# ADC & a transistor circuit connected to the parallel port

• Engineering

## Homework Statement

Hello everybody,i am new member and i hope that you can help me out in understanding the attached circuit.our lecturer gave us this circuit and asked us to do literature research and try to understand how the circuit work (the parallel port,the transistor circuits,the analog-to-digital converter and the capacitor)

## The Attempt at a Solution

i - i have done a lot of researches to understand parallel port and i think i understand nearly 90% of it.

ii - i noticed that the transistor circuits are repeated but with few minor changes (VR1?VR2?VR3?VR4?VR5 and R7?R12?R17?R22?R27).

I would really appreciate it if you guys can enlighten me

## The Attempt at a Solution

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vk6kro
You are welcome.

It is hard to explain this circuit without actually telling you what it does.

One way to measure resistance is to put a voltage across it and measure the current through it.
Another way is to send an exact current through the resistor and measure the voltage across it.

If you used this last method, you could measure a wider range of resistances if you used a number of different current sources and switched them to get different resistance ranges.

Thank you so much for replying and i forgot to mention that our lecturer did tell that this circuit used for measuring resistances.

I would really appreciate it if you be more detailed in your explanation with relation to the circuit.

This i know and correct me if i am wrong:

I - pin(2 to 6) are data lines - output (0 volts or 5 volts) - .

II - pin(1) is nStrobe for data control (active-low) where pin(15) is the input for the digitized analog signal and pin(14) is for chip select - enable chip on active-low-.

III - as for the capacitor (i think) when we place the resistor to be measured ,it forms a RC filter (parallel circuit) but i do not know why we need that.

As for the transistor circuits i think what you mean here is that they act as a constant current source,but how ? why the variable resistors(VR1,VR2,VR3,VR4,VR5) and these resistors (R7,R12,R17,R22,R27) are not the same.

I would really appreciate it if elaborate a little bit more.

vk6kro
I - pin(2 to 6) are data lines - output (0 volts or 5 volts) - .

No, not really.
They select ranges for the current sources. So, they carry DC, not data.

III - as for the capacitor (i think) when we place the resistor to be measured ,it forms a RC filter (parallel circuit) but i do not know why we need that.

This is a stability issue. The wires going to the A to D converter can pick up interference and this capacitor may be there to stop this producing spurious output when there is no resistor conected.

As for the transistor circuits i think what you mean here is that they act as a constant current source,but how ? why the variable resistors(VR1,VR2,VR3,VR4,VR5) and these resistors (R7,R12,R17,R22,R27) are not the same.

Resistors in series with variable resistors are there for a practical reason. They let you use more of the rotation of the pot for adjustment, so they can be set more accurately if there is a series resistor.
For example, look at VR3 and R17. If R17 wasn't there, the pot would cover 0 to 47 K, a very wide range, but with the resistor, it covers 33 K to 80 K, a much smaller ratio of maximum resistance to minimum so it will be covering a smaller range of possible currents and it will be easier to adjust.
Also, they stop anyone accidentally turning the pot to zero ohms and possibly causing the transistor to be destroyed.

256bits
Gold Member
The circuit uses a ladder network.
There are 2 types - 2/2R and Binary Weighted.
This circuit uses the Binary weighted.
Search the internet for these circuits to see how they work.
One is http://www.irctt.com/pdf/LADDERNETWORKS.pdf [Broken] for 2/2R

The resistors R7,R12,R17,R22,R27 from the ladder.
Variable resistors VR1,VR2,VR3,VR4,VR5 are for fine tuning the above resistors to exact value.

I - pin(2 to 6) are data lines - output (0 volts or 5 volts) -
Yes - your computer selects these pins for an output of 5 v or 0v, and as vk6kro said they will carry DC volts. and you can select any combination of pins.

Doing so will give you a voltage output to the resistor and ADC.
Your computer program will have to translate that into a resistance value.

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vk6kro