First thing to correct. In post#3 the pulse time is inaccurate. Using those values (47k, 4.7k, 1uF) you get 39 seconds (not 60 sec).
Now to answer your question of being able to change the speed, change the resistors and capacitor of 555 circuit with the following values:
C=1uF, R2=1k, and replace R1 with two resistors in series. One is fix 5k in series with a potentiometer of 100k.
So in this format R1 value changes from 5k to 105k, changing your time delay from 5 seconds to 74 seconds approx.
As for your LED, putting it in parallel with the diode up there, as you mentioned, might unintentionally create an oscillation. I would rather put the LED and its series resistor in parallel with gate to source of Nmos (this already has a Gate-Source diode of 1v to 1.5v so it wont do much harm).
Hi EG,
I suspect your timing is still too short. The requirement is 30 seconds to 60 seconds on and off period. Being lazy, and using a ball-park calculation, 100K and 1UF give a time constant of 100 milliseconds. Post #4 shows a time constant of 47uF and 2M = 95 seconds when the frequency potentiometer is at its maximum resistance and 47uF and 22K = 1 second when the potentiometer is at its minimum resistance, so the timing provided includes the OPs requirements and more.
By the way, the 47uF timing capacitor is a tantalum type for lower leakage current compared to an aluminum type. The Rolls Royce approach would be to use a polypropylene (PP) timing capacitor, but a 47UF PP capacitor would be big and expensive. The value of the timing capacitor could be reduced by an order by increasing the 2M potentiometer to 20M, but such a high value would not be advisable as any humidity may affect the timing.
AK's suggestion of using a digital period generator would provide a Rolls Royce plus performance but with more cost and complexity.
Notice that I have specified an LMC555 rather than an LM555. An LMC555 has a much higher input impedance on its TRIGGER and THRESHOLD pins and its ouput swings rail to rail. The C MOS version of the 555 has other benefits too.
I looked at a few circuits to implement this function and, so far, the simplest, but not best performing, would be to use an LM339 quad comparator for period generation and driving two NMOSFETs which, in turn, powered the solenoids.
But, as you imply, there are more ways than one of killing a cat than by poking its eyes out with a sharp stick.
spec