Simplifying many window comparators

Question

I have 8 thermistors and I must make sure that each of them is inside a temperature window. They all have the same window and I don't care which or how many are inside the valid range, I just need to know if all of them are inside the (same) window or not. This is to be a hardware-only solution, so software-sequencing of ADC reads is out of the question.

My best solution currently is to use a bunch of comparator ICs and implement a separate window-comparator for each thermistor. To optimize the solution, I can use a number of quad comparators, each with an open-drain output so that I can connect them all up. Still, in essence it is the same circuit. The reference/trigger voltages I can make once, buffer, then supply to all comparators.

I do feel stupid for just simply throwing a bunch of comparators at the problem. I am not sure if there isn't any better way, I'm mostly trying to optimize board space. Is there some creative way you know? For example, select min/max voltages of all thermistors and use a single window comparator (EDIT: two comparators ofc), which IMHO would lead to a larger solution and is thus not a good answer, I'm just mentioning this for inspiration.

EDIT: I know that a software-based solution would be the best. That is why I mentioned it right in the beginning and upfront to prevent everybody suggesting it. The reason the problem is defined this way is because this is a safety circuit, and specifications require me to implement a hardware-only solution in addition to a software-monitor. So the software-based solution is already there, I "just" need to find the best way to implement the hardware-based one.

Answer 1

I think Brian's non-SW answer is the best but here's a purely analogue solution. Use a circuit that inherently chooses the highest voltage input (from several) and puts that to the output. Ditto the circuit for the lowest voltage.

Consider the precision rectifier: -

It produces an output voltage that follows the input voltage throughout its positive range. Now consider what happens when you have two inputs like this: -

Whichever of V1 and V2 is highest will win the battle to drive the output line. Step and repeat for 8 inputs then do the same with reverse connected diodes and you have a solution that generates two output voltages that represent the highest and the lowest levels from several different voltage sources.

I'm not 100% but I think you can reuse the op-amps for the lowest voltages. Maybe I'm wrong on this?

Then use comparators on both lines to determine if one from the eight might be out of range.

Answer 2

One window comparator, a 3 bit counter, and an 8:1 analog mux to connect one thermistor to the comparator.

If all you need to know is that they are all fine; that's 3 chips, job done. (74HC163, 74HC4051, comparator, plus something like a 555 to clock it).

As Andy says, the MUX (e.g. 74HC4051) has fairly low ON resistance, so each thermistor connects to one analog input, and a lone resistor on the other side provides a voltage divider. If the thermistors are all connected to GND, the resistor goes to 5V.

^{simulate this circuit – Schematic created using CircuitLab}

If you need to record/display WHICH ones are out of spec, you start adding circuitry such as a 3-8 line decoder (one more IC, 74HC138) to drive eight LEDs; the guilty parties will take turns blinking on.

If you need more than that, go for the MCU and software.

Answer 3

Suggestions to improve the multiplexed solutions for use as a safety circuit: Have one of the multiplexer inputs preset to an input that is known outside the safe window (or even two inputs, one below one above). Check for there being an "outside window" response from the comparator when the relevant inputs are selected on the multiplexer.

Best drive the multiplexer with a (synchronous is best) counter that has one bit more resolution than you need (and is clocked twice as fast): The least significant counter bit makes a great trigger signal so you can load the state of the comparator into an edge triggered D flipflop after it had plenty time to settle - and this trigger signal can also be safely gated by logic depending on the counter state to different D flipflops (eg one for each thermistor, or one for the thermistors and one for the self-test channels I suggested above).

To be even more safe, duplicate the whole circuit. While the thermistors and associated inputs are still theoretically a single point of failure, the fact you are using a window comparator should prevent a complete short or open on a thermocouple from being misread as a false OK (bias accordingly).

Answer 4

Here is a 6 x window comparator IC and here is a Quad window comparator IC.
No 8's, alas.

You could meet the need with 4 x ye olde LM339 quad open drain comparators.
These can be had in
1.7mm x 1.7mm (3mm x 3mm including pads) QFN,
or 6.4 x 5mm TSSOP

Datasheet here

You'd also need upper and lower window reference voltages (3 resistors) and a single output pullup.

Answer 5

U2+ will be max(inputs) - diode drop.

U1- will be min(inputs) + diode drop.

R3,4,5 sets thresholds. If too low on any input, U1- drops below threshold on U1+, U1out goes high. If too high on any input, U2+ rises above threshold. U2out goes high.

Not sure what you want to actually do when either condition occurs, but drive a transistor/FET/SCR come to mind.

Answer 6

You can buy window comparator ICs to save board space. For example, TPS3700, LTC1042, LMV7231 (hex), or MAX969 (quad).

Answer 7

I'm mostly trying to optimize board space.

I do feel stupid for just simply throwing a bunch of comparators at the problem.

The obvious solution is to use a microcontroller with at least 8 A/D inputs. The rest is firmware.

Even a slow micro can keep up with 8 thermistors. You can trivially create a system that responds in under a millisecond, for example. That should be so much faster than the response time of the thermistors as to be effectively instantaneous.

You say that sequencing A/D reads is "out of the question", but gave no justification for this arbitrary requirement. A small microcontroller will take less board space than a bunch of comparators. Since optimizing board space is mostly what you are aiming for, this is something you need to explore seriously.