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Parallel port - control pins

davidova

Dec 1, 2017
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Dec 1, 2017
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Hello to all,

I would like to ask for help.
I am making a student project which includes a parallel port.
I am controlling the intensity of an RGB diode with D/A converter and connected on Intel 386DX through a parallel port. Its coded in Assembly language.
On the converter I need 5V power supply. I tried to give it from some control pin like Strobe, Initialize, AutoLineFeed, Select Printer, all of which are In/Out.
The problem is, when I send digital high to that pin (eg, Strobe), it gives 2,3V. Also when I send digital low- again 2,3V.
I was thinking maybe that is because it is not defined if it's In, or Out and it needs to be configurated like Out pin, but I couldn't find such code.
Any idea?
Thanks in advance
 

kellys_eye

Jun 25, 2010
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Jun 25, 2010
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4,775
You won't get anything more than a few mA from the parallel port. The usual way is to fit an off-port PSU socket on your external device and power it for a small wall wart or similar.
 

davidova

Dec 1, 2017
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Dec 1, 2017
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Yes, but the next thing I have to do is use 3 control pins like a chip selects for the 3 colors respectively. So I do need 0 and 5V from there.
 

AnalogKid

Jun 10, 2015
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Jun 10, 2015
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2.3 V is a bit low, but still acceptable as a signal level on a parallel port pin.

For some low voltage and low power applications, you can use combined signal pins to power a small circuit. Your project needs too much power for that, so you will need a separate 5 V power supply for your circuit.

ak
 

kellys_eye

Jun 25, 2010
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4,775
What LEDs are you using? I suspect the minimum current for EACH would be 10 or 20mA which, on their own, is far in excess of anything the port itself can supply.

An external supply will be mandatory.
 

davidova

Dec 1, 2017
6
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Dec 1, 2017
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30mA
This is the circuit
 

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kellys_eye

Jun 25, 2010
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4,775
Three LEDs at approx 30mA each plus current required for LM324 (5mA?) and the TDA8702 (20mA) and you're already at over 100mA - no chance.

External PSU or forget it working.
 

AnalogKid

Jun 10, 2015
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The schematic has several problems. The BS170 is shown operating as a source-follower. This means the minimum voltage drop across the device is its threshold voltage, approx. 2.1 V.

What is tye purpose of the capacitor at the input the the LM324? It will need some kind of resistor or something else to discharge it when the transistor is off.

What is the purpose of the LM324? Why not drive the LEDs directly with the transistors?

ak
 

davidova

Dec 1, 2017
6
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Dec 1, 2017
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It's a Sample and Hold circuit. I choose colors with keyboard (r, g and b buttons). When I change the intensity of some color (+ and - buttons), it has to stand so, and then I choose another => I make some combination of the 3.
(full spectrum)
 

AnalogKid

Jun 10, 2015
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If you are using the FETs as analog switches that are charging a capacitor to a positive voltage (above GND), you need to change to p-channel devices.

ak
 
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