The 85 ohms resistor is then likely a "fuse" or a current limiting resistor. So, it's there for "protection".

So the part of the circuit where the voltages are measured across is part of the tester. That's our conclusion so far.

True. Since the tester contains the circuit your building, Why not measure across the integral tester resistors to effectively confirm the numbers that you set?

 

Hey guys!

Was really busy lately at work so i didn't have time to continue with the project. This weekend I had some time so i did some testing.
I was able to connect the sensor properly to the breadboard (had the wrong wire connections), and was able to read some values.

This is what the test box does:

• First you connect the sensor to the test fixture. The test box has two probes named; volts and milivolts, there you connect a multimeter.
• After that is done, you have to turn the voltage adjust knob on the test box, until you read 4.5v on the multimeter connected to the volts probe.
• When you have the 4.5v stabilized, you have to read the value of FC1 and FC2 (there's a switch therefore on the box) on the multimeter connected to the milivolts probe.
• When these values are according to the specifications (this is done by adjusting a plate on the back of the sensor), you have turn the voltage adjust knob again until you read 0.07V or 70mV on the milivolts probe.
• After this is done, you have to read the value on the volts probe, which should be around 0.15v and 0.45v.
  

This is what I did;

First I tested one sensor on the old test box, and got these values;
FC1= 3.90v, FC2= 3.92v, and the last measurent (when you lower the FC values to 0.07v) was of 0.16v.

Then on the breadboard I used 4.5Vcc and measured the voltage across a resistor connected to each of the photransistors (FC1&FC2, what you read on the milivolts probe).
Then I lowered the power supply voltage till I got 0.07V on the FC1&FC2 meters, then I read the Vcc value( volts probe), which should be between 0.15 and 0.45v.

By adjusting a potentiometer connected to each of the phototransistors I was able to get the correct values for FC1 & FC2, but when I lowered these FC values to 0.07v(milivolts probe), I got a voltage value of 1.2v(volts probe).

That last measurement should have a value of 0.16v.

I checked the datasheet of the photransistors and noticed that it has a emitter saturation voltage of 200mV. So that means that the values you read out of the volts probe on the box (4.5v at the beginning and between 0.15v-0.45v at the end) could not be the Vcc.

My question is, what can this voltage than be? I thought the whole time that it was the Vcc.

I also tried with measurement of 4.5v across the LED resistor (instead of 4.5v Vcc), and by the last measurement i got a reading of about 200mV, which is better than 1.2v i got at first, but it's still not accurate so i dont know if i should trust it.

Any ideas what this voltage can be??

Thanks!!

 

Do an old box calibration and take the sensor to your breadboard.

Measure the voltages across R1, Q1, R2, Q2, RLED and LED.

A GENERAL guess is the 4.5 V that you set initially is across one of the resistors. Why? I think you want to be in a linear region, or not near VCE Sat. It's probably unusual to have a 4.5 V supply. The actual supply is really 5V.

So, 4.5+Vce = 5; but you don't want Q1 or Q2 to be in saturation.
Your measurement may be the voltage across the transistor, but you may be varying Vcc.

 

Do an old box calibration and take the sensor to your breadboard.

Measure the voltages across R1, Q1, R2, Q2, RLED and LED.

A GENERAL guess is the 4.5 V that you set initially is across one of the resistors. Why? I think you want to be in a linear region, or not near VCE Sat. It's probably unusual to have a 4.5 V supply. The actual supply is really 5V.

So, 4.5+Vce = 5; but you don't want Q1 or Q2 to be in saturation.
Your measurement may be the voltage across the transistor, but you may be varying Vcc.

Thanks for the tips, I tried all those measurements but didn't get the results I wanted. Which leaves me now to the only option of opening the tester-box and make a schematic, that's the only way to know for sure what the tester really does, otherwise I'm kind of guessing what it does, which is not what i want. I will start tomorrow with the schematic.

 

What values did you get?

 

I don't remember out of my head right now, but they were almost the same as the ones I posted above.

I came close on the last measurement which was supposed to be 0.16v, I got 0.29v one time and one other time 0.22v. It's close but it might also be a coincidence.

