Circuits Question

Patrick

http://www.velleman.be/Downloads/0/Manual_MK104.pdf

I am an ME with a simple EE question.·In the attached schematic, what is the purpose of the resistor in parallel with the buzzer in the upper right corner?· If I wanted an LED to come on when the buzzer was on, I would do the circuit analysis such as (Vsource - Vled) / (rated LED current)· to come up with the correct series resistor right?· With an LED, would I still put that resistor in·parallel with the buzzer?
·
Thanks
Pat

Beau Schwabe

In this application the buzzer is a "piezo"... since the transistor driving the piezo is configured as open collector, you need something to discharge the piezo.
Thus the resistor in parallel with the piezo accomplishes this. It might help to think of a piezo as a capacitor, they both have similar characteristics.

As for the led, the formula you have is·correct, but if you still want to use the buzzer, I would leave the resistor in parallel.

- Beau Schwabe

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔

---

Beau Schwabe - Mask Designer III

National Semiconductor Corporation
(Communication Interface Division)
500 Pinnacle Court, Suite 525
Mail Stop GA1
Norcross,GA 30071

---

Post Edited (Beau Schwabe) : 10/19/2004 3:00:25 PM GMT

Patrick

Thanks for the response.· Does the resistor discharge the piezo by, when the transistor is not allowing current through, the resistor/piezo creates its own little circuit discharging the piezo as it would a cap??· Why is this even necessary?· What would happen if you removed the parallel resistor?

Beau Schwabe

Yes, the resistor discharges the piezo when the transistor is off. When you apply power to a piezo it deflects the crystal shape, and holds that shape as long as there is a charge. If
you removed the resistor you would hear an initial "click", and then perhaps a very weak "buzz" (due to internal resistive leakage)... by applying a resistor, the piezo discharges much
faster relaxing the shape of the crystal so that the next pulse causes it to deflect again. More deflection = more pressure or volume

See...

Parallax Forums > Public Forums > Stamps In Class > Amplifying piezo speaker

...for some other details.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔

---

Beau Schwabe - Mask Designer III

National Semiconductor Corporation
(Communication Interface Division)
500 Pinnacle Court, Suite 525
Mail Stop GA1
Norcross,GA 30071

---

Patrick

When I put the LED in series with the buzzer, I read 5 volts across the buzzer and .8 across the LED.· This is not enough to illuminate the LED of course.· How can I force the right amount of voltage across it?


·

Beau Schwabe

Don't put the LED in series with the buzzer...Connect the cathode (short terminal) of the LED to the collector of T1. Connect the Anode (long terminal) of the LED to
one terminal of a series resistor (Resistor Value determined below) and connect the other resistor termianl to D1's cathode.

A K
--->|---

The resistor value should be....


Rled = ( Vsource - Vled ) / Iled


Where:

Vsource =7.8V (remember you have three diode drops; one across T1 another across D1, and a third across your LED)
A typical transistor diode drop is about .6 Volts, so 9V - 1.2V = 7.8V

Vled =1.7V (typical - you can measure this with a digital volt meter on the diode setting)
Iled =10mA (typical)


Rled = ( 7.8V - 1.7V ) / .010A = 610 Ohms or closest standard value

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔

---

Beau Schwabe - Mask Designer III

National Semiconductor Corporation
(Communication Interface Division)
500 Pinnacle Court, Suite 525
Mail Stop GA1
Norcross,GA 30071

---