Buzzer Circuit Oscillation

[hj47]

Hi Everybody,

I have a circuit that operates a buzzer when triggered by a small voltage change, sorry i have no schematic.
Anyway when i set it to trigger the buzzer with the slightest voltage change i get an oscillation through the buzzer. I can set it so there is no oscillation but then it is not as sensitive.

If i connect an Oscilloscope probe Ground lead on the -12v or 0v Circuit ground the oscillation stops & i can get a very fine adjustment for triggering but with this lead not connected i get the oscillation.

The circuit will be run from a battery when finished but is powered by a small dc power supply at the moment. I have noticed the power supply has no ground pin on the AC plug, only Active & Neutral, could it be a grounding issue, i have no suitable battery on hand to try.

 

[Ross Craney]

You will need to have hysteresis built in.
What is probably happening is you are getting to your trigger voltage & the buzzer is sounding. When the buzzer sounds it causes a small voltage drop to beneath you trigger voltage. So the buzzer stops. The voltage drop from the buzzer is removed , the voltage increases & trips the buzzer again. I think !

 

[hj47]

Ross Craney,

Yes that does seem to sum up what is happening, I found the data sheet on the comparator i am using & it has Hysteresis information.

Thanks

 

[Hero999]

Agreed, adding, hysteresis should help.

Also, te capacitance of the 'scope probe is squelching the oscillation, add a 100pF capacitor in place of the probe and it shouldn't oscillate, if it doesn't work increase the capacitance but 100pF should be enough.

 

[hj47]

Thanks Hero999,

I tried various resistors on the comparator---LM311---across the positive input (Pin 2) to the output (Pin 7) as the data sheet suggests.

I can decrease or increase the oscillation by doing this but i cannot eliminate it, adding resistance increases the oscillation.
By using less resistance across the comparator it eliminates it a little to the stage where the will buzzer stay on constantly when there is not enough resistance.

I will try the capacitor as mentioned, it's quite a frustrating problem as like mentioned with the ground connection of a probe connected, the circuit works perfectly.

 

[hj47]

I found that using a capacitor as suggested did solve the oscillation problem but then the sensitivity went south as well.
I have changed the circuit somewhat & now have no oscillations through the buzzer.

I can adjust the sensitivity with various trim pots until the circuit is very sensitive but the ideal settings seem to drift off & constant adjustment must be made to keep it tuned to perfection.
There are no components at all running hot or even warm but the circuit drifts off the ideal most sensitive settings.

What would cause the drift from the ideal settings & how to fix it?

 

[Hero999]

Please post the schematic.