I'm almost done drawing the schematic, wasnt that much of a problem actually.

I'll try to post it later

 

Hey guys!

I finished drawing the schematic. It's actually a breadboard schematic, I followed all the connections and drew everything I saw.

Now that I'm trying to understand it, I see some strange things. It can be mistakes I made in the drawing, I will try doing it again when I have some time.

The thing that bothers me the most is this section (the FC- switch) :


It's the yellow and orange wires in the schematic I drew:


As you can see the orange and green wires(FC1&FC2) split in two wires, one that goes to the switch and the other that connects to another wire. I find that strange and don't think it that makes any sense.

But one thing I do know now is that the 4,5v set at the beginning and the 0,15-0,45v measured at the end, is the voltage across a resistor connected to the LED.

What I'm not so sure off, is the voltage measured across the other probe, the milivolts probe.

Any ideas??

 

You have the orange and green wires connected together, so the two phototransistors are strapped in parallel. That doesn't seem very likely.
The trimpot in series with two resistors is connected across the regulated supply - but not to anything else - that can't be right.

 

You have the orange and green wires connected together, so the two phototransistors are strapped in parallel. That doesn't seem very likely

Yes that's what I also think. That's this part:


Can it be that the two wires coming out of those cables (green & orange), don't belong to the green & orange cables? But have a different function?

I will check again how the trimpot and two resistors are connected when I get a chance.

Thanks!

 

Can it be that the two wires coming out of those cables (green & orange), don't belong to the green & orange cables? But have a different function?

I have noticed that not only are they connected together, they are connected to ground. That part of the circuit makes sense if the ground connections were removed. Marked with red crosses below:

 

Do two of the diodes by the transformer need reversing?

 

Do two of the diodes by the transformer need reversing?

That's right! I just noticed on a picture I have too:

 

Ok, so I drew the schematic all over again:



I had made some mistakes on the first one. What is being measured at the ''milivolts'' probe is still not clear for me.
My question is, now that im having a difficult time understanding the circuit, can this schematic be simulated on any of those free simulation software found online? If so ,which one would you recommend?

One other thing is the transformer. I know it's a stepdown with 110v on the primary winding, but how can I know what's on the secondary winding? I assume it would be 24v. I tried looking the serial number on the transformer up, but didn't get any results.


Thanks!

 

The trimpot still looks wrong - it doesn't trim anything. It has three connections but only two wires. Is one not connected? Do you know what it is used to adjust?

 

The trimpot still looks wrong - it doesn't trim anything. It has three connections but only two wires. Is one not connected? Do you know what it is used to adjust?

Yes I think you are right. The trimpot is connected at the first and third leg, the second one is not connected to anything (I checked that also before removing all the wires):


Is it than just a 2MΩ resistor?

 

Is it than just a 2MΩ resistor?

Yes, and it and the two 1% resistors are simply connected across the regulated supply which achieves nothing.
I can only think that they have modified it at some point and it used to be connected but is no more.

 

Yes, and it and the two 1% resistors are simply connected across the regulated supply which achieves nothing.
I can only think that they have modified it at some point and it used to be connected but is no more.

That's interesting, I also thought they had modified the trimpot at some point. But why actually don't you achieve anything if you place resistors across the regulated voltage supply? If I follow the wires they clearly go that way..

Should I forget about this part of the schematic?

so from the node before the 7.5k to the node in front of the 2.55k.

 

Across the supply, they would draw a very, very small current from that supply. It wouldn't make any difference to anything.
You can safely ignore them.

 

Across the supply, they would draw a very, very small current from that supply. It wouldn't make any difference to anything.
You can safely ignore them.

Ok I understand. I see that the 11k resistor is also connected across the supply, could I ignore that one also?

I'm trying to simplify the circuit as much as possible before drawing it on a simulation program.

 

The transistor provides a variable voltage via the voltmeter to the LED. The 11k resistor is from that variable voltage to ground and there may be a very good reason why that is there to do with the way the voltage varies when it set low. Leave that one in there.