TSOP1738 - Test Circuit, led is perpetually lit

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I've recently returned to this hobby, well hobby for me, and I am taking another crack at learning IR. All the way from the basics, I've forgotten quite a bit it seems. Making a simple IR detector circuit seemed like a good place to start. I've found a few on google and on the Vishay TSOP datasheet I found this one; attached, that I build on a breadboard.

 

Thing is, it seems to work, but nowhere near as good as some of the examples I've seen on youtube and google. It's is supposed to work like this, you point any IR remote at it, push a button and the leds lites up. I've build mine, but the leds is perpetually on, very dimly, and when I aim an IR device at it, the led flickers fast. So it does detect something.

 

Only differences to the circuit I made is that I used a 300 Ohm res. instead of the 330 and 100 instead of 120. Alternative circuits, using transistors and such, give the same result. Led is always on, and flickers when detecting IR. Any idea what could be wrong?

 

 

Attachment(s): 

Code, justify, code - Pitr Dubovich

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Eloque wrote:
Thing is, it seems to work, but nowhere near as good as some of the examples I've seen on youtube and google. It's is supposed to work like this, you point any IR remote at it, push a button and the leds lites up. I've build mine, but the leds is perpetually on, very dimly, and when I aim an IR device at it, the led flickers fast. So it does detect something.

 

The only LEDs I see in the application circuit are the transmitter (which you're not building) and inside the receiver. You can't see the receiver LED. And they're both IR so unless your vision can detect IR, you won't see a flicker. How about posting the circuit you actually built?

 

 

 

 

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Hi, ....that is the circuit I built, or so I thought. I now see it's obvious that I didn't built it that way, staring me in the face. I replaced the Uc component with a led. So it's more like this.

 

The led on the right is a normal red led, no IR. Still, the questions remains.

Code, justify, code - Pitr Dubovich

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Time to study the TSOP datasheet to see what signal should b expected at pin 3 ...

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Oh, I see.

 
Well, as Andy says, RTFDS for the TSOP17 thing.

 

It has an open-collector output. When idling (no IR signal hitting it), the transistor is off and the 10k pulls pin 3 up. This turns the LED on, albeit very weakly. 

 

When you blast the thing with an IR signal, the TSOP decodes it and the output transistor turns on for logic 0. That turns the LED off for the duration of the logic 0.

This is why the LED blinks when you light it up with an appropriate IR source.

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Thanks for the information, I've tried to read the datasheet, http://www.micropik.com/PDF/tsop... , it's also where I got the test circuit. But my knowledge isn't advanced enough for me to figure out what the default state of pin 3 should be. 

 

Also, I was thinking, isn't this the kind of circuit that could benefit from adding a transistor? Have Pin 3 connect to the base and then switching the led on and off? Idealy, I would want the led to flash when IR light hits it, not when it's idle.

Code, justify, code - Pitr Dubovich

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If I wanted to drive an LED with the output of the IR receiver, I would buffer the output (pin 3, with pull-up) with a Schmitt-trigger input inverter (74xx14).

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Since it is open collector, put the indicator LED anode to Vcc. Add a 270 to 330 resistor in series with the cathode of the LED. Other end of resistor to pin 3. 

 

LED will go on every time the output transistor turns on and creates an (effective) short circuit to ground.

 

Jim

 

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

Last Edited: Sun. Jul 19, 2015 - 01:25 AM
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Reversing the Led basicly. That works, thanks.

 

Okay, I now have this part 'working', time to see if I can hook it up to an AVR and do something with the signal.

Code, justify, code - Pitr Dubovich