt supply.
Personally I don't see the need.

 

That goes without saying as the TS mentioned: " I would like to do the monitoring of SW4 with 5Volt logic."
No problem using a neon optocoupler other then it would require a 100 volt supply.
Personally I don't see the need.

I must have missed that part. But "you can't always get what you want" .

 

OK, the witch can also be monitored using only a low coil current 5 volt relay, a diode rated at least 50 PRV, and an isolated power supply of about 5 to 6 volts. The diode installed to block the 24 volt supply voltage when the witch is open and the PB circuit is closed. No electronics needed. The relay should have a fairly high coil resistance to avoid having much current pass through the switch. AND no spike-prevention diode can be used here.

 

Here is the full schematic, diagram. I guess by trying to simplifying the circuit diagram, I may have confused the question.
The pinspotter states are controlled with a 6 level mechanical step relay. Access to the controll circuits for monitoring must be limited to the J#s, as these are the chassis interconnect on the machine, to the 6525 chassis.

 

You are right that is a lot of circuitry. I don't it changes anything as far as the suggestions that have been proposed.