Resistors and capacitors calculation for 555 timer circuit

I am new to electronics and I want to try an experiment.

I want to use a 555 timer to create an automatic food dispenser for my aquarium (something like opening a small door when the timer has elapsed) and I need some help in timing here.

To have a configurable amount of time (lets say 6 hrs, 12 hrs or 24 hrs), I understand that I need to modify resistors and capacitors accordingly.

Can you please let me know if there is any standard formula or tool for calculating the resistance value required for my resistors and the capacitance value required for my capacitors in my experiment.

Note: If I am completely wrong with my approach, please let me know a better way for creating an automatic timer. I am completely new to electronics and just got this basic info over the net.

• This is a good calculator to use: houseofjeff.com/555-timer-oscillator-frequency-calculator Jun 24, 2015 at 12:59
• Unless you want to feed your fish every 10 minutes I would advise against using a simple timer such as the NE555. You cannot easily achieve such long times due to leakage currents. For longer times like a couple of hours either use a microcontroller or a faster timer (which could be a 555) combined with a digital counter. Have a look at schematics using the CD4060, that can do very long timings ! Jun 24, 2015 at 13:03

For timing in the hours, and if you really don't want to use a microcontroller, you could consider using something like a CD4060, which contains an oscillator and digital divider chain. You could also use a 555 and a counter such as a CD4040, but the 4060 has it all on one chip.

Here's one example circuit.

If you increase the value of C1 to 2.2uF you'll get 10x the times shown.

You may want to trigger another timer (such as a 555) monostable to provide a pulse for the feeder.

One advantage of the divider chain is that if you want to calibrate the time by using an adjustable resistor (trimpot) you can look at the timing on the earlier divider stages and not have to wait hours for each incremental adjustment of the time.

It's not necessarily impossible to achieve timing in hours with a 555, especially a CMOS type, but it's not very easy or rewarding.

I would consider this to be a poor approach. The 555 is not a good choice for time intervals of more than about an hour — the resistor and capacitor values get to be too large, making it difficult to maintain any kind of accuracy.

You might be better off exploring a solution based on a microcontroller, such as Arduino.