Other Parts Discussed in Thread: TLC555
The LM555 is to generate the square waveform with duty 50% and freq=240Hz.
Pease help to check if the circuitry is ok or not?
Application: Monitor TFT LCD
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Brian,
You are really close in concept. Your circuit is 60Hz 50%. So decrease capacitor 4X or decrease resistors 4X or do a combination.
The TLC555 model is good for simulating LM555 at lower frequencies.
User,
The original schematic is the best way to get 50% duty cycle but the math is complex.
The good news is that the formula for RA and C is simple.
The charge time (output high) is given by:
t1 = 0.693 (RA + RB) C
t1 = 0.693 (RA + 0 ) C [RB is zero]
C = 0.1uF [my choice]
T1 = 1/240/2 = 2.0833ms
RA = t1 / (0.693 * C) [FYI 0.693 = ln(2)]
RA = t1 / (0.693 * 0.1uF)
RA = 30,056 = 1/240/2/LN(2)/0.0000001
RA1% = 30.1k [closest 1% value]
The calculation for resistor "RD" is complicated. However, I found that the 50% duty cycle for RD is 42.33% of RA
RD = RA * 42.33%
RD = 30,056 * 0.4233 = 12,723 ohms
RD1% = 12.7k [closest 1% value]
All of this ignores propagation delay, but this won't matter at 240 Hz.
Hi Ronald,
Thanks for your reply.
Can I use the valuse as below for my request?
RA=2Kohm
RB=1K ohm
C=1.5uF
User,
Make RA 3.57k 1%
Make RB 294k 1%
Make CM2 10nF
Nominal output frequency 241 Hz, duty cycle 50.4%
Hi Ron,
Thank you for your suggestion RC values.
Because I don't have a suitable resistor value and capacitor value to use in my company.
I have to pick the common RC values for product design.
Otherwise.
About my circuit, RA2 ,RB2 , and CM5 are not mount (SMT) on PCB.
The formula should be modified as below.
Frequency = 1.44 / ( (RA + 2 * RB)* C)
RA = 200 ohms
RB = 60.4k ohms
C = 100nF
F = 119Hz
Duty= 49.92%
Is it right ?
What is negative duty cycle?
Negative duty cycle = 100% - positive duty cycle
Your blue text numbers work with the formula.
Can you increase RA to something close to 1000 ohms? This will help with supply current and better low discharge voltage for better accuracy.
At 200 ohms and 50% duty cycle the average RA current is 5V/200*50% = 12.5mA
At 1000 ohms and 50% duty cycle the average RA current is 5V/1000*50% = 2.5mA
The expected discharge low voltage is marked in the chart below.