Understood, okay then, I will rebuild it with 3AA batteries supply, thanks for pointing out the potential issue.
What about the voltage/current spike that may be caused by induction coil in parallel circuit? Is that even a concern I should have at all?
PS: Also, just thinking out loud, the pro mini 3.3v has MIC5205 voltage regulator and just looked into datasheet, seems that it can take input voltage in 2.5-16V range, so if I understand correctly powering form 3V pack should be okay then, or still not a good idea?
Still not a good idea. You can power a pro Mini with 4.5 V on the Vin pin. The 3.3 V regulator is an LDO.
(Low DropOut )
With no actual waveform data on the coil, no vendor's link for the coil , no electrical specificatiins on the coil, not even a photo of the coil, all I can say is :
"did you already post these or am I supposed to have a crystal ball ?"
You'll need a 1N4001 flyback diode across the coil: flyback diode
as well as a decoupling cap and electrolytic cap across the supply that will power the coil.
You should have at least a 220 uF filter cap on that supply and a 0.1 uF decoupling cap also.
It is quite possible the diode might affect the timing for the coil. You'll have to do some empirical testing and try the coil without a diode , then try it with a diode to see what effect it has on the sound. If if does affect the sound you can try inserting a resistor in series with the diode. Experiment with the resistor value, starting low (47 ohms) and increasing it to 100, 220, 330, 470, 1 k 2.2 k until the effect on the piezo sound decreases , (if at all). I have never tried it but worst case, if none of the above is successful in elimating the effect of the diode on the coil turn-off time, you will have to eliminate the diode and use a transistor rated for Vce breakdown of 60V+.
What is the switching frequency of the coil driving squarewave on the base resistor ?
raschemmel:
You'll need a 1N4001 flyback diode across the coil: flyback diode
as well as a decoupling cap and electrolytic cap across the supply that will power the coil.
You should have at least a 220 uF filter cap on that supply and a 0.1 uF decoupling cap also.
It is quite possible the diode might affect the timing for the coil. You'll have to do some empirical testing and try the coil without a diode , then try it with a diode to see what effect it has on the sound. If if does affect the sound you can try inserting a resistor in series with the diode. Experiment with the resistor value, starting low (47 ohms) and increasing it to 100, 220, 330, 470, 1 k 2.2 k until the effect on the piezo sound decreases , (if at all). I have never tried it but worst case, if none of the above is successful in elimating the effect of the diode on the coil turn-off time, you will have to eliminate the diode and use a transistor rated for Vce breakdown of 60V+.
What is the switching frequency of the coil driving squarewave on the base resistor ?
Switching frequency is 2.5 - 3.5kHz, thanks for all the great info.
raschemmel:
Part number of coil driver transistor ?
I am not sure yet, I need to buy most parts so I will perhaps be ordering a bunch of transistors and picking up what fit the best, whatever you suggest basically :). I'm new to this, what you already know and can calculate in your mind will take me some time to wrap my mind around.
This is the transistor I have immediately available: BD679AS onsemi | Discrete Semiconductor Products | DigiKey