Nuts & Volts - September 2004

Q&A

the popular EPIC from microEngineering Labs (shop around for the best price). Kits and DIY plans for PIC burners are also plentiful.

Be aware, though, that not all PICs are created equal and the HEX code for the “Hand-held Messenger” won’t work with just any PIC microprocessor. Make sure you get a burner that supports the PIC16C84 called for in the project. Here’s a short list of suppliers:

Dontronics www.dontronics.com/ diyk96.html

up the cabin’s drafty interior. (Car heaters in those days left a lot to be desired.)

If you’d rather not waste the power, you can build a simple, step-down converter using an LM2678 switching regulator (Figure 7). The circuit was created using National Semiconductor’s Webench online design software ( www.national. com/appinfo/power/webench/). The only critical component is the inductor, which must be rated at 12 µH at 5. 5 amps, like the PM2120- 120K from JW Miller (Digi-Key #M6184-ND).

the lamp using a rather intricate formula.

To spare you the calculations, I’ve assembled a table for a few of the most popular lamps. If you don’t find your lamp listed, cross it to the voltage and wattage columns to find the inrush thermistor that’s right for you. The thermistor should be inserted between the fan and the bulb so that it doesn’t limit the fan’s cooling air.

Thanks ... For Nothing!

EPIC Plus www.jameco.com

Protecting Projection Lamps

NOPPP www.covingtoninnovations.com/ noppp

PICALL, available from Amazon Electronics www.picallw.com

Q. Can you provide a circuit to slowly power-up projection lamps in video and movie projectors? These high wattage lamps are expensive and short lived; I would bet a slow turn on would extend the life of the lamps by limiting inrush currents.

Classic Car Conversion

Mike via Internet

Q. A friend of mind has an old Ford that he restored to excellent condition. Against my advice, he converted the car to 12 volts. The original radio works fine on the 6 volts it was designed for. I can change the six parallel filaments to three sets of two series filaments (7A7s, 7B6, 7B8, 7C5, and 7Y4). I was wondering, if I hit the power transformer with short, 12 volt pulses, would it work as if it was seeing 6 volt vibrator pulses? I want to take this route before designing and building a 60 watt switching power supply.

Merlin Baker via Internet

A. I assume you are talking about tungsten filament lamps and not the halogen or metal halide versions. The answer is an NTC thermistor. This thermistor starts out with a high resistance that decreases as it heats up from current flow. You can buy them from Digi-Key for under $3.00. The trick is to match the thermistor to

Q. Some time ago, I requested a scramble/descramble diagram for my 2.4 GHz wireless video camera, but you’ve entirely ignored my request.

Then, I really wanted an USB trans-receiver with an USB module from www.linxtechnologies.com They are actually a Nuts & Volts advertiser. So much for helping them increase their sales by featuring a diagram with their USB module! Are they aware of your help in not helping them sell such modules — especially when they indirectly sign your pay check?! Once again, you ignored my request. Maybe other readers like me would have been interested in building such a project. Have you thought of that?

Then, I requested a self-powered timer for an auto ceramic heater (I even sent you a JPEG photo of the heater) to heat up inside a coil and

Lamp Type

Lamp Volts

Lamp Watts

Lamp Amps

GE Thermistor Model Number

Cold Ohms

Hot Ohms

CLS

120

300

2.5

CL- 80

47

0.6

CZA

120

500

4.2

CL- 50

7

0.13

CZX

120

500

4.2

CL- 50

7

0.13

A. When I converted my ‘ 55 Ford to 12 volts, I used a 1 ohm, 50 watt resistor in series with the hot lead of the 6 volt radio. If you remember the cars of that era, the resistor served double duty by dropping the voltage from 12 to 6 — and heating