Monitoring 3 different voltages for simple On/Off

Hi All - I have a project for which I'm toggling 3 latching single coil relays at the same time and I want a simple single input signal to validate voltage is available at the all the relay outputs. Other specifics: Voltages: +5vdc, +12vdc and -12vdc and I want the them to be opto-isolated. The on/off frequency can be hours to weeks, so speed is not a factor. And FWIW, I don't care to know if the voltage is wrong, or which relay might not bet set or if it's one, two or all three are dead, I just need a simple On/Off.

The attachment is a schematic of the simplest arrangement I've come up with but I'll admit this is a little out of my realm. The design is a basic extension of the numerous simple optoisolator breakout boards on the market, I just simply added 2 more stages joining the emitter/collector between each pair and wiring them up to their respective voltages. It works on the breadboard but I'm here to make sure I'm not too far off base.

Q1: Excusing the choice of components, is the logical design ok?
Q2: When it comes to Optos, what's the science in sizing them properly?
Q3: Other options?
Q4: I don't know what I don't know, so what else do I need to know?

And of course feed back on components is welcome too...

Thanks in advance,




Looks good to me, although I am not sure if 3V3 will be enough for 3 opto transistors in series. What is their on Vce?

[Edit] More I think about it the more I think it won't work because the voltage across the 3 opto transistors might be too much to clamp the base of Q6 off. How about opto MOSFETs?

Do you realise you made a 3 input AND gate?

Right, sorry, I checked the LTV826 datasheet and the collector emitter saturation voltage (page 11) is about 0.5V, so 3 of them in series cannot clamp the base of Q6 to anything like 0V, so Q6 will not turn off.

Perry - thank you for brining the "collector emitter saturation voltage" to my attention. I don't know what it is about transistor, but I never seem to get my head wrapped around 'em... LOL. I also have a 5v rail on the microcontroller side of things, and sitting here thinking about it, I changed my IO expander to the PCAL6524 and have some spare ports too. Just hate to add 2 more wires to the connector and harness. I'll put some more thought into it and spend some time on transistor theory.

Good catch on the AND gate, it was exactly what I was going for.