Describimos un circuito que puede funcionar como un termómetro acústico o para demostrar la sensibilidad de los empalmes de los semiconductores a las variaciones de temperatura. Se trata de un proyecto didáctico-experimental simple de montar.

   Los empalmes de los dispositivos semiconductores, como los diodos, hacen que estos dispositivos puedan ser usados como sensores de temperatura.

   De hecho, en microprocesadores encontramos diodos que monitorea la temperatura del chip, activando el ventilador cuando se eleva y en muchas otras aplicaciones.

   El circuito que describimos sirve para demostrar esta sensibilidad de las uniones, con la utilización de un diodo común como sensor.

   En nuestro proyecto, un oscilador cambia de frecuencia de acuerdo con la temperatura del diodo sensor.

 

    Como funciona

   El sensor usado es un diodo común que al ser polarizado en sentido inverso tiene la corriente circulante determinada por la temperatura de su unión.

   En la figura 1 mostramos el comportamiento de este dispositivo en un gráfico simple.

 

Figura 1 - Corriente de fuga de un diodo en función de la temperatura
Figura 1 - Corriente de fuga de un diodo en función de la temperatura | Haga click en la imagen para ampliar |

 

   De esta forma, nuestro proyecto tiene el diodo sensor cuya corriente de fuga es amplificada por el transistor Q1 y aplicada en la base del primer transistor de un oscilador de audio.

   En este oscilador, la frecuencia depende de R2 y C1.

   El circuito puede ser alimentado por dos o cuatro pilas y su sonido se vuelve más agudo cuando la temperatura del sensor sube.

   

Montaje

   En la figura 2 damos el diagrama completo del oscilador.

 

   Figura 2 - diagrama completo del oscilador
   Figura 2 - diagrama completo del oscilador | Haga click en la imagen para ampliar |

 

    Como se trata de montaje muy simple, podemos utilizar un puente de terminales, como muestra la figura 3.

 

Figura 3 - Montaje en puente de terminales
Figura 3 - Montaje en puente de terminales | Haga click en la imagen para ampliar |

 

   En el montaje, observe la posición de los transistores y principalmente la polaridad del diodo sensor.

  El conjunto sensor se puede instalar en un tubo para un manejo más fácil, como se muestra en la figura 4.

 

   Figura 4 - Montaje del sensor
   Figura 4 - Montaje del sensor | Haga click en la imagen para ampliar |

 

   

Para probar el aparato, coloque inicialmente P1 en la posición de máxima resistencia.

    Batiendo sobre el sensor el sonido emitido debe cambiar.

   Podemos entonces ajustar P1 para que el circuito quede en el umbral de las oscilaciones.

   Aproximando el sensor de cuerpos calentados (sin tocar) y esperando un poco notaremos cambio en la emisión de sonido.

 

Q1, Q2 - BC548 - transistores NPN de uso general

Q3 - BC558 - transistores PNP de uso general

D1 - 1N4148 o equivalente - cualquier diodo de uso general

FTE - 4 ó 8 ohms - altavoz pequeño

S1 - Interruptor simple

B1 - 3 o 6 V - 2 o 4 pilas pequeñas

P1 - 1M2 - trimpot

R1, R2 - 10 k ohms x 1/8 W - resistores - marrón, negro, naranja

R2 - 1 k ohms x 1/8 W - resistor - marrón, negro, rojo

C1 - 47 nF - capacitor de cerámica o poliéster

 

Varios:

Puente de terminales, soporte de pilas, hilos, soldadura, etc.

 

 

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