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I'm designing a circuit that needs a 2 kHz square wave, which will be generated by a 555 timer.

Looking at the reference circuit schematic...:

555 astable timer schematic

I've worked out compatible values for R_A, R_B, and C that will work via an online calculator. My goal is to have as close to a 50/50 duty cycle as possible. In the limit, if R_A = 0R, then both charging and discharging are through R_B only and it's 50/50. This would suggest R_A = 0R, R_B = 3k52 (i.e., a 3k3 and 220R in series), C = 100nF.

Is there a limit to how low R_A can be? Can I use R_A = 100R? 1R? Eliminate it and short DISCH to Vcc with a jumper? (Would this fry the circuit? Or be very power-wasteful? Or..?)

As a second related question, I can't find guidance in the datasheet for a good value for R_L. If OUT is a push-pull output from the internal op-amp, why is R_L necessary? Should I just add a 1k0 (I assume, to improve the OUT rise time?) and call it a day, or is there a relevant equation here? If it matters, Vcc = 6V as I'll be using 4x AA batteries to power the circuit.

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2 Answers 2

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If you need 50% duty cycle there are a few ways to do it.

You can use a CMOS version and a circuit like this:

schematic

simulate this circuit – Schematic created using CircuitLab

As shown in this datasheet Fig. 1a

Or you can use diode steering around the frequency determining resistors like this:

schematic

simulate this circuit

Another option is to double the frequency of the 555 and use a D type flip-flop to divide the output by two.

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  • \$\begingroup\$ You can also use a potentiometer with the arm to threshold and trigger, and diodes on each end to the discharge. Then you can adjust duty cycle to 50% or whatever is desired. Here is an article showing this (although the schematic is a bit sloppy): skinnyrd.com/… \$\endgroup\$
    – PStechPaul
    Dec 11, 2022 at 8:45
  • \$\begingroup\$ I like the flip-flop method best - as long as the original 4kHz is stable, it produces a perfect 50% duty cycle, which seemed to be very important to op \$\endgroup\$ Dec 11, 2022 at 9:08
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as low as you want bearing in mind the maximum discharge current given in the datasheet, 200mA for many parts.

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