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Optocoupler Switch
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Optocoupler Switch
cackland
I'm currently prototyping an audio switch circuit, with the same characteristics as a Vactrol switch, however wanting more of a smaller compact design.

Looking for some quad smd optocoupler chips and hoping this single channel circuit would be suitable, which I can then replicate.

The time constant is dependent on the the value of C1, so that can be varied depending on my 'off' time.

The below schematic is using a Photo-Transistor Optocoupler, however I'm wondering if its better to use a Photo-Triac Optocoupler????

All feedback welcome.
Thanks

guest
optofets are the best (for some definitin of "best" - but they are low distortion) for audio signals.
cackland
Thanks guest. Found the 'H11F1SM' by ON Semiconductor.

Assuming I can replace it into my current schematic above with the correct pinout and it should work? Using 'C1' in my schematic as my timing for the off time.

Edit: Removed the incorrect circuit reference.
tarandfeathers
That's a sample and hold circuit (as indicated by the words "sample and hold circuit"). The cap is for storage, not timing, and the buffer provides a high impedance load so that the voltage on the storage cap sags at the lowest possible rate. This is a different application than what you want to do, so there's no point using it as a reference.
cackland
tarandfeathers wrote:
That's a sample and hold circuit (as indicated by the words "sample and hold circuit"). The cap is for storage, not timing, and the buffer provides a high impedance load so that the voltage on the storage cap sags at the lowest possible rate. This is a different application than what you want to do, so there's no point using it as a reference.


Whoops.. my brain completely ignored the other text and just read 'Analog Switch'. Thanks for pointing that out.
Grumble
cackland
Grumble wrote:


Saw that schematic Grumble, assume its a simple as hooking it up as shown with the audio passing in from either pin 6, out pin 4 or reverse as it mentions in the data sheet its bipolar.
guest
one thing to note, i think the H11 is only linear over about +/50mV or so, and then becomes a constant current source (varying with drive current). so you may need to knock your signal down, and then amp it back up.
nigel
cackland wrote:
Saw that schematic Grumble, assume its a simple as hooking it up as shown with the audio passing in from either pin 6, out pin 4 or reverse as it mentions in the data sheet its bipolar.

That's not a switch, it's just an isolator - the voltage at Vin (coming from the 1K pot) appears at Vout, but Vout can be on a completely separate power supply to Vin. (Although that depends on the two supply voltages, and the two 50k resistors, being matched.)
cackland
Ok, I'm well I'm lost now. Not sure how to implement this ic as a switch using my above schematic.
Grumble
Ah, sorry got carried away hyper
You can use the schematic of the first post, but switch position of the opto coupler with the resistor and use the opto fet.
HERE the datasheet.
cackland
Sorry I'm confused. So place the optofet where the 500k resistor is and place the 500k with the optofet is?

Which is the same as the high frequency diagram only with a lower resistor.

Isn't this the same as my drawn schematic above, except, I have a timing rc circuit for the internal led?
Grumble
As the title says, these are schematics for attenuation of the input signal.
I think you should use the diagram on the left, but with a lower resistor (you have to test what resistor is best).
Also you have to test what current you need for the lowest resistance of the conducting fet, also test this while the datasheet could point you in a certain direction.
Afterall you want an on/off type of schematic.

This is what i mean:

Grumble
Hmmzz... but....
When the following circuit has a low input impedance this will not work, because the input impedance of the following circuit is parallel to the impedance of the FET so if for instance the input impedance of the following circuit is 10k than the output signal is 1/2 the input signal or less.
Maybe it is better to intercange the resistor of 10k with the FET...
The off-state is minimal 300M hyper


edit: switch on = audio out on
cackland
Right, thanks Grumble. Your reference schematic is the same as mine. Just a change in type of optocoupler and physical switch type.

This circuit would be close to the input stage of a large circuit, so the impedance would be high.

I'm going to order some and test.
Grumble
The main difference between the FET and the Transistor (in this circuit) is that the FET act as a resistor and the Transistor as a diode.
cackland
Grumble wrote:
The main difference between the FET and the Transistor (in this circuit) is that the FET act as a resistor and the Transistor as a diode.


Definitely need to read more up on these things. Thanks again Grumble.
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