How to turn on a optoisolator with a desired resistance

Question

I am learning electronics so this is learners question. I wired up the LM339N quad voltage comparator (powered using a 5v power supply through a current limiting resistor of 40 ohms)-in a straightforward standard way- like at

http://www.learningaboutelectronics.com/Articles/LM339-quad-voltage-comparator-circuit.php

but for the potential divider I used two 4.7k resistors-not a pot- and I only used one comparator. I also had an output LED too.I added the standard 0.1uf capacitor near the power supply. So the circuit worked as it should (for the input voltages to the comparator I used AA batteries that went through a current limiting resistor of 500 ohm into the comparator input) at 1.5 volts or less the LED turned off. At higher input voltages the LED turned on.

I want to replace the LED with a pc123 optoisolator, so the optoisolator behaves like a switch (just on or off, with a resistance of between 1 and 60 ohms when on, or zero ohms when off)-so what is the circuit to do this? So when the opto is off there is no resistance .

There maybe other components (im not sure of this) required in addition to the pc123 for answering this question . I replaced the led with a pc123 optoisolator but smallest resistance I could get (directly from the phototransistor) out of it was between 5k ohms and 6k ohms and I used a 40 ohm resistor for limiting the current into the opto. If its not possible to get between 1-60 ohms , then whats the the best that can be done to control the output resistance?

here is data sheet for the opto

http://www.ges.cz/sheets/p/pc123.pdf

Also, for a different and related circuit, I want to replace the LED with a bc547 transistor to turn on a 5v relay -so what is the circuit to do this?

I dont need a fancy answer. I want an answer that show clearly how to answer the question using those components in a minimal way.

Come on folks, try and answer the question!

Answer 1

You cannot change the characteristics of the transistor inside the optoisolator. If you need to switch more current than it can handle (and you need the isolation that the opto provides), then you will have to use the opto to drive a larger transistor that can switch the load you have.

Measuring the "resistance" of a transistor C/E junction is not meaningful.

That part of your question, as posed, cannot be answered.

As to the second part -- if you want to switch a relay with the output of the comparator using a small NPN transistor is a fine choice.

Consider that you are going to want to use active-high output from the comparator in that case, so you will need to reverse the sense of the inverting and non-inverting inputs to the comparator. Simply drive the base of the BC547 through a suitable current-limiting resistor. Keep in mind - the maximum collector current is rated at 100 mA - just like with the photocoupler above, you're going to be limited how large a load you can switch with such a small device.