Timer is the key controlling component of an aeroponic setup. The common timers available in the market says 24/7 on it. Don’t get deceived by it. They are actually drip irrigation timers which has a set number on/off times per day. Typically at the max 8 times per day. Those timers cannot be used for aeroponics. Aeroponic system needs cyclic type of timer that repeats the cycle continuously. Heavy duty industrial use timers like “SELEC Digital Din rail Timer – 600DT” are available for around Rs.1200. They are multi functional. Refer the manual and set the interval as needed.
If you wish to do it yourself, it is quite easy to build a 555 timer circuit. It needs to run in astable mode. Here is a sample 555 astable multivibrator circuit. Just add a relay (to run the pump) to the Output Pin 3. I have used the following components to achieve 20 seconds ON (t1) and 20 minutes OFF (t2):
R1 = 47K
R2 = 1M
C1 = 470uF
R1 and R2 can be variable potentiometers and you can adjust them to vary the t1 and t2 duration.