signal processing component values?

yarisboy

I'm building a tachometer with both analog and digital readouts. The signal from the car is a 12 volt square wave from the OBD-II port. I soldered a 2.7 volt zener diode on the board fed from a 10 K resistor. Will the input on my Propeller like this signal level?

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MOORE'S LAW: The capabilities of electronics shall double every 18 months.
cloyd's corollary: Hardware is easy, software is hard.

Post Edited (yarisboy) : 12/6/2009 12:24:32 AM GMT

Leon

From the data sheet (page 26) Vih is 0.6Vdd min. 0.6 * 3.3 = 2.58 V, so you should be OK with 2.7 V input if your supply is 3.3 V. If the zener diode tolerance is the usual +/-5%, you could have a problem with a worst case device (2.565 V).

You might need to put more current through the zener diode.

Leon

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Amateur radio callsign: G1HSM

Post Edited (Leon) : 12/6/2009 12:56:25 AM GMT

Phil Pilgrim (PhiPi)

You could also use an optoisolator, like an H11L1. They're not expensive and will give your Prop circuit some additional protection.

-Phil

yarisboy

Thanks Phil,
I put the data sheet on my desktop for the next parts order. I've already got 2 "oops wires" on the bottom of the first gen layout. I've been upgrading the layout as I go on this first assembly, so incorporating this 6-pin chip will be easy on version two. The dual display Propeller tachometer is looking good so two of these three prototypes go in the Santa bag. One goes in my car. When I figure out the quirks of posting an image file I'll post. My first attempt was a bust. I just have to determine what value of current limiting resistors are best for my display then its "let the programming begin" time. My photo shows the Jon Williams Propeller Platform (Gadget Gangster) with my board (that plugs into the top) next to it. Neat!
Stan

Leon,
I'll drop the value of my current limiting resistor down for these first three protos and just let the zener heat up a bit more as a quick fix. On the other two boards I'll slip in 3.3 zeners. I need to put a parts order together as noted above. I'm using a Propeller to simulate my car tach signal and feeding it to my prototype. Makes me glad I'm using a crystal.
Thx all,
Stan

P.S. This silly system keeps telling me I don't know how to spell zener.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
MOORE'S LAW: The capabilities of electronics shall double every 18 months.
cloyd's corollary: Hardware is easy, software is hard.

Post Edited (yarisboy) : 12/6/2009 1:47:35 PM GMT

Leon

Choose a resistor value that gives you the current specified in the zener diode data sheet. You can add zener to the dictionary, if the spelling checker annoys you.

Leon

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Amateur radio callsign: G1HSM

yarisboy

Quite correct. This isn't a breadboard build out of the junk drawer. I've got a calculator around here somewhere..............
I've got the opto laid into the board and still have good component & lead spacing. One good usage of the jumper cables could have wiped out my Propeller. The power dissipation rating of the 2.7 V Zener is on the package but while I'm out xmas shopping with the ladies today I'm picking up some 3.3 or 3 volt zeners. The analog LEDs I'm using are clear and pump out enough light for clear visibility in the typical Phoenix summer day. That's why they get their own dimmer pot on the CC return. They suck so much current I've got Op-Amps in there that can source 160 ma per output. That means the feed to my board should be fused at 5 amps.
The whole shebang gets the high temp field test this summer. I've got the typical gangsta window tint but you still can't touch the wheel here in summer until the AC catches up when parked.
Thanks again all.
Stan

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
MOORE'S LAW: The capabilities of electronics shall double every 18 months.
cloyd's corollary: Hardware is easy, software is hard.

Post Edited (yarisboy) : 12/6/2009 4:31:37 PM GMT