Nuts & Volts - January 2006

■ FIGURE 2

■ FIGURE 3

Range

10M 1M 100K

100pF

UV-C Photometer

+9V

DVM

DC Volts

CAL _1K500

+

PDU-S101

LMC662

500 1K

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.

Charles Forman

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 REGULATION

QI'm looking to eliminate/ minimize fluctuations in critical voltages in a guitar tube amp. The particular amp has

15 MINUTES, AGAIN AND AGAIN

■ FIGURE 4

+9V

16

QI am looking for a 15-minute 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?