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Subject: Off to get a Schmitt trigger chip...

Date: 08/12/02 at 9:22 AM
Posted by: Mike
E-mail: msleger@mindspring.com
Message Posted:

In Reply to: Re: umm, Schmitt what? posted by David Ashby on 08/12/02 at 8:22 AM:

David,

Thanks so much for your help. I will be including a 74HCT14 on my Digi-Key order today and when it arrives later this week I'll play with it to see if it will work. I'm using th '04 simply to invert the output of a Decoder/Driver before feeding it into a quad NAND latch to control counting direction (up vs. down). After I have a functional breadboard circuit with the 74HCT14 DIP, I'll look for a triple-Schmitt trigger chip to avoid unused gates on the board. Worst case would be using individual single-gate surface-mount components, but I'm pretty sure I can find a triple gate chip.

Would it be possible to receive a schematic of how you're saying to connect the trigger? (I'm not seeing what you mean by turning the R-C upside down.) Then I could see exactly what you mean for connections. Nothing fancy is needed, just a scanned hand sketch would work.

I really like the thought of being able to bring down the 10uF capacitor. It's the largest one in the circuit. I'm currently in the process of drafting the schematic and defining custom components (diode and transistor array chips, etc) in the software, and my hand-drawn version is already obsolete so how 'bout I send you one after I test out the Schmitt trigger. Then you can see everything in place as a final design before telling me how over-engineered the circuit is. lol

One thing I'm particularly puzzled by is how the LED array doesn't require a current-limiting resistor. The 1.7-volt LEDs are each connected to the +5 V line with no resistor, and they don't blow out or even look like they might. I suspect the current is being limited somewhere in the driver circuit, but I don't know where. When you see the driver, it might be evident to you what's happening.

Thanks again, and I'll let you know how the trigger worked.
Mike


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