4Ohm 3W Audio Speaker interfering with NRF24L01+ modules reconnecting?


I'm hoping this has not already been covered before. I've been replicating a project of a Wireless Laser-Gate Timing System for Track and Field

The project involved 2 x Arduinos and 2 x NRF24L01+ (Finish and Start Modules), plus various other components in those modules. I originally had issues with connecting the modules for the first time and poor range which had been resolved by using a 10uf capacitors on each NRF24 and aluminium foil.

The issue i'm having now is with the Start module that has the speaker. The programme will run for the first time. A '3...2.....Bleep' sound is made which starts the timer. (Which is correct)

After resetting the timer on the finish model the two NR24's won't reconnect. You will get the odd random time when it does connect. However, I noticed when the speaker is either disconnected (which means no '3...2....Bleep' sound to start the timer) the NR24's will reconnect after resetting the timer. Or if the battery in the Start Module is low which meant the sound would not come out properly, the NR24's will reconnect properly.

The only way I can get the NRF24L01+ to reconnect again is if I turn off and on the Start Module.

So it makes me think that the speaker is interfering with the reconnection of the NRF24 modules. I'm using a 9V battery, BD139 transistor and a 3W 4Ohm speaker (which is the same as the person who done this project). Connected to a custom PCB that connects to the arduino.

Below are some pictures of the speaker modules and schematic. If I'm right in thinking that speaker is interfering with the reconnection of the NRF24 modules, does anyone have any ideas to resolve this issue. Thanks.

What’s your 9v battery?

If it’s a square PP3, it’s probably to small to drive the speakers directly, while keeping everything else up.

Look at some current limiting on the speaker.

Thanks for replying. Yes, its a square PP3 Battery.

Can you explain what you mean by 'current limiting on the speaker' and if possible give some examples. I'm new to all this.

Thanks again

Here is example of what the setup looks like.

Looking at your schematic, your 9v goes straight to the transistor, then the speaker, and returns to ground.

In my book, when the transistor goes ‘on’, you’ll have 4 ohms across the battery.

Probably not desirable.

Ok, what can I do to rectify this?

Below is a attachment of the small PCB the speaker and battery is connected too.

The PP3 battery is suitable for fire alarms consuming very low current.
Get a more powerful power supply.

ok, thanks.

May I ask, what alternative power supply do you recommend?

Still have to limit the current through the soon to be burned out speaker.

The real solution is a current limiting resistor (which may reduce the volume), or replace the transistor with a small simple amplifier.

If the sounds are simple tones, a piezo speaker or self-driven buzzer might suffice.

The sequence is Two 'Bleeps' followed by 'Three..Two' and then a final 'Bleep'
..So yes, the the sounds are simple.

So going back to this Current limiting resistor. Is this something I can add to my current circuit and what is it exactly to need to get?


If they’re bleeps, I’d lose the speaker and maybe the transistor, and use a simple 5V piezo buzzer (with internal driver)

I will still need the 'Three' & 'Two' voice so I will need the speaker. According to the code i'm its using "Talkie.h" and "Vocab_US_TI99.h" which i'm assuming is for the speaker.

I would to go down the 'current limiting resistor' route as you kindly mentioned. However, I don't know what I need. That's what I was hoping you guys can help me with.

I'm assuming I need to add some sort of resistor on that tiny PCB to reduce the current to the speaker?

Put capacitor and resistor as on schematic, decrease value of resistor if loudness is to low.

That's how it was done 50+ years ago, when we didn't have better parts.
Class-A is very inefficient, so requires a lot of power.
Nowadays you would use a class-D amplifier, like a PAM8302, (and a lower supply voltage).
A 50 ohm resistor in series with a 4 ohm speaker is a poor solution.
You will loose a lot of volume.
Providing more current from a lower supply voltage source would be better.
Try a 4AA battery pack.
Using a smaller 8 ohm speaker also helps.

Don't you see - decrease value of resistor if loudness is to low.

Not much as you think, check yourself.

I'm using USB power banks or phone chargers.

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ok, thanks.

May I ask, what alternative power supply do you recommend?

A battery with more current capability, like 6 AA batteries in series. You can get battery holders that put the batteries in series.

You could also make them rechargeable batteries with a high mAH rating, that is milli Amps per hour.

not milliamps per hour, milliamp-hours - the product of current and time (ie charge) 1mAh = 3.6C (coulomb, the unit of charge, equiv to 1 amp-second)

Thanks, I will try 6 x AA batteries. I was finding that the 9V battery was draining fast.

Thanks for plotting this information on the diagram. I was looking online for a 50 Ohm resistor and the closest I could find is a 47 Ohm. Is it different kind of resistor I need to be looking for?