Pulse output dynamic time for relais

How to make a pulse output where the 1 time is 250 milli seconds and the 0 time is dynamic.
The 0 time can variate from 0.75 seconds to 5 seconds.

The output is for a diaphragm pump.

Or set the frequency on PWM output.

Maybe you can use the delay node for that.
See also recent discussion : Delay node with variable delay

... followed by a trigger node set to 250mS for the actual pulse.