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computer printer port interface

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bogdanfirst

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i am making a computer interface wich i will use to monitor some switches attached different things, and to control different lights and other stuff on the house.
but i am facing a problem. the circuit i will use to power the interface must not have the same ground as the computer, basicly i want to insulate the interface and the things attached to it from the computer. is there a way to do that, by other means than optocouplers?
please adivice me.
 

kinjalgp

Active Member
Opto-isolators are the only device which satisfy your need of different grounds.

The other option may be op-amp which has very high input impedance (which is equal to isolation) but still, it requires common ground between input and output.
 

bogdanfirst

New Member
got any schematics from an insulated interface with optocouplers? i want to use the 8 bits as output, and 4 bits as input to the computer.
also, if i use 74LSxx(x) combined with 74HC(T)xxx is there a problem?
is a LS capable of driving 3 HC?
 

tdb

New Member
check out this tutorial for connecting optoisolators: http://www.hut.fi/Misc/Electronics/circuits/parallel_output.html

You will need pull-up resistors to drive HC with LS. You do not need pull-ups to driver LS with HC. If you want clarification, check out the V(ih), V(il), V(oh), and V(ol) specs in the IC's data sheet. These values describe the max/min voltage required to turn ON/OFF inputs, and the max/min voltage at the IC's outputs.

What are you using for the PC software? Are you using VB? If so, is this Windows NT, 2000, or XP? I've been trying to find some free drivers for accessing the parallel port using Visual Basic 6 under these operating systems. If you are, could you write me back and let me know how you are doing this? Thanks.
 

bogdanfirst

New Member
thanks for the links, but it seems that i cannot access it.
well, im not going to use VB, i'll use Pascal for testing the interface and then i'll ask someone to help me develop some software using Visual C.
well, about the ICs now. If ill use all of the same type, either HC or LS, do i need resistors?
 

tdb

New Member
sorry, the text editor included the period (.) after the end of the link. Try this: http://www.hut.fi/Misc/Electronics/circuits/parallel_output.html

You won't need any pull-ups if use all the same type. For LS, the typical voltages are:

V(oh) = 2.7V (minimum output voltage for binary HIGH)
V(ol) = 0.5V (maximum output voltage for binary LOW)
V(ih) = 2.0V (minimum input voltage for binary HIGH)
V(il) = 0.8V (maximum input voltage for binary LOW)

In other words, a HIGH output of an 'LS chip will be greater than 2.7V. In order for another 'LS chip to detect this HIGH, the voltage at the input must be greater than 2.0V. If you think there might be a lot of stray capacitance on the line (e.g. it's part of a large bus, or in a long cable), it can't hurt to put pull-ups on each line.
 

bogdanfirst

New Member
here is why!
i only have 8 bits, wich is too little, i need at least 10 relays.
here is how i want to do it:
1. have 2 4 to 16 line decoders(demux) controlled by the 8 bits. i'll connect some of the 30 outputs to flip flops and then drive the relays and i'll use the rest to control other devices wich dont require relays.
2. i will use 2 8 bit to 1 bit multiplexors to have a 16 inputs. these inputs will be from various reed switches and other stuff.
so the problem is to insulate the computer ground from my groud, the suply and the other devices.
optocouplers could habe been simple, but i have to use for each bit an optocoupler, some resistors and a transistors. and the reason why i cannot use them is space. i dont have enough space on the board. i want to make it as small as possible.
so i will have to think of another solution. maybe i could use a buffer somehow?
 

olcal

Member
No, I dont think you can use a buffer. One other alternative is to use SMD devices (expensive and difficult to work with for the hobbyist) since small size is a prime concern for you. You could also try quad optocouplers (4 optocouplers on one chip). There doesnt seem to be any way of getting around optocouplers for circuit isolation.
 

leprien

New Member
a possability

well i do know if you make a microphone you dont need to attach the power that goes to the speaker to anything. insulate the referance votage to the speaker. then use the input connection for your input. You need a mic for each input. lower the voltage and amperage for input to a voltage not much higher than you reference. Then connect your wires from the printer port to each input. Each mic can light leds based on any voltage higher than the reference, but dont over power it you may fry somthin. Lost my book or i would be able to get you a skamatic. hope i was a help. Useing premade devices. Walky talkies can make this wireless just connect to the input wire on the mices insulate the power wire to the speaker. very small voltage is neaded and very small voltage changes are detected if you can do anything with that.
oh you need 10 bits you will need 3 op amps this will give you 12 bits.

good luck


ps do you know how i can send info to the serial port and receive through it. I want to hook up a system that creates frequency from the serial port every differnt combo having a different tone, but i want it to be simple and transferable hooking up remote use for music match. I just nead audio out and room for simple commands back to that program without interfering with its regular operation no data back to remote terminal except for a start # of tracks, and when i change tracks a return stating i did.
 
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