Usb-UIRT 5v to Xantech 12V


Active Member
I need to use the stereo jack (2 zones) in the back of a usb-UIRT to send IR out to two Xantech IR emitter blasters (model 380-20).

I believe that the UIRT operates at 5v, while I know the Xantech is a 12V device. I have verified (via a stereo headset) that I can send IR signals to zone 1 (right headphone), and zone 2 (left headphone).

In searching the forum and net about this topic, I found an amplifier circuit I thought should work (designed by Guy Lavoie). Here is the LINK


I realize that this circuit would only amplify 1 zone (not both). I built the ciruit and connected the 5v end to the tip and barrel of a stero jack (left channel, zone 2). I connected the 12V end to the Xantech, and it works....SORT OF.

Here is the ISSUE. With ONLY ZONE 2 connected to the amp circuit, when I send a IR command out to zone 1 (which is not connect to the amp circuit), I still see the IR blaster emmit a signal.

OK, Lets rethink this........................................
Using my multi-meter and the three wires from the stereo jack (left, right and barrel/common) I do see a voltage fluctuation on zone 1 when I send an IR signal out on Zone 2. This is odd, with the headset I only hear a tone in the zone that the IR was sent to....what gives???

Question is, why does the headset tones seem to work, but I see voltage fluctuations in both zones? It appears these voltage fluctuations cause the amp circuit to fire no matter what zone the IR was coming from.

Any Ideas
Hi Smarty

I wonder if its being caused by some sort of induction.......

Maybe try putting a resistor between the base of the first transistor and common (gnd) and see if it makes any difference....try something like a 4k7......

Additional info:

When measuring between the barrel and tip or barrel and ring, the voltage I am seeing from the usb-uirt is about 2.5 Vdc (this seems low to me)???

Also, I per Frank's suggestion, I put a 2.2K ohm resistor between ground and the first transistor (this did seem to work). A 1K resistor made the emmitter dim down (on the zone it SHOULDN'T have fired on), while the 2.2K made it only fire (brightly) when the proper zone was sent a signal.

BTW, I am using my cell phone camera to view the infra-red emmitter brighness...this works well.

Question: I am NOT a EE. Am I using the right circuit? Am I using this circuit in the way it was designed? Is there a simpler way to tie the UIRT to the Xantech??

OK, that does seem simpler. It says one device is needed per IR zone. I guess I would use a stereo to mono (dual) adapter out of the usb-uirt. 4 zones times $18 each.... :eek:
Hi Smarty

>When measuring between the barrel and tip or barrel and ring, the voltage I am seeing >from the usb-uirt is about 2.5 Vdc (this seems low to me)???

I think its ok your probably reading via a series resistor and the signal is being modulated as well so your reading the average voltage out ....

>Also, I per Frank's suggestion, I put a 2.2K ohm resistor between ground and the first >transistor (this did seem to work). A 1K resistor made the emmitter dim down (on the >zone it SHOULDN'T have fired on), while the 2.2K made it only fire (brightly) when the >proper zone was sent a signal.

So it didnt make any difference putting the suggested resistor in place ???

> Question: I am NOT a EE.
Neither am i ;-)

> I using the right circuit?

Im sure others have used Guys circuit with no issues ...

A resistor placed between ground and the first transistor this DID seem to work (I just want to make sure that this is the PROPER fix). At first I tried a 1k resistor, but a larger resistor seemed to work better.

When sending a signal (on the UNCONNECTED zone) out on the UIRT, the 1K resistor still allowed the emmitter to flash (dimly). The larger resistor prevented the emmitter from firing at all.

I did double check that the emmitter still fired properly when sending a signal on the CONNECTED zone. So, (I guess) this is indeed the fix..... I just want to make sure I am using this circuit right.
The circuit is a 5 to 12 volt level-shifter and assumes the source and load can share a common ground. If adding a resistor between the "common" connections caused it to work when it didn't, then it could be that your two circuits are somehow related in their power sources but do not actually share a common ground, or maybe the voltage levels are not really 5 and 12 volts. The 5 volt side will usually be somewhat lower then 5 volts as you already noticed. This usually means that there is some type of current limiting resistor in series with the output.

