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Two PCBs off of one power source


CALAHAN
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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   

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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

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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.

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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

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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.

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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

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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.

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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

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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






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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.

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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

Schematic-1.jpg

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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.

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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

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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.

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Audioguru,

So I put an 8 ohm (can I used 10 ohm?) resistor at the end of the speaker + of the sound effects chip and connect it right to the 0.1 capacitor, which is connected to the 3.3k resistor right?  We don't use the 10k ohm anymore?

Here is the best I can do for the FRS schematic in way of clarity. I cropped the schematic for the speaker output circuit.  Also, can you look at that pot switch and tell me what value it is if I need to order more? 1k, 5, 10k...

Speakeroutenlargment.jpg

I need to ask this again: 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?

I'll have to go out and buy these components or orer them on-line, as these are not common Radio Shack stock.  1/4 watt resistors o.k.?

Thanks,

Bruce

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Hi Bruce,
Of course one or both amplifiers will fry if you connect them to the same speaker and the same battery. Their output DC voltages are completely different and all the battery current will flow between them.

I have sketched how to connect to the units with shielded audio cable and how they connect to the MC34119 power amplifier circuit as I showed here yesterday.

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Audioguru,

You stated"

"Of course one or both amplifiers will fry if you connect them to the same speaker and the same battery. Their output DC voltages are completely different and all the battery current will flow between them."

What I sated is that I have both runing to a common speaker, but are running off of "seperate" powere sources, not the same. Will they still fry? They have worked good so far.


As for this new schamtic. I'm confused. I'm supposed to hack apart shielded audio wire? I don't have room for that stuff. Why I can't I use regular wire? Are you telling me to find audio wire that is rated at 8 ohms?

I have to tap into the FRS's tiny printed circuit board looking for that line off the ground before a resistor?

You show the ground off the FRS negative connected to the shielding, but where does the center of the shielded wire come off of? I hope it is not the headphone jack. The headphone jack on the FRS board has been destroyed and is used for the mount of the Xmit switch.  I need to be able to hook this up off the hanging FRS speaker wires.

What number pin is the P- on the MC34119?


Thank You,

Bruce



 

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What I sated is that I have both runing to a common speaker, but are running off of "seperate" power sources, not the same. Will they still fry? They have worked good so far.

Their speaker outputs have completely different circuits and have completely different DC voltages. A high current will flow between them. The batteries and the amplifier circuits will suffer.

As for this new schamtic. I'm confused. I'm supposed to hack apart shielded audio wire?

Shielded audio cable is used to connect together all audio equipment. It is not hacked, it is simply connected. It is not needed if the signal wires are less than about 5cm long.

Why I can't I use regular wire?

Regular wire longer than about 5cm picks up mains hum and other interference.

Are you telling me to find audio wire that is rated at 8 ohms?

Audio shielded wire has RCA connectors in North america and has DIN connectors in Europe. The cables come with all audio electronic equipment. The cables are sold in The Dollar Store at a low price. Radio Shack sells the same ones at a very high price. Cut off the connectors and use the shielded cable. It is not rated at 8 ohms.

I have to tap into the FRS's tiny printed circuit board looking for that line off the ground before a resistor?

All grounds in a circuit are connected together, Connect the shield of the shielded cable to any ground point. Connect an 8 ohm or 10 ohm resistor to where the speaker was connected.

You show the ground off the FRS negative connected to the shielding, but where does the center of the shielded wire come off of? I hope it is not the headphone jack. The headphone jack on the FRS board has been destroyed and is used for the mount of the Xmit switch.  I need to be able to hook this up off the hanging FRS speaker wires.

The center wire of the shielded cable needs to connect to the output of the headphone amplifier that connected to the headphones jack. One side of the speaker connected to the positive battery terminal and cannot be used as a ground point if you use a single battery because then the battery will be shorted.

What number pin is the P- on the MC34119?

Pin 7 is the ground pin for the input signals and the P- pin.

Here is the connections diagram with the headphones jack removed from the FRS radio.
also is a pic of the MC34119 power amplifier.
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Audioguru,

I have some headphone cable around the house, but as far as I know, it is not shielded. It is two wire or three wire. These connections are about 5 cm or just over (but not by much), so I'll use regular wire.

O.K. on the rest I'll buy the 3.3k (I have the 10's already) resistors and give it a shot over this week and let you know.

Do I realy need those capacitors at the other pins?  The 1.0, 5.0 etc?

Thanks,

Bruce   

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The value of the resistors for the MC34119 power amplifier don't matter much if they are all the same. Use 10k and it will be fine.

The same for the capacitors.
The 0.1uF can be 0.1uF to 10uF.
The 1uF can be 0.1uF to 10uF.
The 5uF can be 0.1uF to 22uf.
The 100uF can be 10uF to 1000uF.

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Audioguru,

Just an observation, but the line off the headphone lead you have me connected to alreay is being fed through a 3.3k and what appears to be a capacitor on the circuit board. I'll have to figure out which solder pont is which on that headphone jack where it is connected to the board. As I said, th ejack itself has been destroyed and used a mount for the Xmit switch.

Bruce

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Just an observation, but the line off the headphone lead you have me connected to alreay is being fed through a 3.3k and what appears to be a capacitor on the circuit board. I'll have to figure out which solder pont is which on that headphone jack where it is connected to the board.

R107 is 3.3 ohms, not 3.3 thousand ohms. It and the capacitor connected to it keeps the power amplifier IC from oscillating. The 3.3 ohms resistor and the output for the headphones (and for your MC34119 power amplifier) is at the power amplifier IC pin 1. The jack had a built-in switch that disconnected the speaker when the headphones were plugged in.
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Audioguru,

Well I breadboarded it tonight. I wired it up to the values as you have it. I tried a few variables with the capacitors - both the 100 and 10 uf. I also used two 1.0 ufs instead of the 1.0 and 5.0 at pin 2 and 3 to ground. I found that the capacitors at these pins either diminished the volume or caused inconsistent sound. I removed them all together and left those two pins unconnected and it worked. It is at the brink of distortion but acceptable. These are regular alkalines by the way. I will be using AAA lithiums in the final version because I need to bring the amperage somewhat back up as I replaced the 3 AA for these 3 AAA's. I wonder what effect those will have.


I have a pristine FRS model of the one I hacked for the communicator, so I used this for the breadboard. I did exactly as you indicated. I took one of those "earbuds" and put into the headphone jack, cut the earpiece end of it off, stripped the wires, ran one to ground and the other to the breadboard as indicated and found that there is no sound coming through the speaker (I used a second FRS and created sound to be able to hear of course). I can hear the other FRS through the earpiece (I have two of those also), but the spliced one is not amplifying through the chip. Other then that, all is working.

Now what?

Bruce

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