does anyone know the voltage across an infrared led when the remote with a 3V battery is sending?
I'm trying to figure out, why a remote stopped working and how to fix it. It's a newly bought remote (for Fatdock Blu). It stopped working two days after receiving it.Initially it would work from about 30cm distance to the dock, the next day, it would only work from 10-15cm distance. The day after, the dock didn't respond at all to the remote.
Since I've imported the dock, getting a replacement will be difficult.
Steps taken so far:
- A digital camera didn't see any light flashes from the remote, when buttons are pressed.
- Disassembled and reassembled the remote.
- Replaced battery. Checked the batteries with a dmm. One reads 3.2V, the other 3.3V.
- With the dmm, I've also checked most of the curcuitboard, by bridging the sections where the buttons are. The voltage reads around 3.3V across any of the buttons, same as supplied by the battery.
At this point, when the voltage across a button circuit is 3.3V, the led is still not working.
- Oddly, the voltage measured at the two arms of the led, when a button is pressed, measures only 0.2V.
There is a voltage drop across a LED, and for IR LEDs the drop while turned on typically ranges from 1.4 to 1.9 volts. In a remote, while sending an IR signal, the LED is actually off most of the time. That's because the IR signal is broken up into periods of On and Off where the On duration is typically 0.5 milliseconds. The Off duration is usually 3 times that. But even during an On period, the IR light is flashing on and off rapidly-- usually about 8 microseconds on and 18uS off, if it is the typical 38KHz modulation. So the LED is transmitting IR typically only about 10% of the time. A voltage of 0.2 volts DC -- assuming your voltmeter is averaging-- is about right. If you have a voltmeter that is capable of providing true AC measurements, I'd expect something like 1.5 volts AC.
Hey, thanks for the details.
I've used a dmm with a max function (it just displays the highest reading) and measured the voltage across the IR LED again, while pressing multiple buttons. The highest voltage measured was 0.58V. If it should be around 1.4-1.9V for a functioning remote, that could possibly explain the short range for this remote. However, how can that be fixed? Is the low voltage across the LED a sign, that the LED is broken?
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