Assistance with Arduino Inputs

Question

Hi, first time on the forums. Sorry if this has been answered already, but my search-fu isn't returning anything helpful.

Here's my project: I make busy lights.

I have my arduino all coded and happy so that when a momentary switch is pressed (built in pull-up resistor), a relay is turned on and a series of LED lights make a very pretty lighting animation until the momentary switch is let go and the entire board goes silent. It's perfect.

Now comes the challenge. I want the arduino to be "triggered" by an external event.

I've taken apart my wireless headset, desoldered one of the LEDs and extended those leads to my breadboard. So now, when I pick up my phone with the headset, I've got an LED that lights up on my breadboard. Nice!

Now my question, how to connect the two? I want the LED light from the headset to trigger the arduino just like the momentary switch does now.

I've tried using digitalRead on Pin9 (doesn't change the value, shows "0")

I've tried using analogRead on Pin9(doesn't change the value reported to the Arduino)

I feel like I'm missing something simple, a way for the arduino to detect voltage from an outside source (the LED reports either zero voltage (headset is not on) or 2 volts (headset is on, LED is lit up).

If this isn't enough information, I can prototype everything on Fritzing and post the file, or I can post my arduino code, but I have a feeling someone is going to say, "use the BLAH" and maybe point me to a link that I can take and run with.

Sorry for the wall of text, appreciate those who read till the end.

Thanks,

Aaron

bobcroft · Accepted Answer

Aaron, Have you correctly configured your Arduino I/O as inputs? Have you tried setting the internal pull up on the input pin? the output to the LED on the headset may be open collector. That would need a pull resistor to drive the Arduino input high. A 2 volt input may be close to the lower threshold for a '1' (high) if the Arduino is powered at 5 volt. I am not certain of the threshold values. Have you connected the head set ground to Arduino ground? If you have tried the analog input and all is correctly configured I would expect an analog value of about 410 for 2 volt, (2/5 * 1024). This would work even if the voltage was below the threshold voltage for digital inputs because you could use any analog value, say > 100, to turn on your LED display. If you want to use the analog input you could simulate the input from your head set with a potentiometer to test the trip point. Don't despair what you want to do can most definitely be done. With inputs the golden rule is not too little and definitely not too much voltage. Try my suggestions and come back to us if you need more help. Bob

gihu · Answer

Hi Aaron, I think that the atmega8 in the arduino UNO, needs at least 3Vdc for a digital '1', even digital '0' goes from 0volts to 0.2*Vcc, in the UNO Vcc is 5volts, so 2volts is still a digital '0'. Probably you could use a optocoupler, applying the voltage to LED of the optocuopler, then the digital with a pull-down resistor,the emiter connected to the digital input and the collector to VCC, so when you get 0 volts from the headset you will have a digital '0' in the arduino, and when you get 2volts in the headset, you will have a digital '1'. Hope that helps, Miguel

Robert Peter Oakes · Answer

With a 5V uController, 2V may not quite be enough to allow the Arduino to read a one, using one of the alalogue inputs should have read something though, you have got a common ground right ?, connecting the low side (Volts wise) of the LED to the GND of the Arduino Another option is to power a transistor base, say a 2n2222 or similar to drive the input to ground and also allow it to go to the full voltage of the pullup inside the micro Regards Peter

Assistance with Arduino Inputs

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