silvertonessuckbutigotone Posted May 22, 2006 Report Share Posted May 22, 2006 So, I'm in an electronics class at my school, and we're building our own projects for second semester. My project idea was to use a 555 chip, a transistor, and an op-amp to make a tremolo effect. I'll try to explain it as simple as possible for those who arent as in depth into electronics as me or some of the gurus around here. So first we have a 555 chip, which oscillates (varies the voltage at a steady period) at any given frequency, depending on capacitor/resistor sizes. In this circuit we're working between say 2 and 10 Hz (1 Hz= 1 cycle per second). Since the volume of a guitars signal is determined by amplitude, and not just the level of voltage, applying the output of the 555 chip would not be sufficient for raising and lowering the volume. So in order to understand how I am going to use it, you must understand how to wire an inverting amplifier using an op-amp. http://img.photobucket.com/albums/v461/gle...ngamplifier.jpg Thats the schematic for an inverting op-amp. The inverting input (the one with the negative sign on it) is used as an input, through a resistor. R1 in this case would be reffered to as the input resistor. The non-inverting input (with the positive sign) is connected to ground as a reference. The output is connected to the inverting input through a resistor, which would be called the feedback resistor. The beta (gain, or amount of amplification) is determined by the sizes of those resistors. The formula is Ri/Rf=B. In this case we dont actually want to amplify the sound since we are working with line levels, so we would use 2 resistors of the same size (I'm using 1ks, however the size doesn't matter). So in order to change the amplitude, we have to change the Beta, and in order to do that, we must change the size of one of the resistors. But how do we periodically change a resistors size? We can't, however the voltage applied to an NPN transistor's base will change the bias, or internal resistance between the emiter/collector junction. So by hooking the inverting input to the emitter and the output to the collector, and the 555 output to the base, we are effectively steadily moving the resistance between the inverting input and output up and down. http://img.photobucket.com/albums/v461/gle...ngamplifier.jpg That is our schematic so far. As to the size of R3, we use atleast 1K, to limit the current flowing through the transistor. I have spent a ton of time trouble shooting and improving this circuit and its still in the works, I still need to test it using an actual guitar. So far all I've used is a function generator (a peice of equitment that puts out a small AC signal) and an oscilliscope (a peice of testing equitment that allows you to view the actual wave that you are hooked up to) and it seems to be working fine. The wave that the function generator puts out is basically a perfect sine wave, and the output of the op-amp is equally as smooth. Right now I'm doing all this testing on a breadboard, and eventually will transfer it onto a PCB. I will gradually expand on it and let it grow more and more intricate. I believe the first expansion I will do is sending the output signal through a second inverting amplifier because the output of an inverting amplifier is of course... inverted. So sending it through two would give you an in-phase output. Most of the time in a guitar, it doesnt really matter if the signal is inverted or not, considering you're not mixing the signal with anything, but i want to leave myself open if i ever decide to try some parralel effect tricks or something. Something I might also do is have a switch to switch between the two outputs of the 555, one of them is a square wave and the other is more triangular, so i can send it through either more sudden volume changes or more subtle ones. I will also use a potentiometer to determine the frequency of the 555 so i can go slow or fast. And of course, I will add a pot to change the depth also. Any questions/comments would be appreciated, I will probably post the final schematic, including the 555 pinouts and any switches/potentiometers I use, proper voltages and resistor sizes, etc. Quote Link to comment Share on other sites More sharing options...
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