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Two PCBs off of one power source


CALAHAN
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Audioguru,

I have a problem with my first communicator build which we used that MC34119 amp in. Something is sucking the batteries of power even when it is turned off. I'll leave it on the desk after using the sound effects for a few days, and the batteries are dying. Any ideas?

Bruce 

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Something is sucking the batteries of power even when it is turned off.

Then I guess the MC34119 amplifier is still bring powered by the battery when the radio and the sound effects circuit are turned off.

The MC34119 amplifier should receive its power after the on-off switch.
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The FRS radio obviousley has an off/on switch, but the sound effects chip does not. On the original toy it came in there was no on/off switch, the sound effect is activated when a diaphram switch is hit wich completes the circuit - but it is momentary. You hit it once and the sound effect plays through to it's end, and then resets when the diaphram is hit again. Holding the switch closed causes the sound effect to play in a continual loop. I guess that meas the original toy circuit is always hot.

Bruce

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Audioguru,

Where woud you put it?  I use two of the sound effects of the three that came in this unit - there are two diaphram switches. Would my regularly installed switches still activate the souinds as needed? but this capacitor and resistor would switch on when power met it, and switch off when power decreased to it - is that the Idea?

I posted the schamtic I drew for the sound effects unit. Can you tell from that? Otherwise, how do I check - put the tester on the speaker out to the difference in voltage whne operating and not?


Bruce

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Hi Bruce, In order to add a transistor with a coupling capacitor to replace each diaphram switch, we need to know what the diaphram switch connects to. Hopefully the diaphram switch connects either to the supply voltage or to ground. We need to know if it does and we need to know which one.

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Audioguru,

Here is the schematic again. The diaphram switches close the positive supply circuit. I jumped wires off the pads of the diaphram switch to the tac switches in my build, as I need them so I don't want to replace the tac switches.

Schematic-1.jpg

Do you need me to check what the voltage is at the speaker input at the op amp before the capacitor and resistor at idle and then when the sound effect is activated to see what the voltage difference is?

Remember that the sound effects circuit is powered of the 4.5 volt batteries knocked down to a bit over 3 volts through two diodes in series.

Bruce

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Hi Bruce, I am uncertain about what you want to do.
1) Turn on the FRS radio and the MC34119 amplifier also turns on.
Then when you turn off the FRS radio the MC34119 amplifier also turns off.
2) Momentarily push a TAC switch on the sound effects gadget and the MC119 amplifier also turns on. Then the sound effects gadget automatically turns itself off and you want the MC34119 amplifier to also turn off.

About the simplest way to turn on the MC34119 amplifier when a sound effects TAC switch is pushed then have it turn itself off is to use a timer for it which is complicated and uses many parts. A TLC555 Cmos timer IC, 2 transistors and a few resistors and capacitors.

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Audioguru,

The device needs to be able to use the sound effects even if I have the FRS off. That is why I just don't splice the MC34119 power off the FRS switch, because then I would need to turn the FRS on to also power the op amp for the sound effects. The op amp and FRS share the same power source at 4.5 volts, where the sound effects utilizes a reduced voltage shunted off the 4.5v with the two diodes to a bit over 3 volts (at full battery strength of course).  This battery drain must be from the op amp because when the FRS is off, there is no power runing through it. And - the sound effects circuit board, when in it's original toy, did not drain the battery when not being used. Therefore it must be the op amp, which is direct wired to the batteries. Is there an op amp out there that has a low voltage auto turn off feature that is comparable in way of power requirements to the  MC34119?

There is absolutly no more room for yet another IC chip. Is there no way to put a switch in the power line of the op amp by itself? If it comes down to it, I will have to find a very small slide switch - that is acceptible to me, as I can turn on the op amp when I wish to use the sound effects and the radio and turn it off at the end of the day's usage. Is there nothing I can put in line somewhere that will turn on the op amp's power when the signal voltage going into pin 4 reaches over a certain threshold, and then turns back off when the voltage drops below this threshold? I assume that the voltage from the two units feeding into pin 4 is at line level now that we have kocked it down with your modiciations, but does this level not gain in voltage somewhat when the audio signal is being fed into pin 4?

Bruce 

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The MC34119 is a power amplifier with an output current up to 250ma. An opamp has a max output current of only 20mA and therefore cannot drive a speaker.

