I was looking at the circuit and made another attempt at it. What I did was use a multimeter to test the resistance offered by the diode. I then used the voltage-divider rule to solve for the voltage drop at the diode. Looking back at that I am not sure that was the right thing to do because the junction voltage was not included in the voltage drop. So now I might be much further away from fixing the problem. Any more suggestions?

 

The diode is not a simple resistor, the drop for the exact same junction varies from current and temperature. Look up Shockley’s diode equation.

 

Can you tell us what your desired time periods are?

 

What I did was use a multimeter to test the resistance offered by the diode.

That doesn't work to measure its "resistance" since a diode is not Ohmic, there is a logarithmic relation between the diode forward voltage and current [see typical small-diode simulation below that shows the forward voltage (yellow trace) and the pseudo spot resistance as calculated by Ohm's law (blue trace) versus diode current (horizontal axis)].

(Note: Don't confuse this spot resistance with the dynamic resistance of the diode, which is the AC resistance at its (arbitrary) operating point for a small AC signal, and is much smaller than this "spot resistance").

A multimeter will just give one spot on that curve, as determined by the amount of current it uses to measure resistance.
For example, if it used 1mA, then it would measure about 800 ohms of "resistance".
And since a multimeter changes the measurement current to change the resistance range, the diode "resistance" you measure depends upon the resistance scale you use.

 

The diode is not a simple resistor, the drop for the exact same junction varies from current and temperature. Look up Shockley’s diode equation.

I did. How do I determine the reverse saturation current though? If I could determine that I could use the silicon characteristics graph to get the voltage.

 

Can you tell us what your desired time periods are?

I'm trying to get about 10 seconds high and 5 minutes low. The main problem is to understand the theory though. For instance, I would like to understand the correct equation for the high and low times.

 

Keep in mind that the Size of the junction and the type of diode will also affect this ratio. Different diodes of the same part numbers will have slightly different curves. Why not just make it adjustable so you can fine tune it.

 

How do I determine the reverse saturation current though? If I could determine that I could use the silicon characteristics graph to get the voltage.

The diode forward drop doesn't vary that much between diodes.
My simulation graphs should work well enough for your purposes to get the forward drop at the diode current you have.

 

The diode forward drop doesn't vary that much between diodes.
My simulation graphs should work well enough for your purposes to get the forward drop at the diode current you have.

Where can I get access to the graphs?

 

Where can I get access to the graphs?

Why can't you copy it from my post #24?

Or do your own graph using the free LTspice simulator program from Analog Devices.

 

I've seen that same equation. From another tutorial using your circuit they show t1 equal to appx. t1 = .8(R1C1)
View attachment 220698

What I was not taking into consideration was the fact that initially the capacitor charges from 0 to 2/3 Vcc but in corresponding periods it charges from 1/3 to 2/3 Vcc. As a result, I was getting a higher charge time initial runs. I was only teaching those initial runs into consideration and it was up until a few minutes ago that I decided to investigate more trials. I'm getting results in according to the equations shown in sghioto's picture.