I'm looking to build a circuit monitor for use as a fault finder to see what switch drops first in the series before the entire line shuts down.
Explanation I have an escalator that keeps shutting down randomly and the safety string resets and goes back to ready to run mode.
I need to hook up a box that will monitor 8-12 points on the safety string terminal to see which switch is getting tripped first. But each terminal getting monitored can not jump to the next one basically jumping out my safety string. I hope this makes sense. I saw the plc guys talk about a first out annunciator but plc's are out of the question.
voltage is 120 ac
thanks
Andy

 

I'm looking to build a circuit monitor for use as a fault finder to see what switch drops first in the series before the entire line shuts down.
Explanation I have an escalator that keeps shutting down randomly and the safety string resets and goes back to ready to run mode.
I need to hook up a box that will monitor 8-12 points on the safety string terminal to see which switch is getting tripped first. But each terminal getting monitored can not jump to the next one basically jumping out my safety string. I hope this makes sense. I saw the plc guys talk about a first out annunciator but plc's are out of the question.
voltage is 120 ac
thanks
Andy

If pics are out of the question, what else is unacceptable? More importantly, what do you want to work with? Were you wanting logic circuits, mechanical relays? Are you against all microcontroller solutions, or just pics? Give us a little more to go on.

 

Why are PLCs out of the question?
A few questions...
Do you have a circuit of your control system?
And do the various switches have secondary contacts?
Is the safety string 120VAC or is that what is switched by it?
You may have to set up a circuit like used in quiz shows that is triggered with an opto coupler across each switch that lights a lamp and disables all other lamps. I assume the switches are closed normally so when one opens, it turns on the opto LED (via suitable resistor or whatever).
Something like this with the opto outputs as the trigger switches.
https://electronicsclub.info/p_quiz.htm
There are many more but this is just the first I found.
A lot will depend on just what your circuit actually is so more detail is needed.

I found this simple circuit.
I think the SCR gates need a pull down resistor but it may do, once again, use the opto outputs in place of the pushbuttons.

 

I'm looking to build a circuit monitor for use as a fault finder to see what switch drops first in the series before the entire line shuts down.
Explanation I have an escalator that keeps shutting down randomly and the safety string resets and goes back to ready to run mode.
I need to hook up a box that will monitor 8-12 points on the safety string terminal to see which switch is getting tripped first. But each terminal getting monitored can not jump to the next one basically jumping out my safety string. I hope this makes sense. I saw the plc guys talk about a first out annunciator but plc's are out of the question.
voltage is 120 ac
thanks
Andy

I have attached a simple circuit that should do what you want – for simplicity I have only shown the circuit 3 monitoring points (but it is a simple matter to extend the circuit to 8, 12 or any other number).

Circuit operation
Each of the 120Vac monitored points (A, B, C) is fed into a 120Vac relay (RL1, RL2, RL3), each of these relays is driving a corresponding12V relay coil (RL1A, RL2A, RL3A) via normally open contacts.

Each of the 12V relays is configured such that should it be energised it is latched (via a set of its own contacts), additionally the 12V supply to all the 120Vac relay contacts is cut.

So let’s assume the power to RL1 trips out first – contacts of RL1 will now close energising RL1A (illuminating the lamp to show which relay has energised). RL1A will latch and disconnect the common 12V supply to the other 120Vac relay contacts. Should any other 120Vac drop out, there is no longer a 12V supply to energise its associated relay. The diodes D1 –D3 are required to stop the energised relay back-feeding the 120Vac relay contacts.

One potential issue with the circuit is that should the ac supply points drop out in quick succession (before the first relay has cut the common 12V to the ac relay contacts) – then the circuit may indicate two or more supplies as being the first to trip. But given relay operational switching speeds, two circuits would have to trip within less than 100ms for this to occur.

In operation the 120Vac power sources must be present before the 12V low voltage circuit is energised, otherwise all the 12V relays will latch on initial power up.

 

There is an error in my circuit, each of the 120Vac relay contacts should be normally closed (not normally open).

 

So on a fault condition, the 120VAC relay contact closes?
If that is the case, just wire them directly to the "pushbutton" places in the circuit of post #3. Then add more stages as required.
You do not need all the 12V relay circuitry.

