PWM circuit for a dc motor

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burnsy007

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Hi all,
I need abit of help on a simple circuit really but I am struggling. How do 2steering diodes work connected in series with a 47k fixed resistor and 47 k variable resistor connected to the trigger input of the 555 timer. The variable bit of the 47 k goes to pin 3 with the discharge pin controlling a switching transistor and how can I calculate the timing period. This circuit is unlike the ones one the rs datasheets where you can see if it is a monostable or astable. C1 is from trigger to ground 100nF. The 1k from the variable resistor I assume is for protection when the resistor is at zero ohm.
Thanks
 
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