A power amplifier needs to have an idle current to avoid distortion.
The MC34119 has a Chip Disble pin #1 that reduces its supply current to typically only 65uA, but the sound effects circuit can't tell the MC34119 when to be disabled.

I have never seen a bridged low voltage audio power amplifier IC that turns itself on when it has an input signal.

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Audioguru,

two questions:

The chip disable pin. if this is utilized, will it allow the signal to still pass through even though it is not amplified?  I have a switch installed now on the power supply of the MC34119, however, I seem to forget it is off, and when someone is calling me I can't hear them.  I'd rather the audio be low then not present at all. Then when I want full sound effects and FRS amplification, I can then switch off the no 1 pin chip disable and she wil deliver the full volume. Is this possible?


Is there any other IC that is comperable to the MC34119, since the MC34119 is no longer being made?  I need to buy more.

Bruce 

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Hi Bruce,
I am soorry that I didn't know that the MC34119 low voltage bridged power amplifier is not made anymore. I still have a few.
There are not many normal-size low voltage bridged power amplifier ICs still available because new ones are all tiny surface-mount types that are very difficult to solder.

The LM4876 operates about the same as the MC34119 and is in a tiny package.

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Guru,

If I buy a few of these, can I use the same configurment as the MC34119? Are the pins the same etc. I haver a found places on the web that still sell the MC34119, but places like Mouser have them discontinued.

Also, what about the pin 1 option, can I use this as a switchable power down - and of powered down will it allow the unamplified audio through it so I can at least hear it to know to turn the amplifier on?  If it only turns the power off to the chip and the off chip won't alow anything through it , then this would be the same as what I did here; that is hook the power line of the MC34119 to a switch.

Thanks for the time on this.

Bruce

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Hi Bruce,
The LM4876 has the same pins numbers as the MC34119 and uses the same external resistors and capacitors. It has more output power but its max supply voltage is only 5.5V. It needs a resistor from its pin 1 Enable pin to the positive supply to turn it on and grounding pin 1 turns it off (the opposite of an MC34119).
When it is Disabled, it does nothing and has no output.
You might actually have fun trying to solder it!

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You might actually have fun trying to solder it!


I think I'll stay with my switch for what I have now on the MC34119, but if this new IC you recommend has merely those little solder pads used for PCB's I'll have to have new glasses - I have about had it as it is :) 


Bruce
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Guru,

I consider this another success. The first communicator I built, which started this thread, was modeled after the "Alpha" communicator as coined by the folks at www.herocomm.com. This second build is modeled after "Gamma". Of the 10 communicator props built by the prop master for the original 1960's Star Trek TV series, only 4 have been found as of yet.


This radio is using a Motorola Walkabout FV200. It has the LCD screen and the GMRS channel range etc. The channels need to be changed from the back after unscrewing the back shell. However, one need only set the channel and volume and as long as the batteries stay in, the radio will default to whatever is programmed in it.

Because of the use of the Playmtes toy sound effects chip, it has the authentic  "chirp" which sounds with the flip of the lid, and the hailing beeps are activated by a tac switch on the back. I placed a slide switch hidden in the Velcro on the back also. This powers down the MC34119 audio amp so as not to suck battery power when not being used. At the start of the day, just slide the switch over and the speaker is on.


The right control button is Xmit, and the left is the on and off button. The middle jewel lights up when the unit is powered on and flashes when in battery standby.

I found a capacitor on the circuit board and jumped the LED off of it. The LED has a maximum forward voltage of 3.5 volts , and the tester on the one leg of the capacitor shows a fluctuating voltage never over 3.2, but I put a small value resistor on the negative end of ther LED anyway.

I field tested this with a custom built antenna and was able to communicate around a 10 story building in Downtown Chicago, and then stood at 20 S. Wacker and spoke to a guy on another FRS due North at Illinois street which is about 1/2 mile. That is pretty good in the Loop with all of the metal and buildings. As with any FRS, the range is dependent on what is in the way of the signal. FRS radios do not like elevator shafts! At this point I have the original antenna in it, and will be field testing it for range this coming week. The signals are not quite as good as the original radio because of the location of the antenna being now internal, but it will do what I need it to do.

Thanks Guru


Bruce




Gammamontage.jpg

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