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Opto-Isolator problems

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i have included in this post the problems that i have on a word document. i have done this because it contains pictures that are related to the text and i didnt want to post them seperatly, i will host them online and link them here if necessary or requested. a quick run down of my problem:

the opto isolator contains a LED which is used to transmit data and a Phototransistor to recieve it. this phototransistor is set up in a darlington pair array so it has the base of another transistor coming from the emitter of the phototransistor. this is sealed withing a unit and cannot be modified. basically in simulation when i connect these transistors to high and low along with my output the transistors are always on, the output from the darling pair that i use is always high. why is this? when i tried with two regular transistors this didnt happen, is it to do with the phototransistor? is it to do with the simulation? any advice on this would be great, see the word document for better details and pictures.

i have also included my circuit diagram if you wish to look at it, any critisms would be fine. it is important to note that the switiches will be replaced with outputs from a pic chip. on that note, do i need to pull down the outputs from the pic and if so would it be any different from pulling down an normal input (as shown here). also would it be a good idea to put resistors protecting the inputs of the 4014b and 4015b chips so the curent isnt drained into them?
 

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do i need to pull down the outputs from the pic
No, because the PIC outputs can both pull UP and DOWN by themselves.
and if so would it be any different from pulling down an normal input
The reason you need the pulldown resistors in the diagram you submitted is because your switches only source voltage and don't have any means of sinking current to ground. That is what the pulldown resistors are for.


basically in simulation when i connect these transistors to high and low along with my output the transistors are always on, the output from the darling pair that i use is always high. why is this?
I have attached a modified version of your diagram. I've changed the values of some resistors. They are circled in RED. See how that effects the SIM.
 

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oh right sorry i forgot to mention that one of the resitors on the diagram cannot exist in real life because it is connectied to a transistor which is inside a component, so it cannot be reached. i have included an updated diagram with your changes but without that resistor, it does work in simulation but i am still getting a a voltage (of 1.4 volts) on the emitter of the transistor which i have highlighted in the diagram. while this appears to not be a problem, does anyone know why it is happening and how i can get rid of it completely?
 

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Why not get rid of the transistor that is on the optoisolator LED and just drive the LED directly from the 4014?
The LED should only need a few mA to switch the transistors.

Also, whay is the purpose of the optoisolator? You arent using it to isolate anything, so is it really necessary?
 
jrz126 said:
Why not get rid of the transistor that is on the optoisolator LED and just drive the LED directly from the 4014?
The LED should only need a few mA to switch the transistors.

i did think about this myself and i tested it in simulation but it didnt light up as well as i would expect (the simulator simulates the amount the LED would be lit up) so i decided to add a driver. this fixed the problem and i am happy witht he solution.

jrz126 said:
Also, whay is the purpose of the optoisolator? You arent using it to isolate anything, so is it really necessary?

i am using it to send data in free space, i have used this as an easier alternative to an infrared link or a photo diode because i didnt want to research them and i thought a phototransistor would be easier (thought wrong :p). the company i am buying from also do a single transistor version which i might use instead which i could use with my original circuit diagram.


oh and if it helps, this is the component i am buying **broken link removed**

oh and another solution i found was to make the resistor connected to ground from the second transistor on the darling pair only 1k, this made the voltage when the led was off something like .6v which would appear as logic 0 to the 4015b chip.
 
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but i am still getting a a voltage (of 1.4 volts) on the emitter of the transistor which i have highlighted in the diagram
This is probably caused by a poor simulation of leakage current in your opto-isolator. Don't always trust what a circuit simulator tells you.

i am using it to send data in free space, i have used this as an easier alternative to an infrared link
Ok, but how are you going to send the clock signal to sync the 4014 with the 4015? Another problem with this method is ambient light and the resultant noise.
oh and if it helps, this is the component i am buying **broken link removed**
What exactly are you trying to build?
 
i am trying to build a circuit that will convert parallel binary data into serial data which will be sent over free space, then converted back into parallel data and its value shown on a 7 segment display.

the clock to both shift registers comes form the same input so they will have a clock pulse at the same time

i did concider the fact that the LED would switch off before the 4015b could register it and im thinking about how i could solve this. one solution i had would be to use the extra output on the pic im using to have two seperate clock inputs and the other was to put a capcitor across the LED and resistor so it discharges slowly (about .5 of a second). anyways ive inlcuded a picture of my edited circuit diagram, in simulation it works, i was wondering if there would be any problem with this is should be aware of. is the calculation for time delay in this circuit R*C?

i dont need to worry about outside interferance because i will be using it in a dark room :p
 

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use the extra output on the pic im using
Why not do all the bit banging with the PIC and do it using an asynchronous serial stream. This will also save IO pins on the PICs and you can build in an error correction system too.
 
