help with optocouplers

i am trying to switch a 12v current on and off using a 4n25 optocoupler.

12v are hooked up to the collector - but when i turn on the diode - the emitter will only give me around 5 volts. i tried switching power supplies, even gave it 24v, which resulted in a 7v output from the emitter...

i have a digital pin connected to a 220ohm resistor to the anode, cathode goes to arduino ground.

i don't think the resistor between the arduino and the 4n25 is too big, the led should light up nicely... so where is all my current going?

any help would be greatly appreciated!!


How are you wiring up the transistor side?
You might be trying to switch too much current, how much are you trying to switch?

The maximum collector current of a 4N25 is 50ma so you aren't going to get much out of it.
What are you trying to connect it to on the output side? A circuit diagram would be nice.


i am trying to switch 12v LM386amps on and off. so i'm guessing the optocoupler transistor side can't handle 12volts?

It will handle 12V perfectly well

I asked before but you seemed to ignore me:-

What CURRENTare you trying to switch?
LM386 is not a current.
How have you wired it up?

hey grumpy

i dont really know... but here is the LM schematic

is it drawing too much current?

Ok last time.
How have you wired up the opto isolator to your circuit.

hey grumps

here is what my schematic looks like right now, resulting in the puny 5v output from the opto::

i have a TIP120, and wanted to ask you - would this possibly solve my problem?

thanks for hanging in there...

Well it is better. However if you have an 8R load then you are going to be shipping a fair bit of current into the amplifier.
You can try that but you are likely to get a bit of voltage drop with that arrangement. Measure the voltage at pin 6 with a meter when you turn it on.
I would also have a large capacitor of > 100uF across the supply of that amplifier.

I would go for a PNP transistor sourcing current properly.