Priority-based series-connected switches decoding with ADC?

[ricko_uk]

Hi,
with reference to the attached schematic, there are three switches which are normally closed for the power stage of this system to work. The voltage can be as high as 24V and the micro is 3V3. The 100K and 10K voltage divider resistors are high vlaue to avoid turning on the solid state relay. The solid state relay's LED trigger current is shown to be 3mA so I assume a current of 0.5mA is low enough to guarantee the OFF state. SSR datasheet is this: https://www.mouser.co.uk/datasheet/2/408/TLP241A_datasheet_en_20200217-1075431.pdf


But for safety reasons and also to provide user feedback, I need to monitor the status of each switch from a micro. Because they are in series - unless I am missing something - it is not possible to monitor every combination. First question is, is that correct? Or perhaps by adding high value resistors across the switches (as shown) that can be achieved?

But by assigning priority whereby the switches have decreasing priority from left to right (with reference to the schematic) so that if a higher priority switch is open then the status of the other ones does not matter, then an acceptable compromise can be achieved.

My second question, question is about resistor values with the compromise above. To spread the reading of the various combinations relatively wide apart for clear reading by the ADC even with large tolerances of the resistors, how would you choose the resistor values? Perhaps some exponential relationship?

thank you

[Someone]

The 100K and 10K voltage divider resistors are high vlaue to avoid turning on the optocoupler. The opto's minimum current to turn on the opto is 0.5mA.

Is that a specified minimum turn on current in the data sheet? Most opto couplers are highly variable over batch/life/drive conditions.

[ricko_uk]

Thank you Someone,

it is actually a solid state relay (I corrected the wording in the OP). In the datasheet's ( https://www.mouser.co.uk/datasheet/2/408/TLP241A_datasheet_en_20200217-1075431.pdf ) first page it says trigger current is 3mA so my assumption is that 0.5mA is low enough to be off.

I cannot see any other infos about it. Is my assumption correct?

Thank you

[Someone]

I cannot see any other infos about it. Is my assumption correct?

Back to school for you:

IF smaller than minimum IFC is required in the design to ensure that the relay operates [into the OFF-State]

Which for those parts is lower than your (best case!) bleed. But those parameters depend on the load too, optos are a long way from "ideal" or easy to use.

[ricko_uk]

Got it now. Thank you

[ricko_uk]

Any suggestions on how to do it? Increasing the resistances to much higher values might make it too prone to noise. Any ideas/suggestions on how to implement the monitoring fo all switches as mentioned in the OP?

One way would be to use op-amps (high input impedance). Any other options?

Thank you

[Terry Bites]

I think a redesign is in order. What does D8 do?
The SSR is on or it isn't so you need a simple logical decode of the switch settings and probably an NPN to drive its LED.
"Isn't there a chip for that?" SNx4HC148 CD4532 etc?

[free_electron]

use weighted resistor in a 1-2-4 ratio. then you can simply feed this into an adc.

make a simply 0.5 mA current source that drives current into 1k-2k-4k (4k02)  the switches short one resistor each.

so you get 3 bits.
0.5 volt = 1k open
1 volt = 2k open
1.5 volt = 1 and 2 open
2 volt 3 open
2.5 volt ... you get the drill.

[T3sl4co1l]

Easy enough to increase Ioff by putting a shunt resistor across the LED.

Tim

[ricko_uk]

Thank you all!

@Tim,
yes, simple solution for the I 

@free_electron,
yes with those numbers it works nicely

@TerryBites,
yes, but I was trying to keep as few components as possible at the front end to decrease potential failure points