[sdiy] Gates and Triggers

[Veronica Merryfield]

On 12-Aug-09, at 11:55 PM, David G. Dixon wrote:

> Here's a couple of very quick questions for the "team":
>
> 1.  We've already established that 5V keyboard gates are typical.   
> What
> about triggers?  Is 5V generally appropriate for these as well?

Typically.

> 2.  Is there any problem taking these gate and trigger signals  
> directly from
> 74HC14 or CD40106 inverting schmitt triggers (through 1k current- 
> limiting
> resistors, of course)?  Bear in mind that these will be powered from  
> the
> same on-board 78L05 which is powering the matrix scanning circuitry.

The 74HC14 is speced at sinking or sourcing 25mA. The CD40106 sinks or  
sources 0.88mA Vdd=5v, 2.25mA Vdd=10v and 8.8mA Vdd=15v.

The 74HC14 would seem to be a better part, but if you need more, add a  
driver (parallel inverter stages).

Remember that adding inputs to the output will increase the capactive  
load which with the current limit resistor will add as an integrator  
slowing the edges. This is unlikely to be an issue but check.

> I'm presuming that all this is OK, and will allow me to simplify my  
> keyboard
> matrix scanning board fairly substantially, while giving me cleaner  
> gate
> signals.  Unless I hear otherwise, I will be etching and drilling  
> new boards
> tomorrow.

Promises promises!

Vrnc in overcast Port Alberni