Electronics Forum / Basics / April 2010

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I haven't heard much about television detector vans lately but we used
to be told that they could detect a television even if it was switched
off and they could even tell which room it was in.
How do such detectors work?

I was taking apart a broken rechargeable razor, and found a 0.15 uF,
250V capacitor in series with the primary of the (tiny) transformer.
The tiny <1 cu. in capacitor was to charge a NiCd battery.
What is the function of this capacitor?

Does anyone know if one can test a varactor to see what capacitance it is
exhibiting while it is being used to tune a circuit?  I would think that
hooking the meter across the varactor while it is tuned to the desired point
would show what capacitance it had at that point, but ...

I just built Velleman 8061 and got it eventually working - I think?
But I have a question.
Shouldn't I be able to see whether line is active by resistance meter
over the transistor?

I am testing several things and now I would need a
very exact current and voltage source.
It would be nice if it were correct to 0.001mV/mA.
Even more is better but I think that pushing a little.

I need some project boxes and was hoping to get advice on how to make
them.
I ruled out metal because small PCBs will be the contents of the boxes
and for obvious reasons this will not work.

There is no reason to smooth after demodulation in some low noise lock
in amplifier situations.
If you are taking the quotient of two signals that are identical
except for magnitude and noise, i.e., shape, frequency and phase

Any recommendations for wireless headphones I can plug into my radio and
listen while I'm in bed? I see Sony has a pair, but the ear muffs look
awfully big.

I would like to know if there is a relationship other then between cm
and henries in an inductor:
Here is a link where a tesla coil is made and measurements are made in
cm and converted in Henries. Any can illuminate me on this ?

Could some electronics guru please help me ? Suppose I have phase
locked loop in "locked in" state. Now suppose that I wish to use the
reference signal for some further processing ? To be more precise,
what signal has to be monitored on the phase locked loop, so that I

I have a small greenhouse which is mostly constituted by plastic. I
also have a small weather station BLUESKY BWS688, with two receptors.
The manual stipulates that the working range of the receptors is ~30
meters, but I rather think that it is ~3 meters: my greenhouse is at

How does the term invert apply?... in the
use of the phrase inverter technology
at
http://panasonic.com/inverter

The other day I hooked up a LT1016 comparator (live bug prototype on a
6” X 8” piece of copper clad)  to trigger on pulses from a PMT.
Pulses are ~ 300ns long and ~100mV high.   I sent a low level sine
wave into the input (50 ohm terminated to ground) and looked at the

My well pipe comes into the building about 50 feet from where the
electrical service comes in and the well power goes out. Rather than run
100 feet of wire that actually carries current to get to the pressure
switch & back, I've grabbed a somewhat oversized Crydom "brick" SSR to