Design of Window Comparator Circuit w/ Mutually Exclusive LED Indicators

Question

The functionality I seek is as follows:

All LEDs off: Vsignal < 0.118VDC
LED1 fires: 0.118 < Vsignal < 0.1216
LED2 fires: 0.1362 < Vsignal < 0.146
LED3 fires: 0.151 < Vsignal < 0.1607
LED4 fires: 0.1853 < Vsignal < 0.1999

Between these firing windows, all LEDs should be off. The two ICs are an LM339 and a CD40147BE.

The noise from the signal voltage is <0.0002VDC. The accuracy should be to 0.001VDC.

I need to recalculate the resistor values for ref voltages, but I don't know how to connect multiple comparators together; all examples I've seen have been for a single window, not multiple.

Answer 1

You need to make four window comparitors. Fortunately this is easy with the LM339 as it has open-collector outputs which can be joined together to form a wired AND function.

Figure 1. The internals of an LM339 comparitor. Note the open-collector output.

^{simulate this circuit – Schematic created using CircuitLab}

Figure 2. By wiring the outputs together as shown the LED will turn on when the voltage is inside the window set by VR1 and VR2.

How it works:

• If both comparitor outputs are high their output transistors are off and the collector is floating. R1 will pass current to LED D1.
• If the input voltage goes above the upper window voltage set by VR1 then CMP1 will turn off and short out LED D1.
• If the input voltage goes below the lower window voltage set by VR2 then CMP2 will turn off and short out LED D1.

You need four of these circuits. I recommend 10-turn pots rather than a precision resistor divider chain. Add one more pot for calibration and it should be a simple job to set everything up.

Don't skip the decoupling capacitors on each chip.

Answer 2

As Transistor has answered, your best bet is to make 4 independent window conparators. If you are determined to do things the hard way, the following

^{simulate this circuit – Schematic created using CircuitLab}

actually makes use of the 74147 as a priority encoder. Unfortunately, what you want is a dot-mode bar graph, and the 147 doesn't do that. You can, however, add a 2:4 demultiplexer (I've used half the inputs of a 3:8, just because it's convenient) to convert the encoded address to drive the LEDs.

Additionally, LM339s have as much as 3 mV offsets, and although you expect the 4 on a chip to track with temperature, the unit-to-unit offset is not correlated on a chip. This means that your LEDs 2 and 3, which have windows less than 10 mV wide, will potentially have 30% errors on each end of the windows. To get around this, I've added a front end which converts (0.118 to 0.1999) to (0.1 to 3.1) volts. The two op amps should be rail-to-rail output units capable of running at 5 volts. The choice of 3.1 volts as an upper limit comes from the LM339 common mode input limits of 0 to 3.5 volts for a 5 volt supply.

Also note that the accuracy of the windows depends critically on the accuracy of the 5 volt supply. Any variations will directly impact the thresholds of the windows.

Finally, my suggested circuit needs, and does not have, hysteresis. However, I don't know your threshold accuracy requirement nor did you specify the amount of noise which may be found on the input. Without these, there is no way to specify what's needed.

Answer 3

Use the LM339 quad comparator IC to form a bargraph display. Alternately use an LM3194 bargraph IC. The LM3194 will allow bargraph (all lit) or dot mode (only 1 led lit at any time).

This can be achieved with the LM339 by using diodes at each output to short out the previous LED, however unless you fully know what you are doing, you could end up "blowing" the chip altogether