Making LED Normally OFF?

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lwinicki

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Hi all,

I was wondering if there was a way to make an LED normally off when it is wired to a switch? I have connected an LED to an IR receiver module, and when I apply the power source to the circuit, the LED is on and flashes when the IR receiver module receives IR signals.

Is there a way that I can make it so that the LED is off, but turns on when the IR receiver module receives IR signals?

Thanks!
 
use an pnp transistor at the ouput of the IR reciver circuit

could someone confurm this
 
What is the nature of the output from the IR RX?

Is it a 5V logic output?

Does the data sheet say how much current the output pin can source when it is high, and how much current it can sink when it is low?

How do you have the LED wired now? Anode toward the output pin; cathode to ground?

Or anode to +5V; cathode to the output pin?

You do have current limiting resistor in-series with the LED?
 
It's this receiver: here

The LED is wired with the cathode to ground and anode toward the output pin.
I wired a resistor to step up the voltage to the LED, because it was barely even on before I added the resistor
 
lwinicki said:
when I apply the power source to the circuit, the LED is on and flashes when the IR receiver module receives IR signals.
Click to expand...

That means IR module outputs goes low when it receive the signal interrupting the normally +ve output. All you need a PNP general purpose transistor wired as follows:

Connect a 5K resistor to the base and goes to the IR output terminal.
Emitter goes to +ve supply.
Connect LED through a resistor(1K) from collector to GND.

You'll get it blinking only when it receives a signal.
 

I wired an NPN transistor (it's pretty much the same as a PNP, right?) the way you explained, and yes, I only get the LED to blink when it receives a signal, but I was wondering if there was a way to get the LED to stay off, and to blink on when it receives a signal, because right now, it's staying on and blinking off when it receives signal
 
Hi,

You need a PNP transistor like BC556/557,2N3906 to get it work. Try increasing the 'R' value in fig if the LED doesn't turn off at no-signal conditions. R goes to IR module output.

 
Does the IR module have a push-pull output?

You could connect the LED between the output and the +V.
 
The IR receiver is from RadioShack. It is surplus without a manufacturer's name and without a datasheet or any spec's.
Most IR receiver ICs have a very low pullup current (62.5uA) and a max allowed output low current of 5mA.
 
As already pointed out, most IR receivers have a weak pull-up/NPN pull-down.
Assuming a +5 Vcc, and keeping the LED current < 5mA, you could try:
 
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