250mW Power Amp
Colored lenses fall into two categories: diffused and clear
tinted. Diffused LEDs are most often used as indicators on
PC boards, where a wider viewing angle is more important
than illumination or color purity. Clear tinted LEDs are largely
found in instrument panels, where they display on/off status,
operating mode, and other visual information. Water
clear LEDs have the most output power, highest color purity,
and strong IR emission. The chart in Figure 3 shows a
comparison of the different LED lens types.
Power Amp for Earphone Radio
Q. I would like an audio amplifier to replace the earphone
for a battery-powered VHF receiver I have (Figure 4).
I’d like it to provide a LOUD output from an 8 Ω speaker. I
don’t want a Walkman amp because they use external
batteries. I want the amp to be powered by the internal
battery of the receiver. I have a Motorola MC34119 chip
that I’d like to use as the amplifier. Is it a good match?
A. Yes, the MC34119 is a perfect match for this portable
radio because it draws just 4 mA of quiescent current
(volume all the way down) and about 100 µA when shut
down. (Pin 1 can also be used for a squelch control.) The
component count is a sparse six — and it needs no output
capacitor! I don’t think you’re going to get the loud volume
you’re expecting, though. At 9 volts, the output power will
be about 1/4 watt. That’s loud for a radio of its size, but
hardly a boombox. Also, you need a 32 Ω speaker to get
that. With that said, Figure 5 shows the schematic you want.
For louder sound, I recommend the LM386, which
you’ll find in Figure 6. It, too, has a quiescent current of
4 mA, yet puts out three times the wattage. It does that
using an 8 Ω speaker. The downside is that, the louder you
play the radio, the faster you use up your battery.
LCD to VGA?
Q. Is there a way to attach a laptop LCD to a PC via a
video board? I have an old laptop and I’m wondering if
10k 2 5
4 LM386 .05
700mW Power Amp
I should try to sell it on eBay, donate it, or throw it away.
Recently, I’ve wondered if, instead, I should save the LCDs
and use them as monitors for other systems — like a clone
with SVGA or VGA video adapter. Can this be done without
costing me an arm and a leg?
A. I’d donate it and deduct its value from my income
tax, but your question was if it could be used with a PC
video card. No. Again, formats get in the way. If you have
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