Improvement wanted to transistor pin identifier

[carbonzit]

I found this handy little transistor pin identifier (from our friends at RED circuits) that works nicely, except for one thing: the LED drive is much too wimpy, and they're barely visible. (The CD4011 is only capable of about ~9mA drive, and that's on a good day! And this is being powered by a 9-volt battery.) I'd like to beef up the drive a little, using transistors. So I came up with this idea:

**broken link removed**

Would this work? Here's how it should work, as I see it:

The NAND oscillators apply a 3-phase signal to the D.U.T. (device under test). If there's a good connection to that particular leg of the device, then current should flow between the NAND output and the D.U.T., possibly both positive & negative. I'll tap into that to the base of the driving transistor and use that to drive my two LEDs.

Here's where I have problems, and please pardon my ignorance at this point: I still don't know how to properly design a transistor stage, although I'm trying to educate myself in this area. Obviously the transistor needs to be properly biased so that its operating point is set such that its output can swing both positive and negative, in order to light up both LEDs.

I don't know whether it would be better to configure the circuit as shown, or to make it an emitter follower instead. The resistor below the LEDs is for current limiting.

Anyone care to help me out here? Should be a piece of cake for you. Many thanks!

 

[carbonzit]

Certainly someone here must have some ideas? Anyone?

 

[Nigel Goodwin]

It seems a fairly poor circuit, there was a simple 555 based transistor tester years (decades) ago in the now defunkt magazine Practical Television, it tested in-circuit, identified if NPN/PNP, and a basic go/no-go result.

I seem to recall it used a single 555, a couple of transistors, and two LED's?.

 

[carbonzit]

But it's not really a tester, per se; it's just a pin identifier. Can this be done with a 555? If so, can you point me to a circuit or three? Much obliged if you can.

 

[MrAl]

I found this handy little transistor pin identifier (from our friends at RED circuits) that works nicely, except for one thing: the LED drive is much too wimpy, and they're barely visible. (The CD4011 is only capable of about ~9mA drive, and that's on a good day! And this is being powered by a 9-volt battery.) I'd like to beef up the drive a little, using transistors. So I came up with this idea:

**broken link removed**

Would this work? Here's how it should work, as I see it:

The NAND oscillators apply a 3-phase signal to the D.U.T. (device under test). If there's a good connection to that particular leg of the device, then current should flow between the NAND output and the D.U.T., possibly both positive & negative. I'll tap into that to the base of the driving transistor and use that to drive my two LEDs.

Here's where I have problems, and please pardon my ignorance at this point: I still don't know how to properly design a transistor stage, although I'm trying to educate myself in this area. Obviously the transistor needs to be properly biased so that its operating point is set such that its output can swing both positive and negative, in order to light up both LEDs.

I don't know whether it would be better to configure the circuit as shown, or to make it an emitter follower instead. The resistor below the LEDs is for current limiting.

Anyone care to help me out here? Should be a piece of cake for you. Many thanks!

Hi,

Your circuit is not drawn correctly if it is actually working as you say. When the transistor turns on it shorts out the supply voltage.
Come back with a better drawing and we can go from there if you like.

 

[carbonzit]

Your circuit is not drawn correctly if it is actually working as you say. When the transistor turns on it shorts out the supply voltage.
Come back with a better drawing and we can go from there if you like.

OK, fair enough. Tell me which one you prefer, and why, the emitter follower or the LEDs atop the collector:

**broken link removed**

What we're after here is just a current amplifier, correct?

Thanks in advance.

 

[MrAl]

Hi again,

Yes more or less. The second circuit is a little bit better because there is some voltage amplification too though.
I have to ask though what is the purpose of the second LED there, the one with cathode connected to V+. I ask because it should never turn on.

 

[carbonzit]

Read the description of the original circuit (link given in the first message above). The LEDs are different colors (red & green) and light according to the polarity of the transistor pins. That' why I need to preserve the entire waveform, both positive- and negative-going. Will either method above do this? Will the transistor be properly biased?

 

[MrAl]

Hi again,

What is the level of your V+ supply voltage shown there? Does that feed the CMOS too or a different supply?
What is the highest positive voltage you can get in your system?

 

[colin55]

This circuit identifies the base and NPN or PNP then the other two leads
**broken link removed**

 

[carbonzit]

What is the level of your V+ supply voltage shown there? Does that feed the CMOS too or a different supply?
What is the highest positive voltage you can get in your system?

Like the original circuit, it all runs on 9 volts.

You did look at the original circuit, didn't you?

 

[carbonzit]

This circuit identifies the base and NPN or PNP then the other two leads
[snip schematic]

Thanks, but:

1. Your circuit is almost identical to the one I'm using (at least for the part I'm interested in, the pin identifier), except for your choice of IC (Schmitt trigger vs. NAND).

2 Mine will run on a 9-volt battery, while yours would require a 5-volt regulator to do so.

Your circuit will, however, adequately drive the LEDs w/o drive transistors. So I guess it's a toss-up.

 

[colin55]

would require a 5-volt regulator to do so

That is entirely incorrect.

 

[carbonzit]

That is entirely incorrect.

[/quote]

Right, my bad. I was looking at the wrong datasheet (HC part).

 

[carbonzit]

A specific request for a circuit improvement

OK, so I'm coming back to y'all with a specific request for how to improve this circuit.

To recap, here's the circuit I have, a simple transistor-pin identifier:

**broken link removed**

(For parts values and an explanation of the circuit, see this page.)

(I've eliminated the CD4066 switch and associated parts.)

I'd like to boost the output of the LEDs, which are now limited to what the CD4011 can output (not much). So what I'd like to do is to add this to the existing LED pairs:

**broken link removed**

What I need is what is in the box with question marks, indicating "I don't know how to design this circuit". I'm thinking that all that's needed is a single transistor, properly biased, to drive the LEDs. Remember that this is an AC circuit, so it must be capable of preserving and amplifying both halves (negative and positive) of the waveform. (I'm ASS-U-MING that the resistor will be needed for the voltage to/from the D.U.T. to appear across.)

Specs: "square wave" is approx. 5 volts p/p. (You can see from the screen shot below that this is anything but a clean square wave. I don't even know what to call it: spiky chaotic goofy wave?)

The screenshot below shows the LTspice similation of the existing circuit, with a "transistor under test". The .asc file is attached for your simulation pleasure.

 

[colin55]

use 9v to 12v