I have a project where I have an IR LED and an IR detector sensing the light. I'm running the output of the IR detector (which outputs a low voltage when IR light is detected) to the reset pin of a 555 timer that is supposed to flash a visible light LED. I attached the circuit I'm using to flash the LED. The only problem I'm having is that the LED is still on when the detector isn't detecting IR light. How can I make it so the LED is off when the detector doesn't sense IR light? Thanks.

edit: The output goes to a 2N3904 with a pull-up resistor to 5V. That goes to the reset pin of the 555 for the LED flasher, and also to both reset pins of two more 555s to create a siren.

 

How do you know it isn't receiving IR radiation? There are many, many sources of IR everywhere.. that's why most televisions use a modulating scheme on their signals.... why not use a couple 555s or a 556 and send a 38kHz, OOK modulated signal to a 38kHz receiver to limit the amount of noise picked up on the receiving end?

 

I should have said this, but I have a 555 set up for the IR LED to oscillate at 36.7kHz (the same frequency the detector is designed for) and I verified the frequency this morning. That, and I cover the detector and the visible light LED stops blinking and has a low glow.

 

...perhaps you should post your receiver schematic so we can see how you are demodulating the signal....

 

Here it is

edit: Forgot to label the emitter of the 2N3904 goes to GND

 

I checked it with the speaker as well, and it works as expected when the detector is sensing IR light, but when it's not detecting IR light, the sound is just white noise. I need this to be quiet and I need the visible light LED to be completely off. So, I think the voltage is not quite being dropped to 0. But, I don't know what to do to fix it.

The detector is the PNA4611M00XD

When I tested the circuit this morning, the output voltage of the detector was 0.488V when it did not sense IR and 0.0446V when it did sense IR light.

 

You need a 10k pullup resistor from the detector output to V+.
Edit: No, I'm wrong. Datasheet shows an internal pullup.
Show the complete circuit if possible.

 

Here it is. Hopefully its large enough to see.

 

...you may need to add a filter to your output to increase noise immunity

 

The detector is the PNA4611M00XD

When I tested the circuit this morning, the output voltage of the detector was 0.488V when it did not sense IR and 0.0446V when it did sense IR light.

Output voltage should be near V+ with no IR, not 0.488. That's not enough to turn on the transistor to pull the reset's low.

Might want to double check the connections to the detector.

On Siren 1 pin 4 is tied to pin 2. Remove that connection, I don't think it's correct

 

Try increasing the base resistor R10 to 10K. There's no point in it being as low as 100 ohms as the transistor is connected and you may find that the sensor can't source the nearly 50mA needed with the 100 ohm one.

 

Why is there ANOTHER thread about this simple circuit?
Why are the errors still there?
Why is this huge schematic pastel with no contrast and with text too small to be seen?

On the other thread I said:
"In Siren 1 you connected pin 4 to pin 2 and pin 6 which is wrong.
In Siren 2 you connected pin 3 to pin 2 and pin 6 which is also wrong."

 

I think the Japanese datasheet is missing important information, maybe because it could not be translated into English properly.

Similar IR receivers from Vishay say in the datasheet "The output voltage should not be hold continuously at a voltage below 3.3V by the external circuit" which the transistor is doing here." The Germans probably meant to say HELD instead of HOLD.

Vishay's datasheet also explains that the AGC reduces its gain if a continuous ultrasonic IR is received, since it is probably interference from a compact fluorescent light bulb. It says:
"The data signal should fullfill the following condition:
• Carrier frequency should be close to center
frequency of the bandpass (e.g. 38kHz).
• Burst length should be 6 cycles/burst or longer.
• After each burst which is between 6 cycles and 70
cycles a gap time of at least 10 cycles is neccessary.
• For each burst which is longer than 1.8ms a
corresponding gap time is necessary at some time in
the data stream. This gap time should have at least
same length as the burst."

 

Can you post a link to that datasheet, Audioguru?

 

Here is the datasheet of a Vishay IR receiver IC:

 

Any idea what kind of effect holding the output voltage below 3.3V would have on the detector?

 

Any idea what kind of effect holding the output voltage below 3.3V would have on the detector?

I don't know but you should do what they say.

 

Why don't you post a list of which of the posted suggestions you have tried so far.

 

I fixed the errors in the siren subcircuit (pin 4 connected to pin 2 on the first 555, pin 3 connected to pin 2 on the second 555). I changed the base resistor to 10k. Today I tested the 2N3904 NPN transistor to see if it still functions properly, and it does. I have everything soldered to a pc board now, but the visible light LED does not light. The IR LED still oscillates at the right frequency, and I took another sensor and set it up on a breadboard and a transistor just how I have it on my pc board, except I use a simple driver instead of 555 timers for a siren and blinking an LED. That circuit operates as intended. It does not make sense to me why the 555 timers do not turn on when the detector sense light. I attached the driver circuit I used to power the LED in the circuit I tested. The LED goes in parallel with the speaker.

 

You said you tested the 2n3904.
Did you test this way?

1. Disconnect the 2n3904 base resistor from the IR detector output and connect it to Gnd, the 555's should run.

2. Then connect the base resistor to V+, the timers should stop.