Creating a Audio signal with a fixed duty cycle

Thread Starter

gustav2

Joined Mar 22, 2018
26
Hey everyone, i'm working on a solid state tesla coil and im currently trying to make a interrupter for the coil trough which I can play music. My current setup (diagram attached) cannot play music, it is interrupted by a 555 timer with a frequency of 75Hz and a duty cycle of ~15%. Now if i change the timer capacitor on the 555 the tone it plays changes. Now i need to find a way to intterupt my circuit with an audio signal but i want to keep the duty cycle at around 15% to save power. How can i achieve this?

Gustav
 

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You would need to put the audio through a low pass filter first. Then you would use a comparator to generate a trigger signal from the audio.
Use the 555 timer in monostable mode (one-shot) and apply the trigger signal from the comparator to the trigger input of the 555. You would need to add something to hold off additional triggers for a short period in order to achieve the 15% duty cycle.
 

Thread Starter

gustav2

Joined Mar 22, 2018
26
You would need to put the audio through a low pass filter first. Then you would use a comparator to generate a trigger signal from the audio.
Use the 555 timer in monostable mode (one-shot) and apply the trigger signal from the comparator to the trigger input of the 555. You would need to add something to hold off additional triggers for a short period in order to achieve the 15% duty cycle.
Hmm yes this seems like the right thing to do, combined with the circuit posted by @Audioguru I think this is the solution. I'm gonna try it and if I end up failing I can always let go of the whole duty cycle idea since any audio signal has a duty cycle lower than 100%. My variac which i use to power up the tesla coil is 8A max so I hope I can make it. Anyways thanks guys, I will let it know if I succeeded.
 

Thread Starter

gustav2

Joined Mar 22, 2018
26
So after some experimenting I think I found the solution, I will use a op amp to create a schmitt-trigger. This will convert a sine wave into a square wave, with the help of a potentiometer I can set the trigger voltage and therefore set the duty cycle. I tried it with a function generator creating a sine wave and it worked, however i did not try it with actual audio yet.
 

DickCappels

Joined Aug 21, 2008
6,388
Does that mean that you are creating PWM with the audio information and then using the PWM to control the power to the Tesla Coil?
 

Thread Starter

gustav2

Joined Mar 22, 2018
26
Does that mean that you are creating PWM with the audio information and then using the PWM to control the power to the Tesla Coil?
That is basically it, since the coil can only be on or off i can't supply it with any analog info such as a unmodified audio signal. So yes i created a PWM with a continiously changing frequency to create audable sound. I think the audio quality won't be great when i feed the opamp a random mp3 audio signal but midi files would work great i think.
 

DickCappels

Joined Aug 21, 2008
6,388
You could have modulated the B+ with audio and achieved a similar effect, but using PWM seems much more elegant -no need for a high power audio amplifier.
 
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