 

Why are PLCs out of the question?
A few questions...
Do you have a circuit of your control system?
And do the various switches have secondary contacts?
Is the safety string 120VAC or is that what is switched by it?
You may have to set up a circuit like used in quiz shows that is triggered with an opto coupler across each switch that lights a lamp and disables all other lamps. I assume the switches are closed normally so when one opens, it turns on the opto LED (via suitable resistor or whatever).
Something like this with the opto outputs as the trigger switches.
https://electronicsclub.info/p_quiz.htm
There are many more but this is just the first I found.
A lot will depend on just what your circuit actually is so more detail is needed.

I found this simple circuit.
I think the SCR gates need a pull down resistor but it may do, once again, use the opto outputs in place of the pushbuttons.
View attachment 149483

I want something cheap and I don’t see plc being cheap.

 

I want something cheap and I don’t see plc being cheap.

If you're willing/able to do a little programming, microcontrollers (Arduino, etc being the easy starting points) are very cheap, and relays or opto-isolators that you'd need for input conditioning are reasonable as well.

There would be far fewer parts, and lower cost, but the tradeoff is the need for programming.

 

Here's my take on a reasonably simple circuit using SCRs to control LEDs.
When a safety switch opens, all the lights above (to the right of) the open switch will light.
The LEDs stay lit until the battery power is removed, even if the switch recloses.
The circuit is powered by a 9V battery and uses no battery power until the LEDs light.

 

The circuit above may work ok but I think the "quiz" type of circuit that only allows the first one to light will be a better way to go as it will disable all the others so you can see where the first fault cones from.

 

The circuit above may work ok but I think the "quiz" type of circuit that only allows the first one to light will be a better way to go as it will disable all the others so you can see where the first fault cones from.

You only need that if there's more than one fault.

 

Can you just jumper each switch one at a time and let it run for a while.
To see if that switch is causing it.
If its not a safety issue, if it is try to work around it.

You could also put a meter with min max memory across each one to see if it's the one.

Cheep - - - >> Pound on the switches one at a time .

 

Here's my take on a reasonably simple circuit using SCRs to control LEDs.
When a safety switch opens, all the lights above (to the right of) the open switch will light.
The LEDs stay lit until the battery power is removed.
The circuit is powered by a 9V battery and takes no power until the LEDs light.

View attachment 149535

That is an elegant solution, I doubt there is more than one fault in the system.

From the symptoms reported in the original post, one of the circuit’s safety switches is tripping, but then closing and resetting the system (such that the suspect switch cannot be identified).

What the original post did not make clear is that the safety trip switches are wired in series with one another.

If I were the original poster – I would be writing to thank you for the circuit.

 

Years ago I worked with the design of several large control panels having multiple permissive loops. I used commercial type first out fault detectors, I can't recall who made those things but want to say each 1/4 DIN unit monitored 8 channels. It was important to know which fault happened first when a shutdown occurred. .They had 3 LEDs per channel, red, green and amber and logically behaved like this little demo I found online. I cannot for the life of me remember who made the things. Anyway, there are off the shelf solutions or roll your own based on your needs.

<EDIT>
I remembered who made them. The PDF can be seen here.
</EDIT>

For my applications at the time it was important to know which of 16 fault channels triggered the shutdown. Off the shelf was convenient as a few spares were aleays on the shelf if needed.

Ron

 

So here's a variation of the previous circuit that only lights one LED of the switch that opens.
If that closes and a second switch opens, than that LED will light also, but the sequence of which opened first will not be shown.
Again the LED(s) say on until the 9V battery is switched off.

Edit: I found a problem during simulations with this circuit where the SCR's momentarily turned on from the Vdt transient when a switch opens, causing a large transient current through the LEDs.
I added an RC supressor across each SCR circuit which eliminated the problem (at least in the simulation).

 

For cheap PLC s consider "Automation Direct", which offers a few of them for about $100, and the programming software is free. Then which ever fault signal arrives first can latch on it's output and lock out all of the others. Another option is to add a digital input card to an older personal computer and have a program logging the exact time of each input transition. That is probably the cheapest way to go, because older computers are often free.