Some help with transistors?

[robinsonb5]

Hi

I'm new to the forum and wondered if someone could give me a little help with transistors.

I've been playing with one of those Gotek floppy drive emulators and figured that it needed some kind of audio feedback for that authentic retro feel!  So I hacked together a little circuit using BJTs and a piezo sounder.  The result works...

...sort of.

Occasionally I get a parasitic oscillation that makes the sounder squeak when it's not supposed to.  It goes away if I touch the sounder, so I put a capacitor across it, which helped but didn't get rid of it entirely.  Since I don't have a massively solid grasp of transistors most of the component values here were selected by intuition or trial-and-error, and rather than just swap out yet another component in the hope of killing this oscillation, I figured it's time to ask whether there's a better way to achieve this.

I want to keep the circuit as simple as possible since I'm "rats-nesting" it on the underside of the Gotek PCB.

I've attached a schematic of what I've got so far - if anyone can suggest improvements or corrections I'd be grateful

VCC is 12v, taken from the floppy power connector.  The step pin is pulled up to 3v3 by the Gotek (and loaded down a bit by my circuit), and triggered by a very brief low pulse (a few µs) - so T2 has to be a type with a very low storage time.  The much higher storage time of T1 should have the effect of widening this pulse, which I presume is a good thing from the point of view of making the pulse audible.

Any insights gratefully received

[KJDS]

Put a 220R into the base of T1 and a cap, probably about 10n in parallel with R4.

A decoupling cap on the supply would help too.

[madires]

What kind of piezo buzzer do you got? There are different kinds, each with it's own driving method.

[robinsonb5]

Put a 220R into the base of T1 and a cap, probably about 10n in parallel with R4.

OK thanks - I'll give that a go

What kind of piezo buzzer do you got? There are different kinds, each with it's own driving method.

It's a bog-standard piezo transducer, rather than a self-contained buzzer.  This kind of thing: http://www.ebay.co.uk/itm/5-35mm-Piezo-Elements-Sounder-Sensor-Trigger-Drum-Disc-/360853719389?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item54048ec95d