AM Modulation with 555 timer

Thread Starter

Patrishayeah

Joined May 30, 2019
15
Hello,
I'm currently working on a project in which I generate a carrier wave with the colpitts oscillator and a square wave with the 555 timer. I want to modulate the output of the 555 timer onto the carrier wave so that I can receive it with an AM radio, but don't know how. I've tried different things already with simulations, but none seem to work. Could someone please have a look at the two circuits (I'll leave a link and picture below), and give me some suggestions on how to do it? Any help will be greatly appreciated.
http://tinyurl.com/y5oppt4p
Thanks
 

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jpanhalt

Joined Jan 18, 2008
7,891
Welcome to AAC.

Consider using Pin 5 (Control Voltage). It sets the threshold. Been done before.

upload_2019-5-30_19-1-14.png
 

BR-549

Joined Sep 22, 2013
4,936
What are you doing with the signal, after it is received by an AM radio?

In other words, tell us exactly what you are trying to do. All the details.
 

Bordodynov

Joined May 20, 2015
2,392
You can connect the timer and oscillator outputs through a resistor. The size of the resistor you can pick up
 

DickCappels

Joined Aug 21, 2008
5,880
The easiest way to amplitude modulate your colpitts oscillator is to power the oscillator with the output of the NE555.

If you want to be a little more elegant, you can try something like Charles Wenzel's AM transmitter that uses a Gilbert Cell multiplier to perform the amplitude modulation.

Read about it at: http://www.techlib.com/electronics/amxmit.htm
 

Thread Starter

Patrishayeah

Joined May 30, 2019
15
What are you doing with the signal, after it is received by an AM radio?

In other words, tell us exactly what you are trying to do. All the details.
Hi, so what I want to do in this project is hear a tone in an AM radio when the switch of the transmitter is pressed down, and this way be able to send out morse code. The 555 timer would have an output of about 600 Hz, an audible tone, and the carrier wave would be about 1 MHz (In the link and picture it is less but just so that I have a better overview). This is just a school project, so the transmitter is not supposed to be long range.
 

Thread Starter

Patrishayeah

Joined May 30, 2019
15

Thread Starter

Patrishayeah

Joined May 30, 2019
15
The easiest way to amplitude modulate your colpitts oscillator is to power the oscillator with the output of the NE555.

If you want to be a little more elegant, you can try something like Charles Wenzel's AM transmitter that uses a Gilbert Cell multiplier to perform the amplitude modulation.

Read about it at: http://www.techlib.com/electronics/amxmit.htm
Thanks for your reply. Do you mean something like in the picture? I'm pretty sure the modulated carrier wave is not supposed to look like that though. But for amplitude modulation you somehow have to change the amplification of the final transistor, right? My problem is just that I don't know which part of the colpitts oscillator to connect the 555 timer output to, without messing everything up.
 

wayneh

Joined Sep 9, 2010
16,102
Hi, so what I want to do in this project is hear a tone in an AM radio when the switch of the transmitter is pressed down, and this way be able to send out morse code. The 555 timer would have an output of about 600 Hz, an audible tone, and the carrier wave would be about 1 MHz (In the link and picture it is less but just so that I have a better overview). This is just a school project, so the transmitter is not supposed to be long range.
The suggestion by @DickCappels in #6 is the way I'd go. Just turn the carrier on and off with the 555.
 

Thread Starter

Patrishayeah

Joined May 30, 2019
15
The suggestion by @DickCappels in #6 is the way I'd go. Just turn the carrier on and off with the 555.
Ok thanks. Do you have any idea which part of the colpitts oscillator I should connect the 555 with? Because when I substitute the Vcc of the colpitts with the 555, I get a weird result.
 

wayneh

Joined Sep 9, 2010
16,102
Forgot to add the picture, sorry. Here it is.
That looks OK. I don't understand the simulation output. You need to have the 555 at a much lower frequency than the carrier. Then your simulation should look just like the 555 output, except for, when on, a series of oscillations instead of a steady DC output.

And personally, I'd move the 100Ω resistor on the base of the pass transistor.
 
Last edited:

Thread Starter

Patrishayeah

Joined May 30, 2019
15
That looks OK. I don't understand the simulation output. You need to have the 555 at a much lower frequency than the carrier. Then your simulation should look just like the 555 output, except for, when on, a series of oscillations instead of a steady DC output.

And personally, I'd move the 100Ω resistor on the base of the pass transistor.
Thank you so much! Is this what it's supposed to look like? I changed the carrier frequency to about 1 MHz.Screenshot 2019-05-31 at 5.06.17 PM.png
 

bertus

Joined Apr 5, 2008
20,050
Hello,

The pass transistor is mounted the wrong way around.
The collector and emittor should be changed.
Or you should use a PNP transistor in the way you showed.

Bertus
 

panic mode

Joined Oct 10, 2011
1,782
what kind of 555? why bother with transistor and not power oscillator straight from 555 output?
 

Thread Starter

Patrishayeah

Joined May 30, 2019
15
what kind of 555? why bother with transistor and not power oscillator straight from 555 output?
what kind of 555? why bother with transistor and not power oscillator straight from 555 output?
I'll be using the NE555 in astable mode(if that's what you mean). The pass transistor is there to amplify the output of the 555 so it can be modulated properly onto the carrier wave (I think that's supposed to happen, but I'm not sure). I tried doing it without, but the current then flows into the 555 instead of out of it, and no modulation occurs.
 
Modulating with a square wave will give you a buzzing sound, not a tone.
Your 555 output should have a duty cycle of about 50%. Currently it looks like it is more than 90%.
The 555 output is too large and is almost completely killing the oscillator output. This actually becomes On-Off Keying of the carrier.
You need to pass the output through a capacitor to remove the DC component.

Your circuit is too simple to get good results but you will probably be able to hear it in the receiver.
 

Zeeus

Joined Apr 17, 2019
419
Hello,
I'm currently working on a project in which I generate a carrier wave with the colpitts oscillator and a square wave with the 555 timer. I want to modulate the output of the 555 timer onto the carrier wave so that I can receive it with an AM radio, but don't know how. I've tried different things already with simulations, but none seem to work. Could someone please have a look at the two circuits (I'll leave a link and picture below), and give me some suggestions on how to do it? Any help will be greatly appreciated.
http://tinyurl.com/y5oppt4p
Thanks
You must be having fun
 
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