Connecting two op amp outputs?

[brightnight1]

I'm making a design from a schematic that conditions a signal from a PIR circuit and goes to a microcontroller to run code when motion is sensed. At the end of the design before the signal goes to the microcontroller, there are two op amps that are used as a comparator, with both op amp outputs tied to a NAND gate.

The schematic says the NAND gate is optional. If I can build this using less components then I won't use the NAND gate. However, I don't know what to do with the outputs from the op amp. I assume I can't just tie them together and put them into the microcontroller? Any advice appreicated. Thanks in advance.

 

[KlausST]

Hi,

Each Opamp isused as comparator. You may treat the outputs as digutal outputs.
The combination with the NAND creates a window comparator.

Maybe you could use two diodes plus a pullup resistor instead of the NAND, but this increases part count.

Klaus

 

[BradtheRad]

It's best not to exceed the spec amount of Amperes, either source or sink, at the output of the op amp. For an ordinary device this is a few mA. So install resistors for safety, say 2 to 5 k ohms.

 

[brightnight1]

Thank-you for the advice! I might just try the NAND gate, shouldn't add that much cost, current draw, etc. Two more questions:

1) If I put a NAND gate chip on my board that has 4 NAND gates but will only be using 1 gate, if I want to save power, should I ground the rest of the unused NAND gate pins or is that a bad idea?

2) The schematic in my first post also suggests a Diode (D1) between the first and second stage. I'm using surface mount parts but couldn't find a ceramic single diode in a 0805 package on Digikey. There are so many choices...RF diodes, zener diodes, etc. I *think* I just want a "normal" ceramic diode in an 0805 package but there are so many choices I'm not sure what to choose. Any advice?

 

[BradtheRad]

If you work at it you might obtain a Nand gate from an op amp (if you have an unused one on your IC. Create a summing amplifier, adjust things so it changes state at the correct combination of input levels, same as a Nand gate does.

Transistors and resistors can be made into logic gates. Examples:

https://www.4qdtec.com/dlc.html

 

[KlausST]

Hi,

1) If I put a NAND gate chip on my board that has 4 NAND gates but will only be using 1 gate, if I want to save power, should I ground the rest of the unused NAND gate pins or is that a bad idea?

You must connect the unused intputs to valid logic levels. Never leave (unused) inputs floating.
To which level..depends on the desired output state. In your case I see no preferred level.
..
But why not use a single gate logic IC?

2) The schematic in my first post also suggests a Diode (D1) between the first and second stage.

(It has nothing to do with second stage)
No need for "ceramics"
Use a standard signal diode, No RF, no zener, no power rectifier. No schottky needed.
If you look for a size similar to 0805, then look for SOD110.
Use a cheap one. With similar specifications like 1N4148.

Klaus

 

[FvM]

Many slow and medium speed comparators expose open drain outputs. In this and only in this case, direct parallel connection makes sense.

 

[brightnight1]

Many slow and medium speed comparators expose open drain outputs. In this and only in this case, direct parallel connection makes sense.

1) Just to be clear on your wording, you are saying I can combine both outputs and don't need a NAND gate?

At the time I hadn't found a single logic gate IC that did what I wanted, but after more searching I found one that meets my specs

https://www.digikey.com/product-detail/en/texas-instruments/SN74LV1T00DCKR/296-40544-1-ND/5178421

I'm learning a lot on the fly about PCB design, currently using Eagle to make some boards, and really appreciate all the advice on this forum. Does anyone have recommended YouTube video series, books, etc, on the best practices for PCB design? I can make a basic design but am probably missing basics on ground planes, etc.

 

[FvM]

Comparators with open drain output that can be combined without logic gate are e.g. TLV170x, TLC393.