-=GST=- Nemisis (cs/cz) said:
i dont know what you mean? could you give me an example? anyone please im supposed to be programming the chip today and any changes need to be done soon!!!

Personally I don't have the faintest idea what you're trying to do?, you keep mentioning PIC's but then show some bizarre CMOS circuit?. Assuming you're wanting to transfer data between two PIC's?, you just need an IR LED on one, and an IR receiver IC on the other, everything else is software - and covered in my tutorials.
 
heh yeh maybe i have explained this poorly. also this is a rather pointless peice of equipment and i keep finding solutions to my own problems so each time i re-post its different :p sorry for being missleading. i have solved my problem though with the assyncronous clock pulses so thats some good news. if i have another problem i will explain what im trying to do better and repost :) thanks for the help anyways!
 
sorry to bring back a dead thread but i have a question regarding this circuit (and all circuits like this). i have two questions:

1. do i need to use pull down resistors on EVERY output from a chip that goes to be the input to another chip. an example of this may be a pic chip output becoming the input to something like a 4026 chip or another example would be the outputs of a 4520 binary counter going into a 4511 BCD to 7-segment decoder. will the connections between the chips all need to be pulled down?

2. why is it recommended to connect a electrolytic capcitor from high to low on a circuit which has ICs on it?
 
-=GST=- Nemisis (cs/cz) said:
sorry to bring back a dead thread but i have a question regarding this circuit (and all circuits like this). i have two questions:

1. do i need to use pull down resistors on EVERY output from a chip that goes to be the input to another chip. an example of this may be a pic chip output becoming the input to something like a 4026 chip or another example would be the outputs of a 4520 binary counter going into a 4511 BCD to 7-segment decoder. will the connections between the chips all need to be pulled down?

No you don't need resistors like that, their normal use would be on an open CMOS input, where it prevents the gate drifting high (because they have such a larg input impedance). If it's connected to an output, the output will hold it either high or low as required.

2. why is it recommended to connect a electrolytic capcitor from high to low on a circuit which has ICs on it?

Not just IC's, ALL electronic circuits - it's to make sure the supply rails are all an the same AC potential, and to prevent unwanted positive feedback.
 
k thats very useful. i have 2 follow up questions now

1. is there anywhere i can read up about keeping the rails at the same AC potential, does that practice have a name or is there a keyword i could google.

2. in a similar situation where outputs from one chip is going to inputs of another, do i need to put a resistor in serial from the output of a chip to the input of another to stop "current being drained from the chip giving the output, causing it to be blown". i put this in quotations because i have been told this is true.
 
-=GST=- Nemisis (cs/cz) said:
k thats very useful. i have 2 follow up questions now

1. is there anywhere i can read up about keeping the rails at the same AC potential, does that practice have a name or is there a keyword i could google.

You might try ggogling "supply decoupling"?. Or have a look at https://en.wikipedia.org/wiki/Decoupling_capacitor

2. in a similar situation where outputs from one chip is going to inputs of another, do i need to put a resistor in serial from the output of a chip to the input of another to stop "current being drained from the chip giving the output, causing it to be blown". i put this in quotations because i have been told this is true.

Output to input, with similar logic levels, isn't a problem - but with specific chips, and different logic levels, they may be some possibility?, but in general there's no such requirement.
 
Thanks for all your help with this project, i have almost finished it an everything works except the opto-isolator part :(
basically i have built the circuit attached in simulation where it works and in real life where it does not work, is there any reason for this not to work in practice? (note the 4014 chip is connected here because it is the output from this circuit. the thing connected to the clock input is a switch thing.)

also my circuit overall converts parallel data into serial data then back into parallel data. does anyone know any good examples of when this would be useful, dont computers have to convert parallel data into serial data when transmitting it to other devices via connection such as USB and rs-232, then these other devices may convert it back into parallel data?

are there any other real examples?

edit: if this helps some confusion about my circuit i may be wrong calling it an opto-isolator, in a book im reading it calls it an opto-switch and on wikipedia they are called opto-couplers so i hope this makes thing a bit clearer.
 

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