RF M4 Recevier Max Output Current Pins D0 - D3

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alansandersjr
 
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RF M4 Recevier Max Output Current Pins D0 - D3

Post by alansandersjr »

What is the maximum output current that can be provided from pins D0 - D3 on the RF M4 Momentary 315 MHz receiver (with 5V supply)? I'm trying to drive a 2N3904 transistor with 1k Ohm base resistor from pin D3 and have blown up 2 receivers.

On a PT 2272 datasheet I downloaded off the internet (http://www.goldmine-elec.com/pdf/PT2272.pdf) - a 22 page one, not the 8-page one on the Adafruit site - I see the following parameter on page 13:

Dout Driving Current (IOH) = -3 mA with Vcc = 5V and VOH = 3V

Does this mean the maximum current from D0 - D3 is 3 mA? The existing 1k Ohm base resistor I'm using should allow 4.3 mA of base current. Can I increase the base resistor on the 2N3904 to 2k Ohms to reduce the base current to 2.15 mA and still have the 2N3904 operate in the saturation region?

If 2.15 mA base current in the 2N3904 will not put it into saturation, can I place a TIP 120 Darlington transistor in between the RF M4 (on pin D3) and the 2N3904 to keep the current draw below 3 mA?

If I am off on the wrong bunny trail please suggest other options.

Sign me "Tired of Buying Receivers"

Al

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adafruit_support_mike
 
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Re: RF M4 Recevier Max Output Current Pins D0 - D3

Post by adafruit_support_mike »

A 2n3904 with a 1k base resistor shouldn't be capable of killing an M4's output pin. It presents about 1/10th as much load as an LED. Even if the pin can only source 2-3mA, you should just see the output sag by a volt or two.

Whether the transistor saturates depends on the amount of current you want flowing through the collector, but 3mA should be enough to saturate a 2n3904 over its whole operating range. That happens to be my NPN of choice for small-signal work, so I know from experience that the 2n3904's maximum current rating is 200mA and its current gain in that region is around 75.

How much current are you trying to control, and could you please post a photo of how you have things connected? It sounds like something strange is going on.

alansandersjr
 
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Re: RF M4 Recevier Max Output Current Pins D0 - D3

Post by alansandersjr »

Image of project is attached. It looks pretty complicated but it is really just a remote-controlled sound board using the Adafrauit Wave Shield/Arduino Uno R3. Three of the channels on the RF M4 receiver go to the Wave Shield/Arduino boards to make 3 different sounds when channels B, C, D are pushed on the 4-channel transmitter key fob transmitter. This all seems to work fine. Channel A goes to a Sparkfun "Beefcake" relay kit 11042 which consists of a heavy-duty relay driven by a 2N3904 with a 1K Ohm base resistor. Channel A turns on a small 12V DCwater pump, through the Sparkfun relay, when the transmitter button is pushed. I have attached a screen snip of the Sparkfun Beefcake relay since PDF files are not accepted. When I put a new RF M4 receiver in, it seems to work ok for a few minutes. All channels (A, B, C, D) work fine. Then, the receiver just stops working. When the last receiver failed, I noticed the LED on the Beefcake relay glowing dimly just before it stopped working. It had worked fine for a few minutes up until this point. I surmised something in the PT 2272 chip on the RF M4 receiver was breaking down and allowing current to flow to the Beefcake relay even though Channel A was not activated.

This weekend, I'll measure the amount of current being pulled by the base of the 2N2904 on the relay board. I won't have time to get back to the project until then.

Other components in the picture include an audio amplifier, a couple of DC to DC power converters (12V to 5V and 12V to 9V), volume control/ off-off switch for the audio amp, and interconnecting wiring. The white piece of paper with the barcode, at the bottom right, is just a cover for a fuse. The white terminal blocks interface power, speakers, and the pump motor which are remote from the board.

Thanks for your help!

Al
Attachments
Sparkfun Beefcake Relay Board.PNG
Sparkfun Beefcake Relay Board.PNG (18.7 KiB) Viewed 839 times
Photo of remote-control sound board project
Photo of remote-control sound board project
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adafruit_support_mike
 
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Re: RF M4 Recevier Max Output Current Pins D0 - D3

Post by adafruit_support_mike »

Do you have a connection between the GND lines for the two DC-DC converters? If they're floating relative to each other, it could cause all sorts of problems.

alansandersjr
 
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Re: RF M4 Recevier Max Output Current Pins D0 - D3

Post by alansandersjr »

I don't have the two negative outputs of the DC - DC converters tied together. Before I do that, let me describe them to you and ask a couple more questions.

Both converters are encapsulated in epoxy with 2 wires for 12V DC input and 2 wires for regulated output. There is no common ground between input and output as with say a 7805 regulator. None of the wires are black so I was hesitant to tie the negatives of the two outputs together.

If I tie the two negative outputs together, should I also tie them to the -12V lead of the main power input (so all grounds are at the same potential)? Three different voltages are used in the project: 12V for the audio amp, 9V feeding the Arduino/Wave Shield, and 5V for the RF - M4. If need be, I can build 7809 and 7805 regulators that I know will tolerate common grounds.

