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NE555 Formula w/ diode

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ormo

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http://www.kpsec.freeuk.com/555timer.htm#dutycycle

I have a circuit to turn a buzzer on and off at regular intervals. I am using the above circuit (the one with a diode in parallel to R2). The formulas on the site state that:

Tm = 0.7 * R1 * C1
Ts = 0.7 * R2 * C1

Where R1 and R2 are in ohm and C1 is in Farads

I am using a 1000µF cap for C1 and 270Ω and 22000Ω resistors for R1 and R2 respectively.

According to the formulas above I should get:

Tm = 0.7 * 270 * 0.001 = 1.9 seconds
Ts = 0.7 * 22000 * 0.001 = 154.0 seconds

However I get:

Tm < 1 second
Ts ≈ 18 seconds

The circuit is being powered by 1 * 9v battery that is actually giving roughly 8.1 volts.

Can anyone explain where the error I have made could be? I have already tested both resistors and they are correct to within a few Ωs.

Thanks in advance!
 
Ok, I just tried adding another 1000µF capacitor in parallel with the existing one to give 2000µF capacitance and both Tm and Ts pretty much doubled so I'm guessing that the problem is with my formula, not the capacitor.
 
Does anyone have any ideas? I really need to fix this ASAP as I'm on a tight schedule
 
The formula is OK.

What not is OK is the printed value of the cap. Electrolytic caps have high tolerances (+20/-80%)

Boncuk
 
I am using a 1000µF cap for C1 and 270Ω and 22000Ω resistors for R1 and R2 respectively.
R1 should be higher than 1000Ω. 270Ω is too low for pin7 of the 555 to switch.
Multiply the resistor values by 10 so you have 2700Ω and 220KΩ resistors for R1 and R2 respectively. Your 9V battery will last longer too. :)
BTW, you made a miscalculation:
Tm = 0.7 * 270 * 0.001 = 0.189 seconds
Ts = 0.7 * 22000 * 0.001 = 15.4 seconds

Tm = 0.7 * 2700 * 0.001 = 1.89 seconds
Ts = 0.7 * 220000 * 0.001 = 154 seconds
 
Last edited:
Electrolytic caps have high tolerances (-20/+80%)
Fixed.

That's a valid point and is why there's no point in using 2000µF capacitors and 2k7 resistors when 2200µF a capacitor and 2k2 resistor are accurate enough.

2200µF is also far too large, replace it with 220µF and multiply all the resistor values by 10.
 
So if I increase R1 above 1KΩ then the formulas will work properly?
 
Yes, and as I already noted, you originally dropped a decimal point somewhere:
BTW, you made a miscalculation:
Tm = 0.7 * 270 * 0.001 = 0.189 seconds
Ts = 0.7 * 22000 * 0.001 = 15.4 seconds

Tm = 0.7 * 2700 * 0.001 = 1.89 seconds
Ts = 0.7 * 220000 * 0.001 = 154 seconds

The quote from the website:
Choosing R1, R2 and C1
R1 and R2 should be in the range 1kohm to 1Mohm.
 
Why are you bothering to calculate things so accurately?

Given the poor component tollerances you might as well round things:

Tm = 0.2s
Ts = 15s

Tm = 2s
Ts = 150s
 
Why are you bothering to calculate things so accurately?
To avoid the inevitable question of, "How come you calculated 2 seconds when I calculated 1.89 seconds? What am I doing wrong?"
 
I suppose you're not doing anything wrong, I'm just making the point that you shouldn't expect that degree of accuracy.
 
I guess the confusion comes from the fact that I was talking about formulas and you were talking practical circuitry. ORMO's calculations were originally out by apx 1000% which isn't explained by typical electrolytic capacitor tolerances but by a math error. Since Boncuk had already pointed out the typical electrolytic capacitor tolerance ranges, I felt it unnecessary to repeat it.
 
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