Thread: Speed of Optos on BoBs
The particular BoB I'm playing with doesn't seem to be defective as such. (It's almost too simple not to work). I'm commenting on the difference between I/P and O/P 'scope wave forms. To test it I'm feeding it with a 555 astable on a breadboard producing close to a 50/50 square wave. I'm varying the frequency with a simple change of C. What comes out is 'orrible!
What I find odd is the channels outbound from the PC pass through 74HC14s before they get to the optos. No such buffering/inverting on the inbound direction.
not odd at all... those optos need driving and the parallel port on some PCs isnt up to the job. The question I would ask is where are the LEDs and the 74HC14 being powered from?
Also what are the pull up resistors either side of the opto?
What would you recommend for the I/P side? I.e R1.
The reason being, when you reduce R2 to a lower resistance, you increase the collector current therefore you'll need to increase the drive current (If) to the LED. Current I'd guess R1 is 1K, which gives a forward current of (5 - 1.4)/1000 = 3.6mA. with the lowest spec'd opto the transfer ratio (CTR) is 50 which will give a collector current of 130mA but obviously this is limited by R2 to (assuming Vcc = 5v and R2 = 100R) (5 - 0.4)/100 = 46mA so R1 won't need to be changed. However you may find slew rate improves if you reduce it to increase If to, say, 10mA, which needs R1 = (5 - 1.4)/.01 = 360R, so if R1 = 1k try a 560R in parallel.
The fact that these are both 1k may well be an example of offsetting cost of manufacture v breadth of applicability. by making them all 1k it reduces inventory, handling and piece cost, but means there are performance limits that are probably OK for 95+% of cases... you haven't actually said if this is giving you issues, you only asked about the likely maximum speed.
At present I'm not having issues with this kit as I'm presently trying to get acceptable (in my book "best possible") performance out of this equipment.
This takes us back to my original question where I sought the collective wisdom about the maximum clock frequency I could reasonably expect with the active devices fitted.
I overlooked to reply to your question previously about the isolated supply Vcc. It is 5v.
I've now checked the value of R1 on both the inbound at outbound channels. I've found these to both be 330R.
The 74HC14 inputs have 10K pull-ups to stop them floating.
Using ugly construction, I've just tried paralleling the 1K resistor with various values between 100R and 220R. The O/P waveform shape is much improved although reduced in magnitude to circa 3.5v pk-pk.
This is the BoB I've been referring to. (I said it was a simple design).
The plan now is to either obtain some 150R 1206 SMT resistors to mount on top of the existing 1K ones or simply replace them with 120R. With this mod applied tests indicate the O/P voltage swing will now only be approximately 3.5V. This suggests the next step will require a veroboard hack to introduce 74LS14s on all channels ensuring the I/P and O/P isolation isn't compromised.
Last edited by boldford; 24-08-2012 at 07:55 PM.
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