Nuts & Volts - September 2004

Tech Forum

[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.

Dave Sarraf Elizabethtown, PA

[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 easier.

James Vaughan 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?

F o r

E l e c t r o n i c s

NUTS & VOLTS E v e r y t h i n g

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

receiver module. These receivers operate at a frequency of 40 kHz, so you will need an IR transmitter running at that frequency. The circuit shown here should produce enough power, but you