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Low voltage and dim light from IR LED
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Low voltage and dim light from IR LED

by a2rpriest on Tue Jan 02, 2018 11:40 pm

Hi,

I've been working on building a laser tag set for quite some time now. I am using the Feather M0 Lora as the Arduino. And sending a signal to another device using IRLib2. Using the Super Bright 5mm - IR LED - 940nm https://www.adafruit.com/product/387.

I've got it set up currently with the NPN Bipolar Transistor https://www.adafruit.com/product/756.

My problem is, it is not bright at all!!!!!. (I'm using my iPhone's front facing camera to see the IR light).

I've tested the code, and everything works great. But you have to be really close to the receiver (like 3 feet away max) for it to work.

I've used a multimeter to test the voltage. And using the NPN its around 0.2 volts when the light is triggered (granted, I have to trigger the light repeatedly, because IR signals are very fast). Voltage should be around 3.0 volts since I'm using 3.3volt logic.

So, I tried hooking up two transistors in series. That doubled the voltage to 0.4 volts. But still.. not great for sending signals outside, much less a room.

I've tried tutorials like this: https://backyardbrains.com/experiments/transistorDesign, https://learn.adafruit.com/using-an-infrared-library, https://learn.adafruit.com/ir-sensor

I also tried a MOSFET today, which, with just one was able to output 0.4 volts. But still nowhere near the 3.3volts from the Arduino M0.

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I think it has to do with the PWM or the max current or voltage from the GPIO port. Because when I try connecting the Gate lead of the transistor to +3.3volts, the LED is very bright and multimeter reads ~3.0 volts. So I think the transistor is working fine, and the LED is not burned out.

Could the PWM be going on and off so fast that the transistor doesn't have time to reach peak voltage?

Should I just chain 5 transistors in series to reach 3.3volts to the LED?

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I'll provide pictures from fritzing below.

Thanks for your help!
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a2rpriest
 
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Re: Low voltage and dim light from IR LED

by adafruit_support_bill on Wed Jan 03, 2018 7:07 am

It is not clear from your description where you are measuring the voltage. Please post a photo or diagram showing where you are placing the probes.

Also please post the code you are using to pulse the LEDs.

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Joined: Sat Feb 07, 2009 10:11 am

Re: Low voltage and dim light from IR LED

by a2rpriest on Wed Jan 03, 2018 10:49 am

Attached is a diagram of where I'm connecting each lead of the multimeter. Hope it's clear.

For code I'm using IRLib2 https://github.com/cyborg5/IRLib2/. So I'm importing the library, and then just calling:

Code: Select all | TOGGLE FULL SIZE
mySender.send(NEC, IRCode, 38);


whenever a button is pressed.

Thanks for your help. Let me know if you need more info.
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Fritzing_03.png
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a2rpriest
 
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Re: Low voltage and dim light from IR LED

by adafruit_support_bill on Wed Jan 03, 2018 11:10 am

So you are not just turning the LED on, you are sending a series of pulses. Your multimeter is going to average the voltage over a 1/2 second or so, so it will only be a fraction of signal voltage. If you do a digitalWrite on that pin instead of the mySender.send, you will see the actual voltage of the signal being switched.

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Posts: 64073
Joined: Sat Feb 07, 2009 10:11 am

Please be positive and constructive with your questions and comments.