Thoughts?

Many thanks for your help!

Al

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adafruit_support_mike
 
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Re: RF M4 Recevier Max Output Current Pins D0 - D3

Post by adafruit_support_mike »

Power supplies with no direct connection between the input and output are called 'isolated' or 'floating' supplies.

In general, isolation is A Very Good Thing. It means that if something fails in the supply, the load won't suddenly get connected to wall power (or whatever).

It can create problems when you use two floating supplies at the same time though. If there's no common reference point between them, they can take any voltage relative to each other. If we call the supplies A and B, you could connect VCC.A to GND.B and get a combined voltage range of 0v to VCC.A+VCC.B (measured from GND.A).. which could also be called a supply that ranges from -VCC.A to +VCC.B (measured from GND.B).

You could also flip those connections around and hook VCC.B to GND.A.

The point is that the voltages have to be measured relative to some common reference point. In general, we call that reference point GND, or sometimes 0v when we want to talk about negative voltages.

If you don't provide a reference point explicitly, the circuit will find one wherever it can. In this case, I'll bet you can trace at least one strong current loop through the receiver. The receiver's pins can only carry about 10mA, so if something that wants to swing 50-100mA tries to push current through that connection, the receiver will pop like a low-current fuse.

Run a piece of 18-20 gauge wire between the output GNDs of the two power supplies, but don't connect that common GND line to either of the input GND connections. You still want the circuit to be isolated from the outside world, you just don't want the two internal supplies to float relative to each other.

Don't make any kind of low-resistance connection between the two internal VCC lines.. connecting both VCC and GND between two power supplies is a fairly reliable way to start a fire.

Let each supply provide power to its own set of components, then look for the connections between those two sets. Those are the places where current will need to flow through the common GND wire, and they're probably what's been killing receivers.

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Re: RF M4 Recevier Max Output Current Pins D0 - D3

Post by alansandersjr »

Thanks very much for the detailed explanation. I ended up being out of town all weekend and have not had time to make the changes you suggested. Perhaps, I'll have time tonight (Tuesday). I'll let you know how I make out.

Thanks, again!

Al

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Re: RF M4 Recevier Max Output Current Pins D0 - D3

Post by alansandersjr »

I finally got a bit of time to try the wiring changes you suggested. When I tie the negative output terminals of the two DC-DC converters together, they both shut down and stop delivering any voltage. When I remove the tie wire, they both start putting out voltage again.

Any suggestions? I think I may start building some new supplies around 7805 and 7809 regulators.

Al

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Re: RF M4 Recevier Max Output Current Pins D0 - D3

Post by adafruit_support_mike »

alansandersjr wrote:I finally got a bit of time to try the wiring changes you suggested. When I tie the negative output terminals of the two DC-DC converters together, they both shut down and stop delivering any voltage.
Okay, that's a serious problem.

A single connection between supplies shouldn't have any effect. You probably have some other connection between the two supplies that we don't know about.

Disconnect the jumper between the supplies and put a voltmeter between the two GND rails. Let's see what that has to say.

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Re: RF M4 Recevier Max Output Current Pins D0 - D3

Post by alansandersjr »

I went ahead and built supplies to provide 5V and 9V using an LM 7805 and LM 7809. I was under a time constraint as I was building this project for a friend who needed it soon. This seems to have solved the problem of blowing receivers as I tested the unit for quite a while with no problems. Thank you for directing my attention to the two isolated power supplies. You saved me a lot of time and money buying new receivers. I'll get back to figuring out what is going on with the two isolated power supplies when I have a bit more time.

Thank you , again, for all your help!

Al

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Re: RF M4 Recevier Max Output Current Pins D0 - D3

Post by adafruit_support_mike »

Glad to hear you got something that worked, or at least wasn't frying components!

For future projects, I'm a big fan of the test-as-you-go strategy. If you plan to use two power supplies, put them in first, join their GND rails, and do a couple of quick meter tests to make sure everything works as expected. Then add the next piece and test that. Keep working through the add-test (-debug-debug-debug) cycle until you have everything in place.

Debugging is a lot easier when you have a known working configuration to use as a reference. The more pieces you add between tests, the harder it is to work your way back to a known state.

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Re: RF M4 Recevier Max Output Current Pins D0 - D3

Post by alansandersjr »

Thanks, Mike. I'll get back to the two isolated power supplies working together in the next few weeks. I hate to be whipped by something so I'm gonna figure this out! You :an buy these little isolated supplies dirt cheap and they handle more current than something built around a LM 78XX so I'm motivated. Thanks for sharing your build/test strategy.

Until the next time I get in over my head :).

Al

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Re: RF M4 Recevier Max Output Current Pins D0 - D3

Post by adafruit_support_mike »

We'll be here. ;-)

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Re: RF M4 Recevier Max Output Current Pins D0 - D3

Post by Hashem32 »

Hi
Can I drive Dc toy car motor without transistor ? I want connet motor to d0 d1 for left and right

Thanks

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