Hi folks, have a question I'm hoping someone can help with, my electronic skills are terribly rusty and I'm outside my comfort zone on this.
I have a 555 timer circuit I borrowed from a site and got it put together on a breadboad and it works just as I need and expected it to.
But given my stock of parts, I set out to explore how to swap the NPN BC547 transistor with a PNP, in this case I've been trying to use a 2907A.
I've just had no luck. Attached is the original circuit and some of my scrawl.
Ok, again, rusty remember... I think my biggest problem is the right polarity on the base lead. But since this ties directly into the output of the 555 I've gotta fully understand the 555, which I don't. I haven't done this sort of thing in years and I'm flying by the seat of my pants.
I sorta understand that the output is being fed back into the lower comparitor, pin 2 I think and a logic probe on pin 3 with the NPN transistor does show the output in constant hi/lo pulsing. But I'm unable to keep the correct operation of the 555 part of the circuit while I try to modify it for the PNP....
In my case I'm using a piezo motherboard buzzer... and in general it works pretty good.
I'm not using the same caps, I'm using two .47uf caps instead.
They have an effect on tone pitch but unlike in other basic 555 circuits without the transistors where they provide reverse current to keep the buzzer sounding during switching of the timer circuit (that I thought I at least understood fairly well), this one, I think uses the caps to cause the 555 to pulse which drives and undrives the transistor to the buzzer leg. Any confirmation and clarification on this is appreciated.
At this point, I'm just dying to understand the entire thing better. Anyone in the mood for some explaining?
Thanks in advance,
/jim
I have a 555 timer circuit I borrowed from a site and got it put together on a breadboad and it works just as I need and expected it to.
But given my stock of parts, I set out to explore how to swap the NPN BC547 transistor with a PNP, in this case I've been trying to use a 2907A.
I've just had no luck. Attached is the original circuit and some of my scrawl.
Ok, again, rusty remember... I think my biggest problem is the right polarity on the base lead. But since this ties directly into the output of the 555 I've gotta fully understand the 555, which I don't. I haven't done this sort of thing in years and I'm flying by the seat of my pants.
I sorta understand that the output is being fed back into the lower comparitor, pin 2 I think and a logic probe on pin 3 with the NPN transistor does show the output in constant hi/lo pulsing. But I'm unable to keep the correct operation of the 555 part of the circuit while I try to modify it for the PNP....
In my case I'm using a piezo motherboard buzzer... and in general it works pretty good.
I'm not using the same caps, I'm using two .47uf caps instead.
They have an effect on tone pitch but unlike in other basic 555 circuits without the transistors where they provide reverse current to keep the buzzer sounding during switching of the timer circuit (that I thought I at least understood fairly well), this one, I think uses the caps to cause the 555 to pulse which drives and undrives the transistor to the buzzer leg. Any confirmation and clarification on this is appreciated.
At this point, I'm just dying to understand the entire thing better. Anyone in the mood for some explaining?
Thanks in advance,
/jim
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