Communities

Writing
Writing
Codidact Meta
Codidact Meta
The Great Outdoors
The Great Outdoors
Photography & Video
Photography & Video
Scientific Speculation
Scientific Speculation
Cooking
Cooking
Electrical Engineering
Electrical Engineering
Judaism
Judaism
Languages & Linguistics
Languages & Linguistics
Software Development
Software Development
Mathematics
Mathematics
Christianity
Christianity
Code Golf
Code Golf
Music
Music
Physics
Physics

Dashboard
Notifications
Mark all as read
Q&A

555 timer, PAM with digital signal

+1
−0

I'd like to use a 555 timer as carrier wave generator at ~ 50 kHz and modulate the amplitude with an external digital signal at ~ 1 kHz. The pulse width remains constant at 10us (50% duty cycle).

Do you suggest to add MOSFET on the OUT pin of 555 timer, or can I connect the signal directly to a pin of the 555 timer?

I saw here that the digital signal is plugged directly into the RESET pin. Does this have undesirable side effects, or is it a good practice?

Thanks!

Why does this post require moderator attention?
You might want to add some details to your flag.
Why should this post be closed?

5 comments

What are you trying to achieve with your modulated signal? OOK? Non-suppressed carrier? Suppressed carrier? You need to be clear about your goals. 555 timers are normally suited for anything but basic modulation schemes. Andy aka‭ 28 days ago

@Andy aka: it would be OOK (I just learned the term now). mrwavelets‭ 28 days ago

How do you intend to connect it to the MOSFET? If you mean to have the 555 timer on the gate, then you'll have to be very careful with which part to pick, since most MOSFET aren't fast enough to live up to your realtime spec. What 555 timer are you using? In case it's a single chip IC, there should be parts with built-in BJT for the output. Lundin‭ 28 days ago

@Lundin: I would connect the slow signal at the gate, and the fast signal (from 555) at the source. I would use the NE555 (TI). That should work, right? mrwavelets‭ 28 days ago

@mrwavelets‭ So the MOSFET is not there to give a higher voltage needed for the amplitude? In that case it isn't necessary, since your particular part can drive a relatively high current directly. Lundin‭ 28 days ago

1 answer

+2
−0

The 555 timer has a maximum output current rating (for example 250 mA for the NE555, check your 555 version datasheet). So whether or not you have to introduce a mosfet depends on whether your application is current demanding or not. For less than 100 mA (say), you can connect directly your 555 to whatever you intend to.

The link you provided leads to a non related document for me.

the digital signal is plugged directly into the RESET pin

what digital signal? if you want to modulate the carrier signal at the output of the 555, you can simply try to modulate the input voltage at the input supply current pin of the 555. That may work or not (I've not tried). Otherwise, you may want to use a npn transistor mounted as a switch (load above the collector) whose base is driven by the 555 output, and to modulate the input voltage of the "switch" at the desired frequency.

555_carrier

Note: the load above could be the primary of a transformer.

As Andy said, it is difficult to help whenever nothing is known about your application (do you have a positive and negative supply power?, do you need a modulated wave above ground or centered at ground?) etc.

Also, I hope you don't intend to use that for radio communication: at these very low frequencies, that will be difficult.


ADDED:

OK. Now that the problem of the link is fixed and I've read the document, I understand what you are trying to do.

The answer is "Yes", you can connect directly the output pin of the various 555 to the relevant parts, because the various datasheets specify a large maximum reset voltage and a small reset current of about 1mA. So, there is no problem with that. No need to use a mosfet for this application.

And yes, I think this is certainly not a "bad practice" to use the reset pin.

Why does this post require moderator attention?
You might want to add some details to your flag.

5 comments

@coquelicot: apologies, I fixed the link, now it should be clear. The modulated signel will be just used for communication to another device and has negligible loads. mrwavelets‭ 28 days ago

@mrwavelets. A communication signal IS a load. I think you should explain how the output of the modulated carrier wave is supposed to be used (do you want to feed an antenna with the signal?) coquelicot‭ 28 days ago

@coquelicot: I meant that the signal won't be used to consume a substantial amount of power. The signal will be interpreted by a MCU. mrwavelets‭ 28 days ago

OK. See the add in my answer. coquelicot‭ 28 days ago

@coquelicot: Great, thanks! mrwavelets‭ 28 days ago

Sign up to answer this question »

This community is part of the Codidact network. We have other communities too — take a look!

You can also join us in chat!

Want to advertise this community? Use our templates!