Cuando se trata de circuitos para temporizadores es difícil la elección de un modelo que se adapte a ciertas funciones específicas. Algunos temporizadores más simples no son precisos suficientemente y los más avanzados son demasiado caros para una pequeña aplicación.

El Timer propuesto en este artículo, y que puede ser visto en la figura 1, queda en el rango intermedio, entre aquellos que son muy simples y, por lo tanto, no confiables, y aquellos que son demasiado poderosos para ser desperdiciados.

 

Figura 1 - Diagrama del temporizador
Figura 1 - Diagrama del temporizador | Haga click en la imagen para ampliar |

 

En este circuito tenemos inicialmente una base de tiempo hecha con el viejo 555, que será responsable de la unidad básica en que será contado el tiempo.

A continuación, tenemos un divisor de frecuencias programable, hecho sobre la base de un único circuito integrado del tipo 4518 (doble contador de década), y dos llaves selectoras con salidas binarias, o Thumbheel Switches - como se llaman normalmente.

Este circuito dividirá la frecuencia de entrada por el número que está seleccionado en los Thumbheels y así, cuando las salidas del contador 4518 asumen un valor igual al seleccionado, obtendremos en sus salidas un nivel lógico alto.

El nivel lógico, obtenido entonces, activará el relé vía transistores y bloqueará el oscilador de la base de tiempo para que el relé se mantenga disparado indefinidamente.

De esta forma, para una base de tiempo de un minuto, podemos obtener temporizaciones de cualquier valor entero entre 1 y 99 minutos, bastando seleccionar el número deseado en los Thumbheels.

La frecuencia de la base de tiempo se puede ajustar en el trimpot P1, pudiendo situarse entre menos de un segundo hasta unos 5 minutos aproximadamente, la precisión global del temporizador dependerá únicamente de la precisión de ajuste de la base de tiempo.

Indicando el funcionamiento del circuito, tenemos el LED1, que parpadeará en la frecuencia de la base de tiempo, y el LED 2, que se encender después de accionar

El circuito estabilizador de tensión 7806 mantiene la precisión del conteo en función de las variaciones de la tensión de alimentación.

Para utilizar el temporizador, basta con seleccionar el número de minutos (para la base de tiempo de un minuto) después de los cuales se activará el relé.

En la figura 2 tenemos la placa de circuito impreso por nosotros proyectada para el circuito.

 

Figura 2 - Placa para el montaje
Figura 2 - Placa para el montaje

 

 

 

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