Replacement transistors for PUMX1 and PUMT1

[bbagnall]

Hi all,

I'm inexperienced as far as electronics circuit design goes, but have had some limited experience soldering and hacking smaller circuits. One of the circuits I'm trying to make calls for two IC's that have only two transistors in each of them.

The first one is called PUMX1:
www.nxp.com/pip/PUMX1.html?

The other one is PUMT1:
www.nxp.com/pip/PUMT1.html?

I couldn't obtain these specific parts and tried to replace them with individual transistors that I ordered from Mouser, however the circuit does not work so I probably did not choose the correct replacements. The layout of my circuit board is set up for individual SMT transistors with package size SOT-23.

Could anyone recommend some transistors from the Mouser catalog with the same characteristics as PUMT1 and PUMX1?

Thanks.

[theatrus]

Do you have a schematic? Which transistors did you use as a substitute?

[pa2ees]

Theatrus is right, a schematic would be a good starting point.  At least give us some idea of how the transistor is being used, and what is not working, and how it is not working (ie what is happening that you think should not be happening).

  If you need something that does exactly what this transistor does, go to Mouser and use their parametric search to get all the specs you want and you'll have a list of parts that fit the bill.  I know it's a tedious chore, but i've found that when I'm searching for a chip, I'll find other interesting and useful chips that I wouldn't have know about before.

At least some specs of what you're doing with it would be nice.  Such as:
Collector-emitter voltage/current
Base voltage/current
Hfe needed

I'm sure with this information someone could give you a couple of transistors that will do the trick.

[bbagnall]

Thanks for the directions so far. The circuit is for a light detecting sensor which also has an LED light that can be turned on and off. Here's a PDF of the circuit (attached).

I'm able to turn the LED light on and off, but it is unable to read the value from the phototransistor. I think the circuit is at 4.3V -- at least that's the pinout for the LEGO NXT brick sensor ports:
http://en.wikipedia.org/wiki/Lego_Mindstorms_NXT
(see diagram near bottom of this page)

[theatrus]

When you say unable to read, what do you mean? Are you able to use a multimeter to read your output? What transistors did you use as subsitutes?

[bbagnall]

I'm reading the value of the phototransistor by plugging the sensor into a LEGO NXT brick, which is running some code. The brick and code is working as I can test it using an actual LEGO light sensor (the sensor shown in the schematic). The code can turn the LED on and off, but the phototransistor reading fails to change under different light conditions.

I purchased a Fluke 179 multimeter a few months ago if you want me to measure the actual output. I assume it needs ~4.3V to the circuit, perhaps using 3 x AA batteries? I do not have a bench power supply.

Sorry, I forgot to include the transistors I'm currently using:
PUMX1:  2N3904

PUMT1: 2N3906

[theatrus]

The biggest difference between the original parts and the 3904/3906 combo (classic transistors) is the gain (hFE). Its not too different, but with small signals from a phototransistor its possible the circuit is not going to work.

Can you measure voltages at the output connector on the schematic, with and without light?

[bbagnall]

Sorry for the length of time away from this topic. I had another project that took a few months and now I'm happily getting back into this one.

The biggest difference between the original parts and the 3904/3906 combo (classic transistors) is the gain (hFE). Its not too different, but with small signals from a phototransistor its possible the circuit is not going to work.

Thanks for pointing out the hFE differences, which is not something I was aware of before. I'm also not yet understanding how to compare hFE values. Looking at the PUMX1 datasheet, it shows hFE of 120 at 6V and 1mA:
http://www.nxp.com/documents/data_sheet/PUMX1.pdf

Now it gets confusing looking at the datasheet for the 3904 transistor, which shows a whole bunch of voltage and current values:
https://www.fairchildsemi.com/datasheets/2N/2N3904.pdf

Not sure what to make of that. As you mentioned, the values are not too different--to me they look in the same ballpark, but they aren't using the same V and I values. What do you make of this?

Now looking at PUMT1 hFE values, it is also 120:
http://www.nxp.com/documents/data_sheet/PUMT1.pdf

And the 3906 also looks similar:
https://www.fairchildsemi.com/datasheets/2N/2N3906.pdf

BTW The 3904 and 3906 transistors are not Fairchild, they were bought from eBay from China but I assume they have similar specs.

Can you measure voltages at the output connector on the schematic, with and without light?

Ok, measuring both terminals of the phototransistor, the original circuit gives values of 4.1v when ambient room lighting is striking the phototransistor. When I take a DeWalt light and shine it directly on the phototransistor, it measures 0.17V. So there is a large range there which produces nice data readings (on my NXT brick it shows values from 94 in ambient to 245 in bright light).

Now when I'm using my circuit with the 3904 and 3906 transistors, it measures 4.18 volts with ambient room lighting, and 4.08 volts with direct light.  So *some* change is measured, but I would like a greater range to register from this circuit (as it only produces values of 41 to 112 on the NXT brick).

Any ideas on some transistors with better hFE values?