Amplitude modulation with MCU and NE 555

Thread Starter

mrwavelets

Joined Mar 16, 2021
6
Hi,

I want to build an IR transmitter (~ 38kHz).

My plan is to use a NE555 timer to generate the carrier wave (A-stable Operation),
modulate it with a micro controller, and transmit modulated signal via IR LED.

My question:
Can I connect the micro controller output pin directly to the CONT pin of the timer (pin 5) to modulate the wave,
or should I perform the modulation with a MOSFET on the OUT pin (pin 3) ?

Thanks!
 

bertus

Joined Apr 5, 2008
21,530
Hello,

Putting a signal on pin 5 would lead to a kind of Pulse Position Modulation:
NE555_pwm.png
If you want to do AM modulation, you will need to do it on the output.

Bertus
 

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Thread Starter

mrwavelets

Joined Mar 16, 2021
6
Hi Eric,

My plan is to use a digital signal with a frequency in the order of 1 kHz,
similarly to ASK. The signal would be (programmatically) generated by the MCU.

I noticed that in this project the RESET pin of NE555 was used for PAM,
maybe I should use the RESET pin?
 

bertus

Joined Apr 5, 2008
21,530
Hello,

When using the reset pin, keep in mind that the first pulse is longer as the rest of the pulses, due to the startup of the oscillator.

Bertus
 

jjw

Joined Dec 24, 2013
676
Hi,

I want to build an IR transmitter (~ 38kHz).

My plan is to use a NE555 timer to generate the carrier wave (A-stable Operation),
modulate it with a micro controller, and transmit modulated signal via IR LED.

My question:
Can I connect the micro controller output pin directly to the CONT pin of the timer (pin 5) to modulate the wave,
or should I perform the modulation with a MOSFET on the OUT pin (pin 3) ?

Thanks!
You don't need the 555 for the carrier.
The carrier and modulation can be made with the microcontroller.
There are several examples of code on the net for PIC, Arduino etc.
 

Thread Starter

mrwavelets

Joined Mar 16, 2021
6
@bertus: thanks for your help. Could you point me to the startup time "RESET penalty" for A-stable operations in the datasheet?

@jjw: thanks, in this case the MCU is busy doing other things at FOSC 4Mhz.
Continually counting/spin-waiting 13.15 x 2 tics via software would be impractical (it would require complete dedication of the MCU just for waveform generation); the timescale of this interval is just too short to be dealt with interrupts.
 

jjw

Joined Dec 24, 2013
676
@bertus: thanks for your help. Could you point me to the startup time "RESET penalty" for A-stable operations in the datasheet?

@jjw: thanks, in this case the MCU is busy doing other things at FOSC 4Mhz.
Continually counting/spin-waiting 13.15 x 2 tics via software would be impractical (it would require complete dedication of the MCU just for waveform generation), and the period is just too short for interrupts.
The carrier can be made with the PWM module of the micro with no software overhead.
ASK modulating simply by switching PWM on/off ?
 

peterdeco

Joined Oct 8, 2019
391
I've done this with a single PIC micro using the built in hardware pulse width modulator (HPWM). Unlike being capable of doing only one thing at a time, once the HPWM is running at the carrier frequency, the PIC is free to do the rest of the code.
 

bertus

Joined Apr 5, 2008
21,530
Hello,

When the 555 astable does a start-up, the capacitor is at 0 Volts, it takes some more time to reach the 2/3 Vcc.
When it reached 2/3 of Vcc the capacitor is discharged to 1/3 of Vcc.
The astable range is between 1/3 Vcc and 2/3 Vcc.

Bertus
 

crutschow

Joined Mar 14, 2008
27,709
For the standard 38kHz modulation you want all the pulses to be the same width.
So you could let the 555 free run at 38kHz and use a D flip-flop with its clock input from the 555 to turn its output on and off only in sync with the clock as determined by its D input.
The output from the flip-flip than gates the output of the 555.

The LTspice simulation of an example synchronous gating circuit is shown below.
A1 would be the 555 output and A2 the modulating signal.
The gates are needed to avoid spikes at the leading or trailing edges of the output due to gate delays.

1615904469064.png
 
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