Interfacing a loudspeaker priority switch

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Bingo600

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Posted by Bingo600: Sat. Aug 9, 2008 - 11:57 AM

Guyzz

I have a friend that has a boat , and he would like to mount a loudspeaker in his cokpit.

The loudspeaker would play FM-Radio , but when there is activity on his VHF radio he would like it to automaticly come out of the same loudspeaker.

The VHF has a 3watt external speaker connection , and it is there i have to conecct some kind senso circuit to detect activity , above squelch level.

I was thinking of something along the input circuit of this velleman kit.
And then connect it to an AVR that activates a relay , with a timer that keeps the relay (VHF) active for some adjustable delay 30sec ... 2 min. But it seems like this one requires 2 watt to activate , and was hoping for some suggestion to an appropriate input sensor circuit , to the VHF radio.

Maybe a cap in front of the resistor (i just need to trig on ac activity) , and a lower resistor value , and maybe a zener along with the 1n4148 or ????

http://www.vellemanusa.com/us/en...

Schematic http://www.vellemanusa.com/downl...

Any hints are welcome.

/Bingo

Tags:

Learning and Information, General Electronics

DocJC

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Posted by DocJC: Sat. Aug 9, 2008 - 06:22 PM

Hello Bingo,

Fun Project. The Velleman kit is simpler than my approach, but my approach includes a uC!

How good are you with tinkering with op amps?

I'd connect to the VHF radio audio to a 600 ohm to 600 ohm (physically small) coupling transformer. I'd level clamp the output of the transformer, and feed an op amp with the audio signal. I'd feed the audio through a comparator. Low level audio, i.e. background hiss, comes out of the comparator as a low. Speech comes out of the comparator as a string of pulses, (0/5V), roughly matching the audio.

Use a Digital Input on the uC to watch the comparator, if it is high, then the uC drives a transistor to drive a relay to connect the VHF radio signal to the speaker, and disconnect the AM/FM signal from the speaker.

In practice you want to watch for several High pulses to make the decision that the VHF signal is active. You will miss the first few mSec of each transmission, but not generally a problem.

Once triggered, the uC just watches the line for the end of the transmission, and incorporates a timeout delay before returning to music.

The VHF radio may need a power resistor, ~100 ohms, as an alternative load to keep the audio output amplifier happy, when it is not driving the speaker, and to give you a constant load on the radio to monitor for transmissions.

The "nice" thing about this approach is that you are converting the audio to a digital signal for the uC to read. You just need one input bit to read the audio, and no ADC is required.

An option is to feed the isolated, clamped, attenuated audio into an ADC input, and watch for the presence of an audio signal, above the noise floor. Then trigger the relay, speakers, etc.

Analog front end, or digital, both are possible. Either way, however, you need to isolate, clamp, and attenuate your input signal before processing it, to protect your uC.

Bingo600

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Posted by Bingo600: Sat. Aug 9, 2008 - 06:51 PM

Hi Doc

There might be some hints for me here also
http://wolfstone.halloweenhost.c...

But else ...
So i'd rip out a transformer from an old phone/modem.

and ???

I'm not an "analog guy" , so this is where i whish i were :-)

But i have a "kazillion" lm324 in smd , and some hc4050 (15v input)

/Bingo

DocJC

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Posted by DocJC: Sat. Aug 9, 2008 - 07:06 PM

Bingo,

Can you read a MS Powerpoint file?

I don't think it will upload here...

I have a Basic Stamp - I controller simplistic radio repeater, with a digital pot controlled AGC, circuit I could e-mail to you. You could eliminate 90% and use in input stage and the relay driver stage, at least as a starting point. The schematic was drawn in PowerPoint, I don't have it in another format...

jayjay1974

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Posted by jayjay1974: Sat. Aug 9, 2008 - 11:02 PM

Is a uC really needed here? :? I guess a clever 555 timer circuit is all that's needed, or a couple of simple CMOS gates or opamps.

DocJC

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Posted by DocJC: Sun. Aug 10, 2008 - 01:36 AM

JayJay,

Agreed, the entire project could be done in analog, without a micro.

But bingo notes that analog is not his area of expertise. And replacing a 555 with a Tiny is easier these days.

microcarl

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Posted by microcarl: Sun. Aug 10, 2008 - 02:17 AM

I'm thinking that an LF355 set up as a comparator that drives a capacitor on pin 2 of an NE555 (or the CMOS version) that is set up as an monostable (One-Shot) timer. The reference input of the LF355 would be biased at 1/2 VCC.

The idea is that when the squelch of the transceiver activates the audio, the input of the LF355 will cross the reference input, triggering pin 2 of the NE555 one-shot. Of course, then pin 3 of the NE555 would turn the relay on for the predetermined time, disconnecting the FM radio audio output from the speaker, and connecting the FM transceiver audio output, instead.

This is actually a fairly common circuit. I'm sure a Google on NE555 or SE555 will cover the monostable configuration.

In fact, the first page of a Google search on the LM555 reveals the following link

See figure 3 & figure 28 in the following link.
http://www.unitechelectronics.com/NE-555.htm

If the AC noise from your finger in the "Touch-Plate " circuit of figure 28 can activate the NE555 monostable output, then a capacitively coupled audio signal surely should. But realize, that for your application, you'll want the circuit configuration of figure 3.

