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Subject: umm, Schmitt what?

Date: 08/09/02 at 9:39 AM
Posted by: Mike
E-mail: msleger@mindspring.com
Message Posted:

In Reply to: Re: R-C circuit didn't work posted by David Ashby on 08/09/02 at 6:43 AM:

David,

There is no board yet, so I can have any gate needed for this to work. However, I am reluctant to put a hex chip on the board when only 1/6 of the chip would be used. I'd instead use one of those small single-gate surface-mount chips.

The reason for the 100 ohm load on the emitter is to prevent an open-circuit when the transistor shuts off. The reset pin needs to be in one of two states, Vcc or ground. A resistor to ground puts the Reset pin to ground when current isn't passing through the 2N3904.

I've heard of schmitt triggers, but I don't know what they do or how they work. I'm already using two single-gate 74HC04 gates to invert decoder signals into a quad nand latch, and I prefer to use similar components wherever possible to reduce board components. (What's special about schmitt triggers for this application that a normal inverter wouldn't do? Would schmitt inverters work in place of the two inverters already being used?)

The ouput of the 2N3904 presently resets the counter 100% of the time on power-up, and the LED array is working too. The only remaining issue is the pulse length is very short (exists only while the capacitor is charging - just a quick flash). The only thing left I'd like to see before laying out the pcb is a longer reset time. Increasing C above 10uF is not an option because anything higher than 10uF causes the reset to not work. Varying R has no visible effect on the delay, and removing R completely also has no effect on the circuit. (So it's out to save one more component from the tight board space.) Increasing the length of time the 3904 is on would do this.

The present setup is like such: 2N3904 collector to Vcc, base through 10uF to Vcc and reverse-biased 1N4148 diode to ground, emitter to Reset pin, LED array, and through 100 ohm resistor to ground (brings Reset to ground when transistor is off).

I could e-mail a complete schematic if this helps. It would also provide an opportunity to review the entire design for flaws and inefficiences, if you'd be receptive to looking it over. (It's not very large, just a small LED sequencing circuit.) The schematic is only hand-drawn at the moment until the design is finalized, then I'll invest the time in doing a "professional" one in FreeHand. (No, I don't have an electronics CAD system at home for layout - I get to do everything by hand.)

Thanks,
Mike


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