At 3:55 PM -0600 4/23/03, Scott Juskiw wrote:
>The op-amp driving the Random output has a cap in the feedback loop
>which turns it into a lowpass filter (corner frequency about 10.6 Hz
>according to my calcs). With R11=100K and R12=150K the gain is
>-R12/R11 or -1.5, but only for DC and very low frequencies (below the
>cutoff point). Higher frequencies have a much lower gain. The Pink
>output that feeds the lowpass filter has 8 Vpp levels but everything
>over 10.6 Hz gets significantly removed by the filter. In order to
>get a decent Random output level R11 needs to be lowered. I put a pot
>in for R11 and watched the Random output on a scope until I was
>getting, roughly, a 10 Vpp signal on the output, then I measured the
>pot value and it was very close to 27K, so that's what I used for R11.
Doh!! It would have been easier if I wasn't trying to use the formula
for a non-inverting amplifier!!
The only 27K I could find is this 1/2W carbon resistor that is too
big for the hole spacing (from an assortment box I bought from Radio
Shack eons ago). While I'm sure a 20K would work, after reading
Paul's explanation, I think I'll choose a compromise value, like 47K.
I have those :)
So on to the external clock mod. Luckily most MOTM modules have high
input impedances. The average output impedance seems about 1K (eg.
320, 800). One source said that a TLC555 could output 10ma, another
15ma. The output voltage is around 14V, which is too hot. If I put
one 1K from the EXT CLOCK, and another 1K from the new clock out jack
to ground, that should be 7V at 7ma, which should work.
Of course, given my recent luck with math, that's probably wrong :)
The other option would be to replace the TLC555 with a regular 555
that can flash lights and ignite blasting caps, and increase the size
of its bypass cap.