NE555 damaged by ultrasonic operation?

[geeheeb]

Hello:
First of all I must start with the classic "I'm new to this forum so excuse if this post is in the wrong section / this was already discussed etc" I am also inexperienced in electronics as well. By the way, I really did search for this subject and found nothing, so I feel free to post this.

I used an atmega168 programmed to toggle a pin on and off using a delay loop in order to obtain a 40 khz output. Something like:

ldi r20, 0xff
ldi r21, 0x00
loop:
out PORTB, r20
delay_loop
out PORTB, r21
delay_loop
rjmp loop

I tested it using different delay times with a LED and it was working ok.
I then set the delay so that the output was 40 khz (ultrasonic).
I attached a 100 nF capacitor for decoupling a cheap piezoelectric buzzer: PORTB1-----CAP----PIEZO----GND
when I connected the circuit both the atmega168 and my programmer (an usbtiny which was being used for powering the AVR board) died.

While I wait for my programmer's replacement, I tried to obtain an ultrasonic signal using another piezo and a NE555 timer using a rheostat to manually increase the frequency, worked fine for a while and when I brought the NE555 to a theoretical 100 khz the chip died as well, using the same connections: NE555_OUT-----CAP----PIEZO----GND.

Is there any intrinsic limitation for such frequencies using general purpose IC's like the ones mentioned?
Could the CAP and PIEZO combination have generated high voltage spikes?
Could some internal crystal or cap have been damaged by using them this way?

I'd really appreciate any light that could be shed on this issue by any generous light bearer!

Thanks!

 

[duke37]

Post your 555 circuit. To get a high frequency I presume that the pot would have a low resistance. Have you shorted something?

 

[davenn]

Hi geeheeb

welcome to the Electronics Point Forums

I suspect you problem lies in the fact that you were trying to drive the piezo directly from the ATMEGA and 555 timer outputs. I doubt that either of them could supplythe current required to drive the piezo, and in trying to do so they failed.

Generally the way would be to use the output of either chip to drive a transistor which would then drive the piezo element.

I will dig up a cct or 2 as examples .... there may still be some on the forum from previous queries

Dave

 

[davenn]

OK here's a couple of circuits using 555 timer....
you could use the same output method from your ATMEGA chip

cheers
Dave

 

[geeheeb]

That's exactly the circuit I used (the ECELab's). I varied to values of R1 & R2 using rheostats and the cap to obtain different frequencies... But I tried to directly drive the piezo. I omitted R3 and Q1.

According to:

http://old.nabble.com/-EE-:-Piezo-buzzers-td808688.html

"piezos are very capacitive" that makes me think there might have developed a resonance between the "decoupling" cap I used and the piezo leading to a voltage spike... Mybe I'm wrong though...

 

[davenn]

That's exactly the circuit I used (the ECELab's). I varied to values of R1 & R2 using rheostats and the cap to obtain different frequencies... But I tried to directly drive the piezo. I omitted R3 and Q1.
...

thats where you went wrong ... as I commented in my first post... you need to drive the piezo/ultrasonic transduce via a resistor and a transistor, else you are likely to to kill the output of the mpu or 555 through excessive current drawn


Dave