Two PCBs off of one power source

[CALAHAN]

Hi,

I am trying to run a FRS radio (3 AAA's) and a simple printed on board chip sound effects circuit board (generally 2 AA's, but will work with 3 AAA's it seems without frying it, and it gives me added volume which I needed), off of the same 3 AAA power supply. I was told to use a 10 ohm resistor in line with the power end of the sound effects PCB off the positive battery post, and I get reduced volume and distortion. I was then told to use a silicone diode, and that resulted in reduced volume, and the radio not working at all (the radio is not hooked to any of these components by the way). I also tried a Zeener glass diode, and that seems to have fried the receive end of the FRS amplifier. I have tried a few different values of resistor and I can only get low and distorted sound out of the sound effects PCB (mind you that I am over volting this thing, and it is still too quite). Does anyone know what I can use and where to put it, so both of these devices can run off of one power source? The sound effects PCB does not run continuously when the radio is powered on, but it will be used intermittently on occasion when the FRS is on.

Thanks for any help.

Bruce   

[CALAHAN]

I thought this was a simple question. No-one wants to put in their 2-cents as to what to put in the middle to get two electronic devices to run off of one power supply so they don't fight each other for amperage and/or voltage?

Tonight I will attempt the 10 ohm resistor on the FRS posistive to the bateries and see if that works rather then on the sound effects chip. Then perhaps the silicone diode also if the resistor test fails to meet expectations.

Bruce

[audioguru]

Of course they can operate from the same battery power supply.
But it sounds like you are shorting their outputs together that will damage one or both of them.
You should never short outputs of circuits together, even if one of them is turned off.

Add a mixer circuit for both circuits to feed one amplifier/speaker.
Add a power amplifier to the output of the mixer if you want more volume.

[CALAHAN]

Are you saying that I need only to put a transistor at the connection of one of the two circuits at the speaker?  I don't need any amplification, and the two circuits only share a common battery and speaker.

If so, what should I use as a transistor 10k ohm? Since the FRS already has a volume pot, I guess I would put the transistor on the sound effects circuit to the speaker - right?

Bruce

[audioguru]

Two circuits cannot share the same speaker. They will get damaged.
The circuits feed a mixer circuit then the mixer circuit feeds a power amplifier circuit that drives a single speaker.
You can make a mixer circuit with 2 resistors. There are a few low-power power amplifier ICs that work with a supply voltage as low as 4.5V.

[CALAHAN]

I am going to try the two resistors tonight. I found one web page that specified 10k ohms, but did state that this may knock down some of the high end off the sound. If this is no good, then I guess I will try a lower value reistor. As for the low power IC amps. I do not need an amplifier. The volume level is good enough as it is. On top of that, I had experimented with these amps months back, had spent a considerable amount of money and time, and in each case they did not work. I do not have room for them in any case. Both of these devices are already amplified on board, though the sound effects circuit board uses a simple transistor as an amp which is not very powerful. I am told that doubling up on amplifiers does not usually work, and from experience, that seems to be correct.

Thanks,

Bruce

[audioguru]

Bruce,
Each circuit has its own output amplifier to drive a speaker. You cannot short the outputs together to drive a single speaker. Use a separate speaker for each circuit.

I suggested adding resistors in series with the output of each one to isolate them from each other. The resistors become a mixer. The resistors cannot drive a speaker because the level will be much too low. If you select a lower value for the resistors then the amplifiers in the circuits will become shorted together again. The resistors are supposed to drive a power amplifier that should work perfectly if it is connected properly.

[CALAHAN]

Audioguru,

I don't have room for two speakers.


I tried to insert a few different values of resistor in the + side of both circuit's speaker lines. I even used a variable resistor pot on each and played with them for some type of result. Neither will share one speaker, no mater what resistor I use.

I have very little room, and do not know why I need a power amplifier when the volume is fine as is. The FRS has within it a variable resistor volme pot as it is, so I am just doubling the resistance for it's output by adding yet another one, and quite frankly it needs all the volume it can muster. The other circuit will give me sound at about 10 ohms to probably 100. 1k and 100k  - nothing (that is without the other circuit wired in - that is stand alone).

These two circuits will apparentley not allow my to share a speaker when hooked top one power supply - so I have went back to my original configuration where one circuit uses 3 AAA's and the other uses what it needs in way of Lithium coin cells. They share a speaker, with no resistors between, but they have their own power supplies also. This works very well, and I will leave it as it is. If it ain't broke don't fix it as I like to say.  I only hoped to save a little room inside the device I made.  I am going to make a second one like this, and I guess I will have to - once again, use two power supplies.

Thanks,


Bruce

[CALAHAN]

Audioguru,

I decided to take the time and read through the internet, and once again purchase more components to try what you suggested. What you are saying is that the + outputs of these speaker wires need to be brought down to line level and then fed into an amp to bring the volume back up again. Bringing them down to line level with resistors creates a mixing circuit where both circuits are separated from each other from shorting.

