At the start of this contest I thought "this is easy! Last time there were 30 people and there was no task. I have a real chance at this!".
Now I'm not so sure.... I've been trying for 2-3 weeks now to resume work on my Hush-Puppy project.
I started with a commercial product:
I took it apart, analized the board, realized the f*ckers scraped off the IC's markings. WHY?!?! Why would you bother? I assumed it's a timer, obvious choice was 555, but it was DIL14, so 556 maybe? I traced out the pins and I'm 95% sure it's a 556. The second timer chip was used for the flashing LEDs.
So I built a 555 circuit on a breadboard. It worked at flashing a LED, then added a transistor, then faster pace, then added the speaker. It sort of works, as in you can hear the sound, but it way too low. It actually is 12V, just like the "battery" voltage.
OK, I thought, I can up the battery voltage, but up to what point? I's not a viable solution. I have to do something on the output. What I need is a transformer. I tried adding a mains transformer to up the voltage, but that seemed to make little difference.
This is when things started to change: I accidentally connected the transformer while the piezo was still connected. A massive increase in volume. WOOOO!! where did that come from?! Inductive kick-back, probably, as the high peaks were negative. CH2 is the 555 output, CH1 is the onverted piezzo voltage.
All is well untill things start to go south - first a transistor goes up in smoke, then another one, then a 555. At this point I am thinking the transistor has to withstand that high voltage, so I get a MOSFET, being that none of my BJTs go up to 60-70V. I manage to find a 60V FET, that one runs for like 1 minute ultill it goes pop, zener too, 555 too.
I then tried different inductors only to reach the logical conclusion that more inductance yields a louder sound (more kick-back). I settled on a mains filter. Further breadboarding testing got me frustrated because wires kept coming off, contacts were not perfect, so I built a PCB for it.
Next run gets me an exploding elactrolytic (happened to be a low ESR one) and anothe 555. At this point I resorted in adding a socket for the IC and also adapted SMD 555s for DIL usage. I also placed an order for 20 555s, 50 high voltage transistors and lots of caps. Pictured here are only some victims of this carnage:
Now I realize this only happens when I run the circuit without the piezo speaker. Meaning all of the inductive kick-back goes back on the power rail, instead of in the speaker. I become extra carefull in not running the circuit without the piezzo. I also start wearing gogles, since the last capacitor blew right in my face (I mounted it in reverse polarity).
OK, so now I get to thinking why is the voltage settling on 60V? How much voltage does the commercial unit give?
At this point I have to say my yellow commercial unit unit was made deffective by my poor skill at unsoldering the transformer. I broke it, got mad and threw it away. My better half took out the garbage, so by the time I realized I overreacted, it was too late, the transformer was gone. By this time I borrowed a unit from a friend.
Measured the output, it was 240vpp, perfect sinewave. The board looks like this:
and as you can see there is a
my-name-is-Martin-and-I-think-it's-for-calibration potentiometer. And by that very logic I assumed it was for frequency adjustment. I fiddled around with the settings until I realized it was for someting else, perhaps the duty cycle. Working the potentiometer did not really modify the frequency by much, but it made it look way off from a perfect sinewave (on the piezo side).
So I took that as a challange and thought "why 240Vpp?!? The chinese surely got it wrong, I can go higher." I back-traced the circuit and figured the 555 was powerd from a 6V zener (can't remember exactly). I replaced it with a 7V zener and all hell broke loose - the waveform was way off, maxed at some 50Vpp. Replaced the original zener, realized it's sole pupose was to stabilize the 555s frequency, since the piezo was run off the transistor. I never managed to achieve those 240Vpp again, only 220Vpp. I returned the unit to my friend, he stated it "works" just as fine (on shy dogs, if you ask me...)
All of this time I have been using this piezo:
but after a brief shopping spree I got a hold of these
After a few weeks of break, I returned to my project. I started with another aproach - an LCD inverter. I tweaked the components to give me the right frequency (15-30kHz) and all went well, untill I connected the piezo. The output went way low, just like the transformer was shorted. It now comes to mind that maybe a series resistor would have changed thing...
I then reverted back to the 555 circuit and tried a higher supply voltage - 30V from a printer SMPS. The voltage would not go higher that 60V, no matter what.
Unfortunatly, somewhere in here my car broke down. I took it to a specialist that told me X is broken, he can fix it for ~800$. That made me angry, since I knew X was ok. Decided to be my own mechanic, so I started learning shi... stuff. I am still learning and fixing and this is taking most of my spare time. It's not that difficlt, but since I never did it before...
That is why I cannot properly complete my project. I never got to the point where I try Steve Gibson's single FET circuit. I would have liked to do that.
Wohooo, this took some time to post.