You would need to explain how the UIRT and the Xantech blasters are powered to get a clearer picture of what's going on.
The USB-UIRT is powered from the computers USB port. The computer is fed from a UPS plugged into house power. The Xantach blaster is powered from an Elk PD9 12V supply. The wall wart for the PD9 is plugged into house power.

What would be the better way to do this?

See attached photo.



  • DSCN0431.jpg
    136.7 KB · Views: 37
The first thing you would need to determine is how the UIRT device drives its emitter LED. The level shifter circuit assumes that the input is a positive going signal because that's how the Ocelot works. If the UIRT device is NPN open collector and is thus a negative going signal (a probable case) then the circuit would not normally work as-is. To determine if this is the case, try powering off the PC and then with an ohmmeter, measure the resistance between the PC's ground (just touch a probe to a bare metal spot on the PC) and the UIRT's output jack. If you find about 0 ohms going to the barrel side of the mini-jack for the emitter, then it is positive signal out and should work with the circuit. If however you find that neither the barrel or the tip of the jack gives a low resistance reading to the PC's case, then it is probably an open collector output.

If you do find or suspect that it is open collector, then try it with the circuit this way: remove the first transistor (2N3904) and the two 1k resistors completely, and connect the 2.2k resistor between the input plug's barrel (not the tip) and the base of the 2N3906 transistor, where the 1k resistor coming from the first transistor was going. Don't connect the circuit's input plug tip to anything! Then connect the PC's ground to the Xantech's power supply ground ("common" terminal). See if it works that way.
I have confirmed that the PC case and the barrel lead wire of the UIRT ohm out to zero (they have a common ground); and therefore your circuit should work. ;) B)

Is it possible that when I send a signal out the UIRT on the UNCONNECTED zone wire that the connected zone wire acts as an antenna and 60HZ noise actually causes the blaster circuit to fire (per an idea of an EE at work)?

I will add a 1 microFarad cap inline with the tip lead (+5V), and add a 1K resistor between the +5 and Ground to make a 1KHz filter. This should knock out 60Hz line noise and still allow the 15KHz IR signal to pass.

Does this seem OK? Other ideas?

Please note that your circuit DOES work when a signal is sent through it. I am trying to figure out why it also works when I send the signal to somewhere else.
The 60 Hz noise theory sounds a bit far fetched, unless someone already knows something quite specific about the UIRT. I don't have one of these so I'm only guessing as to how it works. If you suspect that the unused output is a source of noise, just put a resistor (such as 1k) across it in order to load it down and suppress any induced noise.

If you have an oscilloscope then it would be much easier to figure out what's really going on. The filter idea sounds quite iffy to me and would likely mess up the signal enough to make it stop working.
Hi Guys

I have a USB UIRT here, so i have pulled it apart to have a look......

The zone driver transistors are smd marked 2A 0..... one leg of each transistor is connected to +5v another leg of each goes via a smd resistor marked 820 which i assume is 820 ohm or maybe 82 ohm..???......82 ohms sounds right ??

Im guessing its a pnp transistor with both collectors tied to +5v and the resistors connected to each emitter...??

Thanks for the help Guy & Frank ;)

Our new house is still "Under Construction", but I wanted to get this piece understood now.


Anyway, (as a "plan B"), if I consider using the SmartHome IR Link mono jack, how would I wire this to the Xantech IR Blaster?

After looking at the (very limited) installation page, I guess I would wire signal to signal, ground to ground, and "posative voltage" to the 12V Xantech blaster connector.

I would then run the power source (12VDC) across the ground and +12V connectors. Right?

Thanks, and Happy New years!!!