Piezo/Mosfet/Keyboard problem

[Peter]

I have a keyboard attached to a PICAXE micro on a lonnnnng cable
(50m). I want to add a piezo to the keyboard so I can buzz it to
indicate invalid keystrokes etc. The Pic has the ability to control
the 3 LEDs on the keyboard so I figured it should be a simple matter
to connect a transistor and piezo to one of the LEDs then flash the
LED rapidly to produce the buzz in the piezo. I tried it last night,
both with a transistor and a mosfet without any luck. Any ideas? (PS.
I know that I could run a separate wire for the piezo but I do NOT
want to do that.)

Here is the LED circuit. The keyboard IC is between the LED and
ground:

+5V -----/\/\/\----->|---- KBD IC --- GND
680

So I figured I could just add a PNP transistor/P channel Mosfet before
the IC like this, and flash the LED at around 500Hz:


+5V -----/\/\/\----->|--.-- KBD IC --- GND
680 |
| +5V
| |
| /
| |/
'-/\/\--|
470 |\
\------- PIEZO ------ GND

I tried it with a PN200 transistor. Also with a MTP2955 MOSFET without
the gate resistor. Absolutely nothing. I know with an N channel Mosfet
you often need a resistor from gate to ground to bleed off the gate
charge. With the P channel I wasn't sure what was required. I tried a
10k between gate and Source and also Gate and Drain but still nothing.
What am I doing wrong? (I didn't have the long cable attached when
testing, just the standard one.) I did notice strangely that when I
put a DMM across the Piezo to check the voltage across it it did give
a very faint hum.

 

[John Popelish]

I have a keyboard attached to a PICAXE micro on a lonnnnng cable
(50m). I want to add a piezo to the keyboard so I can buzz it to
indicate invalid keystrokes etc. The Pic has the ability to control
the 3 LEDs on the keyboard so I figured it should be a simple matter
to connect a transistor and piezo to one of the LEDs then flash the
LED rapidly to produce the buzz in the piezo. I tried it last night,
both with a transistor and a mosfet without any luck. Any ideas? (PS.
I know that I could run a separate wire for the piezo but I do NOT
want to do that.)

Here is the LED circuit. The keyboard IC is between the LED and
ground:

+5V -----/\/\/\----->|---- KBD IC --- GND
680

So I figured I could just add a PNP transistor/P channel Mosfet before
the IC like this, and flash the LED at around 500Hz:

+5V -----/\/\/\----->|--.-- KBD IC --- GND
680 |
| +5V
| |
| /
| |/
'-/\/\--|
470 |\
\------- PIEZO ------ GND

I tried it with a PN200 transistor. Also with a MTP2955 MOSFET without
the gate resistor. Absolutely nothing. I know with an N channel Mosfet
you often need a resistor from gate to ground to bleed off the gate
charge. With the P channel I wasn't sure what was required. I tried a
10k between gate and Source and also Gate and Drain but still nothing.
What am I doing wrong? (I didn't have the long cable attached when
testing, just the standard one.) I did notice strangely that when I
put a DMM across the Piezo to check the voltage across it it did give
a very faint hum.

A piezo is essentially a capacitor that changes shape slightly when
the voltage across it is changed. Your circuit applies 5 volts across
it and then disconnects from it, leaving its capacitance charged to 5
volts. After that, additional pulses do nothing ot the voltage on the
piezo. You need some way to discharge the charge stored on the piezo
when the transistor switches off. A resistor in parallel (perhaps
10k) with it would be the simplest solution. However, piezo devices
last longer if they do not see long term DC. So a more complex
solution might work better. But you should verify that it works at
all with the change I suggested.

 

[Peter]

Thanks for that. That would explain why I got a faint hum when I
paralleled the piezo with the DMM.

At $0 for the piezo (from an old musical Christmas card) I can afford
to replace it if it gives up the ghost.

Maybe I could just add an N channel mosfet to the circuit with the
gate connected to the P channel drain and the drain/source connected
between the piezo input and Gnd. That way when the P channel turns on
the N channel turns off. When the P channel turns off the N channel
turns on and discharges the piezo.

PS. I still cannot work out why the P channel mosfet did not seem to
work at all. I put a DMM across the Drain/Source and it did not change
regardless of whether the LED was on or off. It just sat on 5v and
stayed there I would have expected it to drop to zero when the base
was pulled low. Do I have to do anything with it to drain off the gate
charge, like with an N channel? eg. should I tie the gate high with a
10k say?

 

[John Popelish]

Thanks for that. That would explain why I got a faint hum when I
paralleled the piezo with the DMM.

At $0 for the piezo (from an old musical Christmas card) I can afford
to replace it if it gives up the ghost.

Maybe I could just add an N channel mosfet to the circuit with the
gate connected to the P channel drain and the drain/source connected
between the piezo input and Gnd. That way when the P channel turns on
the N channel turns off. When the P channel turns off the N channel
turns on and discharges the piezo.

Should work.

PS. I still cannot work out why the P channel mosfet did not seem to
work at all. I put a DMM across the Drain/Source and it did not change
regardless of whether the LED was on or off. It just sat on 5v and
stayed there I would have expected it to drop to zero when the base
was pulled low. Do I have to do anything with it to drain off the gate
charge, like with an N channel? eg. should I tie the gate high with a
10k say?

I don't think the problem was with the switch (transistor or mosfet).
Your approach simply did not have a mechanism to discharge the voltage
applied to the piezo after the switch turned off. The first pulse
charged it to 5 volts and it stayed there after the switch opened, and
was still there for all the future pulses.

The two mosfet push pull inverter does not have this problem. But I
doubt it sounds much different than a single transistor driving the
parallel combination of the piezo and a resistor.