help! how to modulate the 555 timer's output signal? - Page 1
Forum section: electronics / beginners

author: ali6x944
author_rank: Frequent Contributor
timestamp: January 26, 2016, 07:05:54 pm
content: help! how to modulate the 555 timer's output signal?i saw some people do it with pin 5 of the 555 timer, but it doesn't work when i tried it & i don't Know WHY?please help!

author: Ian.M
author_rank: Super Contributor
timestamp: January 26, 2016, 07:13:57 pm
content: Modulate how?Amplitude, frequency, duty cycle or some unholy combination of two or more?Show us the (as-built) schematic that didn't work for you.

author: ali6x944
author_rank: Frequent Contributor
timestamp: January 26, 2016, 07:27:15 pm
content: i used this but without the 0.1uf cap on pin 5 and without the led

author: ali6x944
author_rank: Frequent Contributor
timestamp: January 26, 2016, 07:30:34 pm
content: and i feed pin 5 with an audio signal in hope that it will modulate the signal's pulse width and frequency but it did not

author: Ian.M
author_rank: Super Contributor
timestamp: January 26, 2016, 07:35:45 pm
content: Not a good circuit: if you turn the variable resistor right to its minimum value and it has metallised track ends, it can blow the 555's Discharge transistor. It will be OK if the measured minimum resistance of the variable resistor is >100 Ohms.We need a more detailed description of what happened ("doesn't work") does't tell us anything much), what measurements and observations you made and what test equipment was used.Also, *HOW* did you couple the audio signal into pin 5?

author: Zero999
author_rank: Super Contributor
timestamp: January 26, 2016, 08:00:32 pm
content: The circuit will work, if it's designed properly.That circuit will perform pulse position modulation. The length of each pulse is the same but the frequency varies depending on the amplitude of the input signal.The carrier frequency needs to be much higher than audio signal on pin 5, try 20kHz.The signal should be AC coupled to pin 5.

author: ali6x944
author_rank: Frequent Contributor
timestamp: January 26, 2016, 08:44:27 pm
content: what value cap should i use for the coupling cap?

author: Zero999
author_rank: Super Contributor
timestamp: January 26, 2016, 08:52:34 pm
content: You need to look at the internal schematic of the 555 timer to figure that one out.http://www.sentex.ca/~mec1995/gadgets/555/555.htmlHint: the input impedance is equal to the equivalent of R7, in parallel with R8 + R9.

author: ali6x944
author_rank: Frequent Contributor
timestamp: January 26, 2016, 10:03:41 pm
content: but i use an audio signal the impedance would change as frequency changes , I'mto burn my mp3 player am not risking a third one!

author: Zero999
author_rank: Super Contributor
timestamp: January 26, 2016, 11:11:04 pm
content: Quote from: ali6x944 on January 26, 2016, 10:03:41 pmNo.QuoteHow on earth did you do that?

author: Audioguru
author_rank: Super Contributor
timestamp: January 27, 2016, 12:58:13 am
content: 5k parallel to 10k is 3.33k That is the resistance of pin 5 of a 555. It is a hundred times higher than 32 ohm headphones that are normally used with an MP3 player so it can drive the 555 pin 5 perfectly if you use a coupling capacitor to block the DC but pass the signal.The impedance of the coupling capacitor changes with frequency so make its value high enough so it passes all audio frequencies especially low frequencies. A capacitor that passes 20Hz into 3.33k ohms is 2.4uF, use a standard value of 2.2uF or 3.3uF.What will be the output of the 555? It will not be audio and it is too low a frequency to be a radio signal. If the modulation is 20Hz then the carrier frequency of the 555 will go up and down at 20Hz which will make its carrier frequency flutter.

author: ali6x944
author_rank: Frequent Contributor
timestamp: January 27, 2016, 10:20:40 am
content: Oh,

author: ali6x944
author_rank: Frequent Contributor
timestamp: January 27, 2016, 04:36:19 pm
content: sorry, i actually was shorting the mp3 player ground through the power rail vcc that's why the things were smocking !oh also i found a 0.1uf cap to work just fine!thanks everybody thanks a lot