[7041 - July 2004]
I have three radios that use an LM386 for the audio
output. This chip tends to be noisy. (It produces audio
hiss.) Are there any circuit modifications that can be
applied to this chip to make it less noisy?
The LM386 is a very old chip that is still around
because it works well. It is also well known for generating
hiss. The most common solution is a 10K resistor and 0.01
mF capacitor in series between pins 5 and 8. This provides
feedback that reduces the high frequency response and
therefore the hiss heard at the speaker.
[7042 - July 2004]
I have been looking for years for a circuit to sense
cars in my driveway. I know that the traffic lights use a
single wire loop in the pavement and I assume that it is
some kind of tank circuit tied to a PLL. I would
appreciate any information on the subject.
#1 I worked on a loop detector design in 1967. The
technology has changed, but the theory hasn't. Design an
oscillator for about 100 kHz, with the loop as part of the
tank circuit. Cut a slot in the pavement and lay outdoor
insulated house wire in the trench and backfill with epoxy.
A few turns are all that’s needed. When a car enters the
loop, the frequency will increase by between 1% and 3 %.
The frequency will also change with temperature,
moisture, etc. The detection threshold has to be slowly
balanced in 10 minutes or so. We used phase locked loops,
but counting the frequency with a microcomputer is much
Bainbridge Island, WA
#2 Old traffic lights used the inductive wire you mention.
Modern traffic lights use video cameras to sense when
vehicles are present. The cameras are usually mounted on
the horizontal support that the traffic light is on. Most
people think the cameras are monitored by people at a
central traffic office — they are not. They are just local
cameras connected to the traffic signal control box. The
reason they went to video was that it was too expensive to
cut the road to lay the wire. Plus, there were maintenance
problems with the wire method.
If you wish to use the same method to "watch" your
driveway, there is a plug and play solution. Resources Un-Ltd. ( www.resunltd4u.com) has a VM10, which is a video
motion sensor. Attach a video camera to the unit and
whenever a change in the scene is detected, the unit will
buzz. It sells for $179.00 and also has contacts for a VCR.
Jon B. Bushey
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[6042 - June 2004]
I would like to have my garage open automatically
when I pull in the driveway. Maybe an IR transmitter
on my garage and a receiver in my car? The IR on the
garage could transmit a constant signal so that when
the receiver (in my car) picked it up, the door would
automatically open. Any suggestions?
NUTS & VOLTS
Wow! Too lazy to push a button! I understand your
need to have the receiver in the car, so only your car will
open the door. But you need a transmitter in the car and
receiver in the garage to operate the motor that opens the
door. I suggest that you remove the push button from the
remote garage door opener and replace it with an IR
operate at a
40 kHz, so you
will need an
running at that
power, but you
may need some optics to direct it to the driveway.
Circle #55 on the Reader Service Card.