■ FIGURE 2
■ FIGURE 3
for germicidal lamps, as well as photo
resists for printed circuits. My particular
application is for aquarium purification.
After so many hours of use, the UV falls
off and there is no indication of bulb
degradation until fish start getting sick.
The bulbs are very expensive — typically $45 — and the end-of-life varies from
one to another. The ability to monitor the
UV output would save lives and money.
AThese lamps emit UV in the
200 nm to 300 nm range —
that portion of the UV spectrum called UV-C. Also known
as “short-wave” UV, this spectrum includes the target germicidal wavelength
of 254 nm. An optical sensor sensitive
to this wavelength — typically a PIN
diode — is expensive to fabricate and
runs $20 and more. Fortunately, Digi-Key
(800-344-4539; www.digikey.com) stocks
the PDU-S101 from Advanced Photon-ics ($20.17) that matches the UV-C spectrum to a tee (Figure 2). It comes in a
rugged TO- 46 hermetically sealed metal can with a UV transmitting window.
PIN photodiodes are current generators, where the output current is a
linear function of light intensity. The
brighter the light, the greater the current. Because it’s easier to monitor
voltage than current, the first thing
we’ll do is convert the current to a
voltage using a simple op-amp. The
second op-amp in Figure 3 is a
voltage follower that buffers the
current/voltage converter and prevents loading of the voltage converter.
The output of the UV lamp can now be
monitored using a simple voltmeter.
A single nine-volt battery will power
the photometer for about a year if you
power up the instrument only as
needed. For 24/7 monitoring, you’ll
need a 78L09 regulated power supply.
The Range switch is used to give a
reading on the voltmeter; the 10M
position is the most sensitive with the
100K position the least sensitive.
Once a suitable range is found, use
the CAL pot to obtain a voltmeter
reading that makes sense to you. For
example, if you calibrate the output
for one volt with a fresh lamp, then a
reading of 0.5 volts means the lamp
has lost half its light output. You don’t
have to physically mount the photodiode next to the UV lamp — in fact, I
warn against it. Instead, I’d use an
optical fiber to channel the light from
the lamp to the photodiode. Just make
sure it’s UV rated, otherwise your signal will be lost. Shielding the photodiode from ambient light would also
be a smart move; black shrink wrap
tubing is a good solution.
AThis is a common request that
I’ve addressed in past columns
(e.g., May 2004), but one that’s
too frequent to ignore. So here
is this year’s take on the 15-minute timer.
This time, I’ve replaced the analog timers
with a 4060 ripple counter (Figure 4).
The 4060 includes an RC oscillator whose
frequency is set by two resistors and a
single capacitor. (You may recognize this
design from past columns, where I use
two NAND gates to create a square-wave
oscillator.) When the oscillator’s
frequency is 18.2 Hz, the Q14 output (pin
3) will toggle on and off every 15 minutes;
the 100K pot fine-tunes the timer.
TUBE AMP POWER
QI'm looking to eliminate/
minimize fluctuations in critical voltages in a guitar tube
amp. The particular amp has
■ FIGURE 4
QI am looking for
timer, but can’t
find what I what.
I need a timer that’s
powered from a nine-volt
source and toggles a relay
for 15 minutes on and 15
minutes off. I’ve looked all
over the Internet and can’t
find one. Can you help?
15-Minute On/Off Timer
January 2006 13