I tried putting a 10k ohm resistor in the + end of the speaker outs of these two devices, with the - going to ground and even tried a more complex (using one 10k and one 1k resister to create a split) idea where the outputs of these two devices were to be brought down to line level.  I then tried the LM386, and the MC34119, and even played with an TLO82. None of these configurations worked - no sound - nothing.  I tried different values of resistors, capacitors, removing capacitors, and resistors etc...

The TLO sounded promising, but there are no schematics that I could find to wire two circuits, at either line level or amplified output voltage, into this thing and them to a single speaker. I did searches for hours on that thing, and found nothing. The data sheet schematics on it are no help. 

Speaking just of the sound effects circuit, I used the schematics you drew up for me with the MC34119 months ago, and did get a bit of sound out of it, but too low in volume. I even tried putting a 10 ohm resistor at the ground of the circuit's main, and it helped a bit but really only cleaned the sound up a bit, but no added volume.  When I tried adding the FRS to it, the radio would not light up. They seems to fight for power (remember that all of these devices are wired to the breadboard of the 4.5 volt power rail). I tried a 10 ohm resistor on the sound effects circuit negative along the FRS, and still nothing.

These IC's, the sound effects circuit, and FRS have to be powered off the same 4.5 volt bank of AAA's. None of the configurations work. I tried everything I could find on the internet.

I would be willing to pay you, or someone else if I could get a working schematic to work for this project. Give me a quote privately if needed.

As it stands now, my next build I do with this project will have two speakers in it. I'll have to do some grinding to make a second speaker fit as I found that they have to be a certain size in order to maintain decent volume (about one inch diameter minimum).

Question: Currently my project uses the same speaker for the FRS and the sound effects, but different power sources. Will this still eventually fry one of the two amps?

If I were to trace through this small toy sound effects circuit board and jump the sound right where it comes off the POB chip, before it gets to the amplification transistor, would this do anything for me?  Of course this could not be done for the FRS radio.

If you'd like to see what my project is, take a look at these photos.  This thing works quite well with a custom made internal antenna, but the only issue hanging me up is this sound effects mess. I have been on this for a year, and have about $1,000  sunk in the research. Needles to say I have a bunch of expensive and useless garbage sitting here.

Thank you,

Bruce


http://i129.photobucket.com/albums/p209/trma1/GEFRS01.jpg
http://i129.photobucket.com/albums/p209/trma1/Finishingup03.jpg
http://i129.photobucket.com/albums/p209/trma1/CIMG0319.jpg

[audioguru]

We are just guessing at the circuits.
Maybe the speaker is a transistor'e AC and DC load. Then the transistor won't work properly with a 10k mixing resistor, it needs 8 ohms then a mixing resistor.
If the outputs have DC on them then they need coupling capacitors to block their DC from messing up the input of a mixing amplifier.

Post their schematics if you have them.

[CALAHAN]

Audioguru,

Sorry for my inability to create a proper schematic, but this is basically a trace of what I have for this toy sound effects chip.

The colored boxes (other then black) are for wires which led to switches and LEDs for other sound effects which I will not be using. I clipped those wires.

P+ and P- are the power lines

SW indicates switch pads and you can see the corresponding negative pads that surround them. The diaphram covers were removed. I used SWR in the term list which is wrong. It should be "SW".

473 CAP is a capaciter with 473 written on it. and 100uf is of course one of those cylinder capaciters.

the rest is self explanatory. Thanks for taking the time on this one.

Don't forget that I need to power this, any Op Amp needed, and the FRS off of one 4.5 volt battery bank.

Bruce

[audioguru]

Hi Bruce,
The speaker connects directly to the S8050 which is a PNP transistor and to the positive supply voltage. So the speaker is the DC load for the transistor. The speaker must be replaced by a resistor with the same resistance then the output can be capacitor coupled into a resistor that is the input of a mixer.

The output of the FRS radio will need to have the same analysis. Then both signals can feed mixing resistors at the input of an MC34119 power amplifier.

[CALAHAN]

Audioguru,

Thanks, but what kind of capacitor, and where does it go?


I put an 8 ohm resistor at the end of the speaker +, then a ? capacitor, then a 10k ohm resistor, then into pin 4 of the MC34119.


For the FRS,  speaker +, 8 ohm resistor, ? capacitor, 10k ohm resistor, MC34119.

Both the FRS and the effects can be combined at pin 4 of the MC34119?  all grounds to the ground rail.
 

Currently my project uses the same speaker for the FRS and the sound effects, but different power sources. Will this still eventually fry one of the two amps?

Thanks,

Bruce

[audioguru]

Hi Bruce,
You showed the parts layout of the sound effects unit. Its speaker can be replaced by a resistor with the same resistance and it will work. But I have no idea what is the output circuit of your FRS radio.

It is easy to make the input of an MC34119 power amplifier into a mixer by adding a second input.
Here is the MC34119 mixer/power amplifier circuit with a voltage gain of 2.