Este artículo muestra cómo construir un circuito que dispara por el acercamiento de un imán, pero que no utiliza reed-switches. El circuito puede ser utilizado en la apertura de puertas, accionamiento de lámparas y otras aplicaciones similares.

   Los reed switches, o interruptores de lámina, consisten en componentes que se pueden utilizar para encender o apagar un circuito por la acción del campo magnético de un imán, como muestra la figura 1.

 

   Figura 1 - El reed-switch
   Figura 1 - El reed-switch

 

   Sin embargo, estos interruptores poseen una pequeña capacidad de control, de algunos cientos de miliamperios típicamente, lo que limita su aplicación.

   Para controlar cargas mayores se deben utilizar circuitos auxiliares.

   Sin embargo, el reed-switch no es la única opción que tenemos para conectar un circuito desde el campo magnético de un imán.

   Una opción interesante es utilizar una bobina captadora conectada a un SCR y eso es lo que haremos en este proyecto.

    La aproximación de un imán con movimiento rápido cerca de una bobina captadora induce un pulso con intensidad suficiente para disparar un SCR y así controlar un relé.

   En nuestro caso, como el circuito es alimentado por tensión continua, una vez disparado el circuito permanece en conducción.

   Para apagarlo hay que interrumpir por un momento la alimentación.

   

Montaje

   En la figura 2 tenemos el diagrama completo del aparato.

 

   Figura 2 - Diagrama completo de la llave magnética
   Figura 2 - Diagrama completo de la llave magnética

 

   Como se trata de montaje muy simple, optamos por la versión en puente de terminales, mostrada en la figura 3.

 

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

 

   El SCR puede tener cualquier sufijo, y no necesita disipador de calor.

   Observe su posición y la polaridad del diodo.

   La bobina es el devanado primario de cualquier pequeño transformador del cual se haya retirado el núcleo e introducido un bastón de ferrita.

  Para alimentación puede ser necesario utilizar 9 V para compensar la caída de tensión de 2 V en el SCR.

   Para disparar, basta con tocar el bastón o pasar cerca de él un imán permanente.

   En la figura 4 tenemos una sugerencia de un circuito completo de una cerradura electrónica con el sistema de rearmar.

 

   Figura 4 - Una cerradura electrónica
   Figura 4 - Una cerradura electrónica | Haga click en la imagen para ampliar |

 

   Los diodos rectificadores son 1N4002 o equivalente y el trimpot sirve para ajustar la sensibilidad.

   Para probar, basta con aproximar el imán del sensor y ajustar al mismo tiempo el trimpot o el potenciómetro.

  

 

SCR -TIC106 - ver el texto

D1 - 1N4148 - diodo de uso general

P1 - 100k ohmios a 1M ohmios - trimpot o pote

K1 - Relé sensible de 6 V

S1 - Interruptor simple

B1 - 6 V - 4 pilas o fuente

X1 - Sensor - ver texto

R1 - 1 k ohms x 1/8 W - resistor - marrón, negro, rojo

Varios:

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

 

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