Maker Pro
Maker Pro

IR LED and Reciever circuit

Kieran P

Apr 12, 2016
4
Joined
Apr 12, 2016
Messages
4
As part of my level 3 electrical BTEC i have to carry out a project which involves designing and producing a small circuit. For my circuit i wanted to create a IR led powered by a a-stable 555 timing circuit to emit IR light at a set frequency. I then produced a receiver circuit which is designed to detect this IR light and then trigger a normal red LED to illuminate. My receiver outputs the 5v supply i give to it and then this output voltage drops to around 0.8v when it detects the IR light from my LED (normally closed receiver). However, this voltage only drops momentarily and then returns to 5v even though my IR LED is still emitting light.
I thought perhaps my receiver was at fault so i tested it by pointing a tv remote at my emitter and the voltage dropped and remained at this lower voltage so i checked the datasheets for my components but have been unable to find anything to explain why this occurs. However, i am not too familiar with all of the technical vocabulary used on the data sheet so i may have missed something.

Below are links to my IR components:
IR LED - http://www.rapidonline.com/electron...al6200-5mm-940nm-ir-transmitter-diode-49-4513
IR Receiver - http://uk.rs-online.com/web/p/ir-receivers/7085115/

Any help would be appreciated
 

hexreader

Apr 21, 2011
135
Joined
Apr 21, 2011
Messages
135
This receiver is not suitable for receiving a constant 38kHz signal.

You will need to modulate the 38kHz (like a TV remote does), or choose a different receiver that can handle a steady 38kHz signal.


Check out page 5 of the datasheet:
 

Kieran P

Apr 12, 2016
4
Joined
Apr 12, 2016
Messages
4
This receiver is not suitable for receiving a constant 38kHz signal.

You will need to modulate the 38kHz (like a TV remote does), or choose a different receiver that can handle a steady 38kHz signal.


Check out page 5 of the datasheet:

Ah i see! I will look into that, thanks for you help
 

Kieran P

Apr 12, 2016
4
Joined
Apr 12, 2016
Messages
4
This receiver is not suitable for receiving a constant 38kHz signal.

You will need to modulate the 38kHz (like a TV remote does), or choose a different receiver that can handle a steady 38kHz signal.


Check out page 5 of the datasheet:
Am i not modulating my output by feeding it from a 555 then? As producing my square wave signal is that not a form of modulation?
 

hexreader

Apr 21, 2011
135
Joined
Apr 21, 2011
Messages
135
Am i not modulating my output by feeding it from a 555 then?
No, you are producing a carrier with no modulation.

As producing my square wave signal is that not a form of modulation?

No, if you want to modulate your 38kHz carrier, you need to use a second 555 (or the other half of a 556) to turn the carrier on and off at a rate of about 200Hz to 2kHz. Your receiver will then strip off the 38kHz modulation and output just the modulation signal of 200Hz to 2kHZ - or whatever modulation you applied.

I like the diagram shown on this web page: http://www.sbprojects.com/knowledge/ir/

There are receivers that work down to DC (no modulation) but I do not know of a part number off-hand
 

hexreader

Apr 21, 2011
135
Joined
Apr 21, 2011
Messages
135
Hmmmm I may have made a mistake....

The only receiver I have that works with DC is Vishay TSOP4838

...But that is what you are using!!! :( :(

Maybe the receiver you have does work down to DC, but then I don't know why it would be causing problems.

Now I am confused too. Sorry...
 

hexreader

Apr 21, 2011
135
Joined
Apr 21, 2011
Messages
135
Perhaps you could put an oscilloscope on the transmitter output to be sure that you are transmitting 38kHz (+- 1kHz)

If transmission frequency is about right, then try blocking the IR light between Tx and Rx with your hand. If the oscilloscope shows a short pulse low at receiver output when you unblock the IR path, then this suggests that your receiver cannot handle steady 38kHz.

Can not think what else to suggest...
 

Kieran P

Apr 12, 2016
4
Joined
Apr 12, 2016
Messages
4
Perhaps you could put an oscilloscope on the transmitter output to be sure that you are transmitting 38kHz (+- 1kHz)

If transmission frequency is about right, then try blocking the IR light between Tx and Rx with your hand. If the oscilloscope shows a short pulse low at receiver output when you unblock the IR path, then this suggests that your receiver cannot handle steady 38kHz.

Can not think what else to suggest...
I connected my output to an oscilloscope and the waveform i was producing seemed spot on. So i used my mobile phone camera to let me see the IR being produced by the TV remote and it seemed to be quite low frequency. So i tried reducing the frequency of IR i was producing by putting in a larger capacitor on my astable circuit. This seems to have got my circuit working although at a lower frequency than stated on my data sheet, not sure why though.
 

hexreader

Apr 21, 2011
135
Joined
Apr 21, 2011
Messages
135
Not sure why either, but if it works to your satisfaction then that is a good result.
 
Last edited:
Top