josan4321

Hi, I have a cheap, 1.5V battery odometer. I'd like that if a specific LCD digit segment is on, an added piezo transducer starts to beep. The piezo connected to the odometer without anything else vibrates, but quite weak. I can't use big components, is it possible to do that with only transitors and resistors? Any hint/tutorial that explain circuits like that?

josan4321

I meant, ways of attach components on a existing circuit with least repercussion.

audioguru

You cou;d add a complicated circuit to boost the voltage so the piezo transducer will be louder. Then the 1.5V battery will be drained much quicked. Or you can add a single transistor and a higher voltage battery for it.

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kchriste

LCD segments are driven out of phase from the backplane when they are on and in phase when they are off. Do you have the piezo connected between the backplane and the segment in question or does one lead go to ground? You could do something with an exclusive-OR gate, transistor and an inductor if that would fit? The problem is running these things from a 1.5V battery; there is little room for voltage drops.

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josan4321

Thanks for your replies.

I did't supose it works that way. I thought that the chip just sended pulsed signal to the LCD segment when wanted to iluminate it. I remember old LCD did a dim oscilation. I relied in this oscilation to generate audible beeps in the piezo.

The piezo wasn't in the original circuit, I got a diaphragm one from a even cheaper wrist watch. I could put it anywhere.

No room enough, I'm afraid.

Let's supose I have this deadly simplified, DC (no pulse signal) circuit (above). Rectangle is the LCD segment. Small letters are the points where I could connect a lead. Values of resistors no supplied.
Let's also supose:
a) V e-f <0.6V
b) V e-f >=0.6V

The below circuit is an idea for case b) (more resistors might be added). But, in case the switch is off, excess of current could flow through the other branch? Could it dry the battery? I'd like to know how can I design these kinds of circuits with minimum repercussion over original currents (again)!

In case a), I think I can put a second battery.

Ideas?

Attached Images

piezo.GIF (4.3 KB, 12 views)

kchriste

Your problem is that you are trying to drive a piezo element that resonates somewhere between 3-6Khz with a apx 100Hz waveform from the LCD driver circuit. What you need is a piezo with a build in driver that runs on 2-3Vdc and then rectify the 100Hz AC into DC. Something like this may work (You'd have to use germanium or schottky diodes) but it's hard to tell how much current the LCD driver can source:

Attached Images

piezo.GIF (2.3 KB, 9 views)

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--- The days of the digital watch are numbered. ---