portable headphone mixer/switcher

[Doug Kite]

I would like to combine the headphone outputs from a small fm radio, a
FRS two-way radio, and a scanner into one heaphone out. So basically,
you would hear the radio signal unless someone transmitted on the
two-radio or the scanner picked up a signal. The system must be small
and portable, preferrably powered by a 9-volt battery.

I found many examples of small portable headphone amps with an LM386. My
original idea is just to use a simple passive resistive mixer on the
inputs, run the output of that mix through an LM386 amp like so:

input1---/\/\/\-----+
|
input2---/\/\/\-----+---LM386 amp------headphones
|
input3---/\/\/\-----+

What I would *really* like to do is have it so that there is a relay or
vox-type circuit that would "prioritize" the inputs. In other words, you
would hear input3 as long as neither of the other two had signal. If
input2 activates, cut off input3 until that signal is gone. And if you
get signal from input1, cut off both 2 and 3 until it's done. This would
be better than hearing a mix of the signals. Any ideas on how to build
such a circuit?

Thanks,
Doug

 

[John Fields]

I would like to combine the headphone outputs from a small fm radio, a
FRS two-way radio, and a scanner into one heaphone out. So basically,
you would hear the radio signal unless someone transmitted on the
two-radio or the scanner picked up a signal. The system must be small
and portable, preferrably powered by a 9-volt battery.

I found many examples of small portable headphone amps with an LM386. My
original idea is just to use a simple passive resistive mixer on the
inputs, run the output of that mix through an LM386 amp like so:

input1---/\/\/\-----+
|
input2---/\/\/\-----+---LM386 amp------headphones
|
input3---/\/\/\-----+

What I would *really* like to do is have it so that there is a relay or
vox-type circuit that would "prioritize" the inputs. In other words, you
would hear input3 as long as neither of the other two had signal. If
input2 activates, cut off input3 until that signal is gone. And if you
get signal from input1, cut off both 2 and 3 until it's done. This would
be better than hearing a mix of the signals. Any ideas on how to build
such a circuit?

 

[Rich Grise]

I would like to combine the headphone outputs from a small fm radio, a
FRS two-way radio, and a scanner into one heaphone out. So basically,
you would hear the radio signal unless someone transmitted on the
two-radio or the scanner picked up a signal. The system must be small
and portable, preferrably powered by a 9-volt battery.

I found many examples of small portable headphone amps with an LM386. My
original idea is just to use a simple passive resistive mixer on the
inputs, run the output of that mix through an LM386 amp like so:

input1---/\/\/\-----+
|
input2---/\/\/\-----+---LM386 amp------headphones
|
input3---/\/\/\-----+

What I would *really* like to do is have it so that there is a relay or
vox-type circuit that would "prioritize" the inputs. In other words, you
would hear input3 as long as neither of the other two had signal. If
input2 activates, cut off input3 until that signal is gone. And if you
get signal from input1, cut off both 2 and 3 until it's done. This would
be better than hearing a mix of the signals. Any ideas on how to build
such a circuit?

The logic shouldn't be hard - take what you've got there and make
a truth table, then just slap a few gates together, like ands and
inverters, the first one inhibits the next, and so on.

inputs outputs
A B C Qa Qb Qc
0 0 0 0 0 0
0 0 1 0 0 1
0 1 0 0 1 0
0 1 1 0 1 0
1 0 0 1 0 0
1 0 1 1 0 0
1 1 0 1 0 0
1 1 1 1 0 0

Hey - that was easier than I thought. ;-)

And of course, they come from three little VOXes with logic outputs,
which also control analog switches.

Have Fun!
Rich

 

[John Fields]

The logic shouldn't be hard - take what you've got there and make
a truth table, then just slap a few gates together, like ands and
inverters, the first one inhibits the next, and so on.

inputs outputs
A B C Qa Qb Qc
0 0 0 0 0 0
0 0 1 0 0 1
0 1 0 0 1 0
0 1 1 0 1 0
1 0 0 1 0 0
1 0 1 1 0 0
1 1 0 1 0 0
1 1 1 1 0 0

Hey - that was easier than I thought. ;-)

And of course, they come from three little VOXes with logic outputs,
which also control analog switches.

 

[Rich Grise]

Well, we don't like to do homework for people, but since it's not
exactly homework, and it's evident from the snottiness of your remark
that you're not competent to come up with the logic to implement the
truth table as shown, I'll help out:

A ----+-------------------------------Qa
|
+--|>o---+--|
| |AND --------------Qb
B -------------)--|
| |
| +--|
+--|>o------|AND ---------------Qc
C ----------------|

Cheers!
Rich

 

[John Fields]

Well, we don't like to do homework for people,

---
"We"? who the **** made you the spokesman for everyone here?
---

but since it's not exactly homework,