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View Full Version : How to make a line driver to interface a TSL230 light sensor to a Propeller?



ElectricAye
05-24-2010, 10:25 PM
Good morning,

I'm interested in making an optical densitometer array out of TSL230s (Light to Frequency converter) but I notice the data sheet suggests that if the output lines need to be greater than 12 inches, it's necessary to use a line driver or buffer. Can somebody please point me to how such a driver is designed? I'll probably need to direct the TSL230 output through about 5 or 6 feet of wire back to my Propeller, and the line driver (which I presume needs to be very close to the TSL230) needs to endure temperatures up to about 158 F (70 C) for long periods of time.

Also, would twisted wire pairs be good enough for this, or would a coax cable be necessary?

thanks for your help,
Mark
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Data sheet is attached.

Post Edited (ElectricAye) : 5/24/2010 2:41:05 PM GMT

Phil Pilgrim (PhiPi)
05-24-2010, 11:54 PM
Even twisted pair would be overkill at that short distance, I think, unless you've got a really horrible noise situation. A simple open-collector NPN transistor inverter, pulled up to Vdd at the Prop end, should suffice. I might regulate each sensor chip's own Vdd at the sensor of the cable, though.

-Phil

ElectricAye
05-25-2010, 12:24 AM
Phil Pilgrim (PhiPi) said...
....A simple open-collector NPN transistor inverter, pulled up to Vdd at the Prop end, should suffice....


Okay, I'm not experienced with designing transistor circuits, not even basic ones, but I guess it's time to learn. I'll look this up and take your advice on the local voltage regulation, too.
But if you happen to know of a chip prepackaged to perform this sort of function, I'd be much obliged to know about that, too.

thanks, Phil, you are definitely a mood-enhancing forum member,
Mark
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Phil Pilgrim (PhiPi)
05-25-2010, 01:12 AM
In lieu of a transistor, a 74LVC1G07 with a 1K pullup on the Prop end, should provide a reliable connection. A bipolar transistor driver would probably be more rugged, though.

-Phil

kwinn
05-25-2010, 01:41 AM
Since the maximum output frequency is only a bit over 100KHz you could use standard RS485/422 line driver/receiver chips.

ElectricAye
05-25-2010, 01:43 AM
Phil Pilgrim (PhiPi) said...
In lieu of a transistor, a 74LVC1G07 with a 1K pullup on the Prop end, should provide a reliable connection. A bipolar transistor driver would probably be more rugged, though.

-Phil


Thanks very much, Phil. Your advice is as good as gold.

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ElectricAye
05-25-2010, 01:47 AM
kwinn said...
Since the maximum output frequency is only a bit over 100KHz you could use standard RS485/422 line driver/receiver chips.


Thanks, kwinn, I'll have a look at that, too. I just noticed Digikey doesn't have 74LVC1G07 in a through-hole version, so I might have to check out other options.

cheers,
Mark
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StefanL38
05-25-2010, 04:41 AM
Hello Computerguy,

if the electric noise isn't too big you can use something like this
74HC/HCT7541 Octal Schmitt trigger buffer/line driver; 3-state (http://www.nxp.com/documents/data_sheet/74HC_HCT7541_CNV.pdf)

If you want super-maximum protection against electrical noise use RS485 and twisted pair cables.

best regards

Stefan

ElectricAye
05-25-2010, 10:48 AM
StefanL38 said...
....
if the electric noise isn't too big you can use something like this
74HC/HCT7541 Octal Schmitt trigger buffer/line driver; 3-state (http://www.nxp.com/documents/data_sheet/74HC_HCT7541_CNV.pdf)
...


Stefan,

Thank you. This looks like something I can easily solder and understand how to hook up. Indeed, my environment will be low on noise so I think this will be perfect.

thanks again,
Mark



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Phil Pilgrim (PhiPi)
05-25-2010, 11:25 AM
Mark,

I think you're making this harder than it needs to be. You certainly don't need an octal driver on the transmit end to handle one signal output. The other difficulty with the octal driver is its extremely fast rise and fall times which, without proper termination, could cause ringing and false counts at the receiving end. However, the 74HC7541's Schmitt triggered inputs would come in handy on the receiving end. Here's the circuit I would use:

http://forums.parallax.com/attachment.php?attachmentid=70610

The Schmitt trigger shown could be a section of the 74HC7541 or any other device with a Schmitt trigger input.

-Phil

ElectricAye
05-25-2010, 12:53 PM
Phil,

wow, thanks for going to all the trouble to draw that up. I'm embarrassed to confess I still haven't learned how to design anything with transistors so I usually mess up even the simplest of circuit designs. http://forums.parallax.com/images/smilies/blush.gif

I understand that with great knowledge comes great responsibility. I promise to learn from this inscription and to use your circuit for only good and never for evil.

I am eternally indebted to your kind generosity.

Mark


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