# Single transistor IR repeater

I need the IR repeater to pass IR (TV and so on) remote signal to remote devices in some box (pass the IR signal from outside the box). So, I bought a cheap IR repeater on single 2TY transistor, 4838 IR receiver and IR LED. It works just fine doing exactly what I want:

I tried to reproduce the schematic in my project, and I've got this (without power led):

And I assembled it on the breadboard and it does not work. When I use simple ordinary visible light D2 LED, I see it blinking when I use any remote on the sensor. But when I replace it with the IR LED, even from working device I bought or 50mA L-53F3C for example, it does not work with TV etc.

And I don't understand the capacitor C5 capacitance, it actually looks like this: And closer picture: So I wonder, is it capacitor at all? Tried with different capacitors and with no capacitor at all - no success. But I guess it does not matter. Please help, what's wrong and what could be the reason that it does not work?

• R4 on PCB =/= 100$\Omega$ but 10$\Omega$ (Hint: Do use the same components references in your schematic as the are on the PCB) – Huisman May 19 at 20:06
• I see it's 10 and zero zeroes at the end, so it's R10, I actually used it without resistor at all, it's just a schematic mistake - copypasted the previous one and forgot to change the value )) – BbIKTOP May 19 at 20:18
• I suppose it's just a current-limit resistor, and I used 100 mA LED at first and 3v3 VSS, so it shall be 33 Ohm that's why I decided to use it without resistor at all – BbIKTOP May 19 at 20:21
• Q2 in your schematic appears to be upside down. The way you have it doesn't make sense. – JRE May 19 at 22:40
• EC is reversed on Q2 – Sunnyskyguy EE75 May 20 at 4:38

C5 was not installed on the board, what you're seeing is just the drop of glue that was supposed to hold it during soldering. They probably decided the circuit would work without it, so they configured the pick and place machine not to place it, but they didn't bother to modify the configuration of the machine that dispenses the drops of glue.

Now, looking at the schematic, this shouldn't work. The IR signal is modulated: it is not simply a LED blinking on/off for each bit, rather the IR LED blinks at 38kHz for the duration of a "1" bit, and it stays off for a "0" bit. The TSOP receiver demodulates the IR signal and ouptuts bits, not the 38kHz carrier.

If the repeater works, then something on the board is turning the bits back into modulated 38kHz signal. Either the transistor is not a transistor but an IC, or the "LED" that it uses to transmit is not actually a LED but an IR transmitter which will generate 38kHz IR pulses when it is on. Or the TSOP4838 isn't a TSOP4838 but something else that outputs not demodulated signal.

Either way, there's a component in there that isn't what you think it is, that's why your attempt to replicate it fails.

• I thought about it in the same way. I supposed it's just a glue and built it without capacitors, but since it does not work, I supposed it's a kind of capacitor I just know nothing about. I tried to replace the LED in the PCB with another one I bought in local store and it still works. Transistor looks just like a transistor, it's marked as 2TY, so I deceded it's 8550. – BbIKTOP May 19 at 20:14
• And just found the same you wrote about - in the 4838 datasheet it's clearly stated that it outputs demodulated signal. So probably it's really not 4838 (although it looks exactly as it is) - will try to replace it with the one I definitely know it is 4838. Also will try to replace 2TY transistor with another 8550, probably it would make everything clear. Thank you! – BbIKTOP May 19 at 20:14
• By the way, is there any receiver that does not demodulate the signal? Just a kind of IR sensor with amplifier? – BbIKTOP May 19 at 20:16
• I have no idea, but if you got an oscilloscope you can probe the output, see if you find bits or 38kHz pulses... – peufeu May 19 at 20:39
• Sure, you can make a question like "help identify component" and put a picture – peufeu May 20 at 9:56