does anyone still chat?

@UbuntuUser can i see the quection page?

Hi all - I'm new to circuit design, but not programming. I'm trying to read from an Infrared detector (sparkfun.com/products/241) -- datasheet here: sparkfun.com/datasheets/Components/LTR-301.pdf

Wiring Schematic: reconnsworld.com/ir_ultrasonic_basicirdetectemit.html

Would it be possible to use the digital GPIO to monitor the output of that?

Feels like I need to convert Analog -> Digital, but I read something that lead me to believe that I might not need additional circuitry.

Ideally, I set this up in such a way that it's a discrete 0 or 1, and then use an interrupt when the value changes (as opposed to polling)

Howdy

connect the phototransistor (like shown in the schematics) via a resistor to 3.3V not 5V

that way you should be able to directly connect it to the Pi's GPIO pin (in the schematics labeled "output")

@Ghanima so -- 3.3v+ -> resistor -> phototransistor -> output to gpio?

1k ohm?

it reads 10k

no need to significantly change that

Ah, ok

So, just use a 3.3v source instead of 5v?

What does that achieve?

drive the LED like shown in the schematics but the transistor from 3.3V

reason is this:

the maximum input voltage of the Pi's GPIO pins is 3.3V not 5V

so you want to limit the upper voltage level to 3.3V

Makes sense. And will the output from the phototransistor be discrete? (0 or 1) ?

it sure is possible to have an output that is inbetween high and low

(0v or 3.3v)?

so it depends a little bit on the rest of your setup

i.e. have the phototransistor properly illuminated by the LED to have the transistor properly saturated

Here's the actualy application - this is a racetrack "finish gate". I need to know when a car is passing over the phototransistor, and throw an interrupt when it happens.

hmm

Feel free to suggest a different approach.

(including entirely different hardware.)

In the end, I need accurate timing.

the thing with this setup is that it is fairly sensitive to ambient light

My old circuit (using a netduino) polled all 4 lanes. It was two slow (3.153s and 3.157s could be a tie.)

as the gap between the sender (LED) and the detector (phototransistor) gets larger your more prone to get trouble with ambient light

how did you read the detector with the *duino ?

Analog input

(hang on, github link incoming)

so you actually have a setup of the the LED / transistor pair up and running

... and with all that being attached to an analog input you could actually test its output levels for both cases

github.com/lcrumbling/derby-racing/blob/master/RaceTimer/…

It was completely analog with that code ... fluctuated all over the place between 80 and 1200 or so.

@LynnCrumbling btw: it doesn't need to be exactly 0 and 3.3 V

I think a difference of 500 was my magic number.

see here: raspberrypi.stackexchange.com/a/40404/19949

lower than 0.5 V is likely low and higher than 2 V is likely high

@LynnCrumbling but you have a number for "no car" and for "car" and you could calculate the voltage from that

Can I still use an interrupt, and set a threshold for the interrupt to fire only when it crosses that number?

that I do not know for sure, maybe they know better there: arduino.stackexchange.com

Ah, ok.

Is there an "easy" way to get 5v -> 3.3v?

(My power source for all of that is 5v atm)

hmm, if you use a Pi, you could use the 3.3V from the GPIO connector for things like this transistor here

It has its own 5v power supply to power leds + phototransistors atm