Schmitt Trigger or other means to trigger input

I have a device that normally uses a contact closure (signal to ground) input on the microprocessor I/O. When the input is open it measures +3.29vdc.

I have an EMA OB1202 photo sensor that I would like to trigger the input with. The signal wire on the photo sensor fluctuates between +0.7vdc and +0.5vdc, not enough change to directly trigger the input. Also, when I have the trigger attached to the input and measure it goes from about 3.2vdc to 1.2vdc, again not enough to trigger the input.

So in researching, and I don’t really know what I’m doing, it looked like a Schmitt trigger (inverting or non inverting) might be what I need. Or I’m assuming there’s one that just takes the signal to zero.

I did purchase a few of the NC7WV14P6XCT-ND to try and test with, however, even using a stereo scope these are crazy small and tough to soldier.

So I thought I’d come to the experts for some thoughts on how I might approach this.

Thank you,


As I’m researching this more, I am wondering if I need a schmitt trigger or if a basic comparator would work? Maybe something like this? MCP6546-I/P-ND

Welcome to the TechForum! I am looking into this to see what I can find out for you.

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Hi bobtx,

I think we need a little more info on the parts you have. Does your microprocessor have pull-ups enabled? What microprocessor are you using, and at what I/O voltage does it operate?

Do you have any data on your EMA OB1202? I tried to find a datasheet for it, but I could not find the part with a Google search. Using a meter to measure voltages does not give one the full picture. We need to know what the internal structure is on the respective input and output pins of each part to know how to make the two talk to each other.

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David, I appreciate the help. This is using a Microchip PIC18F6520 and it does have 10k pullup resistors in place. I don’t have the OB1202 specs with me. We have an ice storm in TX so I may not be able to get to the shop till tomorrow. I’ve attached 2 photos from the info in the catalog.

One interesting note is that we have another board that uses the same pic and pullup resistors but the whole board is smaller and the sensor works on this one. I’m guessing the voltage drop must almost be enough to trigger the input but due to other noise or signal loss, it’s not quite enough on this board.

EMA Photosensor1
EMA Photosensor2

Without the data sheet I can’t be certain, but it looks like a fairly ordinary open collector output on/off system. Similar to the common hall-effect digital switch components.

If that’s correct then the connections are:
Brown to 3.3Vdc
Blue to common
Black to micro plus an external pull-up resistor (often needs to be much stiffer than the built-in pull-ups of a micro)

For the NPN version a very common value is to use a 10K resistor for the pull-up to 3.3Vdc.

In most PIC micros the built-in pull-ups are 100K or higher so won’t work for many open collector sensors.


I feel for you with the ice storm. I’ve seen a few of them over the years and they are definitely nothing to be trifled with.

Implied in Paul’s reply is that the PIC is running on 3.3V. The PIC18F6520 requires at least 4.2V to operate, whereas the PIC18LF6520 can operate down to 2.0V.

Assuming you are using the “LF” version and that you are powering it with 3.3V, then I believe his response is spot on. It is an NPN open-collector arrangement, so the only other thing I could suggest is to maybe play with the pull-up values a bit. If 10K is still a little weak for some reason, it could probably be dropped down to 4.7K or possibly as low as 1K, though that’s typically lower than necessary.


Thank you, I will try this out and let you know.

Thank you, I will test this out when I can get back to the shop.

Nice catch on the power requirement. I haven’t worked with PIC18’s in many years and forgot that the old lines of “F” series parts where effectively 5V only.

With the modern PIC16’s I’ve used the past few years, the “F” series work at 2.3V/2.5V to 5.5V and the “LF” series work at 1.8V/2.5V to 3.6V, showing the distinct trend of 3.3V being the new common power source low cost micro-controllers.


That’s interesting. This board according to the BOM is the PIC18F6520. The other board that the sensor works with is using the PIC18LF6520 pic. I’ll need to see if in the future we can switch to the LF chip on both units.

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I honestly hadn’t noticed this difference before but it makes sense.