"John Popelish" wrote in message
...
tempus fugit wrote:
"John Popelish" wrote in message
news
tempus fugit wrote:
Here's the schematic you were asking about John. This is the section
from my
amp that is used for remote switching. The original design was to have
the
'effects 1+2 footswitch' connected to a mechanical switch, but I
wanted
to
change that to transistor switching so that I could incorporate it
into
my
pedalboard.
Where is the LED you mentioned in your post to
sci,electronics.basics,
"Thanks for the reply John.
I'm switching 12v at the collector
with 5v at the base.
The 12v is coming through a 100K resistor
from the previous device, so the current
is pretty small.
I've found that using a standard (2N2222 type)
transistor doesn't quite switch things
all the way off
( there is an LED indicator
that still glows faintly)."
I am assuming that it is something you are adding at the
transistor switch replacement part. I also need to see the
schematic for all that, to understand what you are testing.
If you just wanted transistors to replace the mechanical
switches, they could easily be 2N7000 type mosfets or
something with even higher resistance, since they only
switch the 150 uA current through the 100k resistors in the
amp. The harder problem may be keeping the off state
leakage current well below something like 15 uA to let the
effect be all the way on. If the LED gets involved in this
current, it will easily interfere with such a high impedance
deal.
]
Woops. That schem isn't covering enough ground. Here's a bigger chunk.
The
LEDs in question are marked 'Lead LED' and 'Rhy LED', and are part of
the
amp itself, not my switching system.
Not there yet. The post at the top talks about the effects
1+2 foots witch, but the LEDs are connected to the channel
select foot switch. I need to see the experimental
transistor circuit you connected to this amp that didn't act
as you expected, so I can help you figure out how to fix it.
Did you just relocate the Q7 Q8 circuit to outside of the amp?
It's getting late so I'll post more tomorrow, but I'll try to answer a
couple questions.
The 2 switches in question are the Channel select (at the bottom of the
schem - the rest of it is to the left, which I'll post tomorrow - sorry
again about that), and the volume boost (closer to the top). The reverb is
not switched remotely, since I leave it on all the time. I misposted in
saying that it is the effects 1+2 footswitch, since it is in fact the
channel select and volume boost (effect 1) that I'm actually working with.
The rhy LED is the one that is still (very) dimly lit when I switch. I
should also note that when I use a MAX4662 anaog switch
(
http://datasheets.maxim-ic.com/en/ds...61-MAX4663.pdf) (Ron = 2.5 ohms)
that the LED is completely out when switched off. I didn't relocate Q7 and
Q8 outside the amp; the LEDs are on the front panel, and if I am correct in
their purpose, they are the drive transistors for the LEDs.
You mentioned the MOSFETs as a possible alternative - this is what got me
posting in the first place, along with the other post about transistors. I
had thought of replacing my BJTs with MOSFETs, but then learned that they
shouldn't have any voltage on them before they were powered up. SInce the
possibility exists that the amp may get turned on before the pedalboard (the
switching system), that would put 12v on the drain with no voltage on
anything else until the pedalboard was switched on. Looking at the circuit,
though (which would be basically the same as my BJT circuit - using a
transistor as a switch, only with much higher resistor values) I couldn't
see why that would be an issue, since I wouldn't have any voltage at the
gate unless I was actually switching the MOSFET on anyway.
I've included a partial schematic of my transistor switching circuit. The
ones in question are in the lower left.
Thanks again, and I should have more tomorrow....