I hope this helps.

EDIT:
The NE555 timer and I go back to about 1976, or so. Since that time, I've used and relied on a book that goes by the title:

Quote:

"The 555 Timer Applications Sourcebook, With Experiments. Techniques, applications, and Experiments using the 555 IC timer "
By Howard M. Berlin, 1976.
Published by E&L Instruments, Inc.
61 First Street
Derby, Connecticut 06418
Telephone: (203) 735-8774

The book carries no ISBN number, and I don't know if E&L Instruments is still in business.

Yes, a Google gives the following web-link:
http://www.elinstruments.com/

This is one of the few books that I've actually worked through most of the experiments.

And then, to my surprise, I also found this:
http://bugbookcomputermuseum.com/The555Timer.html
Which is a newer version of the copy that I have.

I can only assume that this is the most accurate ordering information...

Quote:

"The 555 Timer Applications Sourcebook, With Experiments. Techniques, applications, and Experiments using the 555 IC timer "

ISBN: 0-672-21538-1

Authors: Howard M. Berlin

Publisher: Howard W. Sams & Co., Inc.
4300 West 62nd St.
Indianapolis, IN 46268

Copyright: 1976

In my copy of the book, there are several circuits that could easily meet your requirements, without modification.

You can avoid reality, for a while. But you can't avoid the consequences of reality! - C.W. Livingston

Bingo600

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Posted by Bingo600: Sun. Aug 10, 2008 - 08:50 AM

DocJC wrote:

Bingo,

Can you read a MS Powerpoint file?

I don't think it will upload here...

I have a Basic Stamp - I controller simplistic radio repeater, with a digital pot controlled AGC, circuit I could e-mail to you. You could eliminate 90% and use in input stage and the relay driver stage, at least as a starting point. The schematic was drawn in PowerPoint, I don't have it in another format...

JC

@Doc

Yepp OpenOffice can read .ppt fine ....

@jayjay & Carl

I have a few Tiny13 just waiting for this project (This is AVR freaks :-) ). And they have an analog comp. That i could use.

I just need the "analog" part ...

/Bingo

johnrk

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Location: Missouri

Posted by johnrk: Sun. Aug 10, 2008 - 04:07 PM

Hi Bingo,

How about the Maxim 4905?

John

My thought was that maybe you could use the internal comparator with different voltage levels? to "trick" it into sensing the VHF audio??

Attachment(s):

Here is a screenshot...I had to search for the datasheet:-)

Just some guy

emuler

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#10

Posted by emuler: Tue. Aug 12, 2008 - 02:51 PM

Some VHF sets have an LED that lights up when a signal is received. Detecting that might be even easier. Otherwise a simple peak detector across the speaker leads driving a schmitt trigger gate or op-amp or 555 should do the job. If you want to use a uC, any Tiny AVR with an ADC should be sufficient.

If you think education is expensive, try ignorance.

jayjay1974

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#11

Posted by jayjay1974: Tue. Aug 12, 2008 - 02:59 PM

Or use the analog comparator, which is even available on a (obsoleted) tiny11; but even a tiny11 is overkill IMHO :P

I think it would be more fun to design a circuit that only uses transistors instead ;) An opportunity to learn a bit more about analog stuff.

jayjay1974

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#12

Posted by jayjay1974: Tue. Aug 12, 2008 - 05:30 PM

Just for fun I designed a little circuit, see attachment...

Only disadvantage is that it will turn on the relay for the delay time after power on before it falls off again.

Resistor R10 sets the time that passes before the relay is turned off after the input signal has gone away. The relay will is turned on as long as there is a signal on the input. Minimum input voltage can be set with R9 (100K=~1.2V).

R8 is the relay.

How well it works in practice I don't know; I just simulated it :)

Attachment(s):

transistor_based_radio_switchover_circuit.png

Bingo600

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#13

Posted by Bingo600: Wed. Aug 13, 2008 - 10:36 PM

emuler wrote:

Some VHF sets have an LED that lights up when a signal is received. Detecting that might be even easier. Otherwise a simple peak detector across the speaker leads driving a schmitt trigger gate or op-amp or 555 should do the job. If you want to use a uC, any Tiny AVR with an ADC should be sufficient.

@emuler
I have a Tiny13 waiting for this :-)

But guess i need a circuit to protect the ADC from the voltages on the speaker leds.

DocJC was kind enough to mail a schematic , w. a 600ohm transformer and some opamps. But i was hoping for something simpler :-)

/Bingo

emuler

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#14

Posted by emuler: Thu. Aug 14, 2008 - 07:14 AM

A simple voltage divider should be sufficient to bring down the voltages to safe levels. The speaker is probably driven in a bridge configuration (both leads are driven, in opposite phase) so you only need to monitor any one speaker lead.

Adding a diode and a small capacitor will make a working peak detector. Remember to put a resistor across the capacitor to let the charge bleed away, or the circuit will never reset itself.

The ADC method is probably too elaborate for this application (you just need to convert the peak detector output to a logic signal) but hey, it's there, so it's already paid for. ;)

If you think education is expensive, try ignorance.

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