easy vox circuit

[KE7TBB]

hello i have googled several vox projects and came up with some good stuff but nothing seemed to work. usually it is a op amp with a rectifier. im also limited to a lm-386, tlc-272, or lm-324 and some transistors and a breadboard. im using it to drive a 5v relay for a mini-peater. all the projects ether had feedback problems or overloaded from the am bc 1/4 mile away. o and 1 more thing it has to be somewhat easy

[KL7AJ]

hello i have googled several vox projects and came up with some good stuff but nothing seemed to work. usually it is a op amp with a rectifier. im also limited to a lm-386, tlc-272, or lm-324 and some transistors and a breadboard. im using it to drive a 5v relay for a mini-peater. all the projects ether had feedback problems or overloaded from the am bc 1/4 mile away. o and 1 more thing it has to be somewhat easy

The real trick to making Vox work seamlessly is to have a good ANTIVOX to go with it. Making a voice operated relay is trivial. I'd try to duplicate the vox circuitry from one of the many successful ham rigs rather than trying to design your own.


Eric

[K7JEM]

hello i have googled several vox projects and came up with some good stuff but nothing seemed to work. usually it is a op amp with a rectifier. im also limited to a lm-386, tlc-272, or lm-324 and some transistors and a breadboard. im using it to drive a 5v relay for a mini-peater. all the projects ether had feedback problems or overloaded from the am bc 1/4 mile away. o and 1 more thing it has to be somewhat easy

Why don't you post what you've tried, and tell us why it didn't work. It could likely be fixed with simple component changes or additional bypassing, etc.

Joe

[KE7TBB]

i dont need anti vox this is for a repeater not contesting and remember its got to be easy

[K9STH]

Question:

What are you planning on using the VOX circuit to operate? When used for activating an amateur radio transmitter a VOX is very often problematic. Getting the time constant correct is a real hard thing to do. If the time constant is too short then the transmitter drops out between words and sometimes even between sylibles. If the time constant is too long then there is often quite a delay before the receiver comes back on.

Way too many operators often keep the transmitter operating by adding some sound, often an "ahhhhh", between sentences and sometimes even between words in a sentence.

Using push to talk is usually a better method of controlling the transmitter. Many operators use a foot switch so that both hands remain free. After just a few minutes of operating when a foot switch is first installed one gets the "hang" of using the foot switch and then one performs the action necessary to activate the transmitter automatically. Over the years I have found a foot switch to be very handy even when working CW. I can immediately return to receive when the last dit or dah is sent and not have to wait for the VOX circuit to recover.

Glen, K9STH

[KE7TBB]

this is what i tried

[KE7TBB]

im using it to drive a 5v relay for a mini-peater. interfacing receiver to transmitter

[K7JEM]

this is what i tried

And did it not work at all, or was it plagued by AM interference? You might try bypassing the input and voltage line with some .01 caps or so.

Joe

[KE7TBB]

did not work and i built and rebuilt it 10 or more times the circuit makes sense but when i put a speaker on the output all get is feedback eben the feedback should key the relay but it doesnt

[K7JEM]

I would use an LM324 as an amp, then rectify it, then feed it to a switching transistor to drive the relay. Your 386 amp probably has too much gain, but I don't know why it won't pull the relay.

[KE7TBB]

what kind of diodes shhould i be using? do i need to have a full bridge and not just a single diode

[KE7TBB]

i would do the circuit in the arrl 1994 hand book ch 14-6 fig. 6 but i dont have a negative 12v power supply

[KE7TBB]

come on you guys left me hanging i tried the arrl HB circuit and just my luck
NOTHING!!!!!!

[K7JEM]

My first question is what type of radios are being used, and is the intent to make a repeater out of them? This can be done pretty easily, but a relay isn't really needed. Basically, all the VOX is doing is supplying a high audio amplification, then rectifying that signal, and applying it to a switch or relay.

I don't know why the circuit shown is not working, possibly it is oscillating on it's own due to some design issues. But something like that should work.

A couple of things that don't look right with the 386 circuit shown are that there is usually a .05 uF cap in series with a 10 ohm resistor from the output to ground, and the + input should not need the input coupling cap. Those things could be causing the oscillation.

Joe

[VK2AKG]

some things to consider:

- power supply bypass cap at or near the chip
- Zobel network pin 5 to ground per K7JEM (this and supply bypass can stop the chip taking off)
- RC low pass on input if leads are long or unshielded (LM386 can easily amplify stray RF) place it between the 10k pot and pin 3 and roll it off around 1.5 kHz
- output impedance of audio source (if high Z then Vin can droop when the relay pulls in and places 1k8 to ground)
- remove the cap from pin 1 to pin 8 (this drops gain from 46 to 26dB), maybe you can use this cap between pins 4 and 6 (for power supply bypass)

There is probably more I should write but it is 1AM and I'm writing this in my sleep (hence I may have said something stupid).

73 Frank vk2akg ZZZzzzz.....

[W8XV]

i would do the circuit in the arrl 1994 hand book ch 14-6 fig. 6 but i dont have a negative 12v power supply





It doesn’t get any easier than this. Crude but simple. This will work well for what you want to do. Now you can build the one in the ARRL handbook.

T1 Radio Shack http://www.radioshack.com/product/index.jsp?productId=2102703
D1 Radio Shack http://www.radioshack.com/product/index.jsp?productId=2062580
C1, C2, 1000uf Radio Shack http://www.radioshack.com/product/index.jsp?productId=2102507
C1, C4, 10uf Radio Shack http://www.radioshack.com/product/index.jsp?productId=2102510
U1, Radio Shack http://www.radioshack.com/product/index.jsp?productId=2062600
U2, (7912CT) This isn’t listed on their website, but I have found them at my local Radio Shack.
U1, U2, Heat sink, http://www.radioshack.com/product/index.jsp?productId=2102857
Heat sink compound, http://www.radioshack.com/product/index.jsp?productId=2102858

[KA8RAW]

I home brewed a VOX many years ago, If memory serves me correctly, I got the schematic from the 1976 or 1978 radio amateurs handbook. Somewhere in all my notes, I still have the schematic. If I can locate it, I'll post it here.

[KE7TBB]

ill make it simple i need a circuit that when i feed line level into (like headphones) it keys a relay and the relay keys a ptt to ground and yes its for a repeater. Parts: 5v relay and lm-386, tlc-272, or lm-324 and some transistors and a breadboard caps and resistors and id rather try a new circuit than shoot the old one:confused:

[K7JEM]

ill make it simple i need a circuit that when i feed line level into (like headphones) it keys a relay and the relay keys a ptt to ground and yes its for a repeater. Parts: 5v relay and lm-386, tlc-272, or lm-324 and some transistors and a breadboard caps and resistors and id rather try a new circuit than shoot the old one:confused:

You can try this, it should work for you:

[KE7TBB]

its a separate ptt line

[AI3V]

Holy open loop batman!:eek:

With pin2 grounded like that you have full gain of the chip.

Try a 10KΩ from pin 5 to pin 2, (remove ground from pin 2)

Rege

Edit: the datasheet (http://www.national.com/ds/LM/LM386.pdf) also talks about a resistor between pins 1 and 8 for gain control.

[KE7TBB]

600 z to the tx 8z to the rx ima use a isolation transformer