New to forum and electronics, need help understanding a circuit

[Shane1974]

Hi everyone, new to electronics and new to this forum. I have a little understanding of electricity as it applies to motorcycles and powersports but I am trying to expand on my knowledge by using one of those Elenco electronic kits that does 130 projects on a single board. Been enjoying it quite a bit but I am currently stuck on a certain concept that is being used by several of the beginning projects. The current project is a single transistor metronome. I have uploaded the wiring diagram. I understand that the capacitor and transistor is what is turning the speaker on and off but I don't fully understand how the electrons are flowing. I believe that once the circuit is closed the 10uF capacitor charges through the collector/base diode of the transistor and once fully charged it creates a positive charge that shuts down the base of the transistor to stop the flow of electrons through the collector/emitter which would turn off the speaker and then the capacitor discharges until the base side of the transistor reaches a negative state compared to the collector. The potentiometer controls the rate of discharge and therefore the rate at which the metronome is heard. Maybe I am getting that right, maybe not. Where I get really confused iis the other side of the 10uF capacitor and its relationship to the transformer powering the speaker.

Anyway i really appreciate any help you can give.

 

[Bluejets]

Don't get discouraged over this one.
For a start, reading circuit diagrams that are drawn unconventional can be a task for the best of us.
Then using pnp thrown in with negative battery switching.

I am pretty sure I worked out how it works but I'll avoid any further confusion and let one of the more experienced explain the operation.

 

[CDRIVE]

It may interest you to know that I Spiced your circuit and it in fact produced a "Tick , Tick" sound on my laptop speakers.

Chris

 

[Shane1974]

What does Spiced mean?

Anyway Bluejets no one else has stepped up so I am totally wanting to hear your explaination, please.

 

[(*steve*)]

What does Spiced mean?

It means that he emulated your circuit using Spice. Spice is an electronic circuit simulator.

 

[wingnut]

Hi Shane1974

You asked what was happening on the + side of the 10u cap....
1. Close the key and you close the circuit - the big square with two side branches, one going through the top coil of the speaker and the other branch going to the base of the npn transistor.
2. The npn transistor is biased negative through the pot to start with, so the transistor is a closed switch allowing conventional current to flow through the lower half of the speaker (5 to 4). Electrons flow opposite to conventional current, i.e. from 4 to 5 through the lower part of the speaker, from the -ve terminal of the battery, through the npn to the +ve terminal.
3 For a metronome, this condition has to be unstable, so something has to make the base +ve to make the npn into an open switch.
4. As you rightly diagnosed, that is the charging of the 10u cap, which as you rightly said depends on the value of the pot (and if you increased the cap to 20u it would slow the metronome).
5. At the start, when the key switch was closed, electrons flowed through the top part of the speaker coil to the 10u cap making the top of the cap start to become -ve. In exactly the same amount as the top becomes -ve, the top of the cap repels electrons from the bottom of the cap, making the bottom half of the cap increasingly more +ve.
6. The +ve side of the cap is attached to the base which then assumes the same charge as the +ve side of the cap, and stops the flow of current, turning the npn transistor into an open switch.
7. But slowly, depending on the value of the variable pot (rheostat, variable resistor) at the bottom, electrons seep through the pot, back to the base and the +ve side of the 10u cap, slowly turning this, and the base -ve again.
8. As the base, and +ve side of the 10u pot turn negative, the npn's base becomes negatively biased, conducts current, turns into a closed switch.
9. So we see the transistor opening and closing, sending current or not through the speaker.

 

[Shane1974]

That was a fantastic explanation Wingnut. I was getting confused because, being a motorcycle mechanic, I was taught to follow conventional theory (even though we are told about electron theory). I tried to use conventional theory to understand the circuit but now using the electron theory, with your explanation, it totally makes sense. I going to use this explanation to go back through some other, similar circuits I built with my kit to see if they make more sense now.

Again thanks Wingnut and everyone else who checked out and/or commented on the thread.

I have another question now, lol. Is Spice the same as Autodesks Circuits? Which one do you prefer?