■ FIGURE 3
closely controlled, I made R8 a pot.
To adjust R8, set R2 for maximum
voltage and adjust R8 for 500 volts out.
A QUDIO TO TTL CIRCUIT
Is there a 5-6 VDC circuit
which can convert audio
input (from typical line
input) to a TTL signal that
can drive a transistor or a single LED
— sort of a simple color organ?
— Robert Christopher
AAn LM393 comparator
should do the job; check
out Figure 4. The diodes
are for input protection.
I sometimes parallel the inputs and
outputs of this dual comparator for
more drive to the load, but that also
doubles the input bias current. So, in
this case, I just grounded the inputs of
the unused comparator.
Note - A larger copy of this schematic can
be viewed at the Nuts & Volts website.
www nutsvolts.com
■ FIGURE 5
28 February 2009
CO2 LASER POWER
SUPPLY QUESTION
QI have been working on a
CO2 laser engraver/cutter
project for a couple of
months now. Although I
didn't realize how little I understood
when I started this project, I have
managed to glean most of the
knowledge I needed off the web. At
this point, I have a working X/Y robot
controlled by a PIC18F2550 that also
monitors all of the safety features,
coolant flow, hood open state, etc.
However, I'm running into one major
hurdle that I'm hoping you can help
me with.
I need a power supply that can
be controlled by my microcontroller
so I can pulse the laser and control
the power output. I have been able to
find all kinds of power supply plans
■ FIGURE 4
for CO2 lasers but none of them
give me the control I desire. If I
understand what I have been reading,
I need 20 KV and roughly 15 mA. The
laser tube has a negative resistance so
current control is critical. I'm a little
fuzzy on this, but I guess it is the
current that controls the actual laser
power output, so a simple switching
power supply does not work. The
tube I am using is a 20W, sealed,
DC excited laser tube.
Any help will be appreciated.
— Jeremy
AThe negative resistance of
the CO2 laser is a control
problem, so the solution is
to put a positive resistance
in series such that the net resistance is
positive. My research indicates that
the negative resistance runs -25K to
-50K ohms. Putting 100K in series
should guarantee positive net
resistance of about 50K. The voltage
