AVR-Chat

Hi Mark,

I don't know these optos but, as a general rule, there are two things 
to watch out for. Current Trandfer Ratio (CTR) and speed. CTR is a 
bit like bipolar transistor beta - it is the ratio of output 
(collector) current to input (LED) current and is commonly as low as 
20% (0.2). You mention a 330R input resistor but not the voltage of 
the receiver output. Do you have an oscilloscope to view the receiver 
output?

There are a few factors that can influence speed. A trap is to use a 
darlington output - they are very slow compared to single transistor 
outputs.

Why are you isolating the input?

I hope that helps.

Regards,
Les.

On 30 Nov 2003 at 21:28, Mark Pepper wrote:

> Hi
> 
> I am currently driving an AVR (mega 8) input from a SFH618A 
> optoisolator being driven from the output of a radio control 
> receiver. The input LED is driven from the R/C receiver via a 330R 
> series resistor and the output transistors emmitter is taken to 
> ground with a 3K3 resistor. The transistor collector is taken to +5V 
> and to AVR is driven directly from the transistor emmitter pin. The 
> LED is connected to ground on the R/C receiver. This works fine.
> 
> The problem is that I cannot find any other optoisolator that will 
> work in this circuit and cannot find any new resistor values to work 
> with the other optoisolators I have, (e.g SFH615, TLP504). The 
> SFH618 is relatively expensive you see.
> 
> Can anyone recommend a good way of choosing resistor values or point 
> me to a good "how to use optoisolator" type article please?
> 
> Mark
> 
> 
> 
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