# 555 timer circuit with d type flip-flop

Found this circuit for a 555 timer. I can't understand the use of the d-type flip-flop. Please explain.. And is the variable resistor used to adjust the frequency?

The D-types just divides the 555 output by two generating a square wave output from both.

The issue with that though is there is nothing in the circuit to ensure the two D-Types do not start out 180 degrees out of phase.

As you suspect the pot adjusts the frequency.

• still unclear to me. Isn't the output of the 555 timer already a square wave. Mar 3, 2018 at 18:03
• Not when you want to have a variable frequency. Mar 3, 2018 at 18:05
• @JOHNDOE getting an exact square wave out of a 555 is actually a bit of an art especially with variable frequency. Mar 3, 2018 at 18:16
• @Trevor_G. Not to be picky, but I would point out that a flip-flop can ONLY put out a perfect 50% duty cycle, as long as the frequency is not changing.
– user105652
Mar 3, 2018 at 20:46
• @Sparky256 true that :) Mar 4, 2018 at 13:55

The D-flipflops are connected to provide a divide-by 2. I assume the main reason for those is so that the output duty cycle remains 50/50 even when the frequency varies (Which is not what comes out of the 555.)

The FET gives and inverted output but they could have gotten that by using the Q-bar output so maybe they needed a specific drive characteristic.

• so when you vary the frequency the duty cycle of the 555 does not remain constant Mar 3, 2018 at 18:07
• one more question I want the output square pulse to have a pnp output and a npn output how can I modify the circuit to achieve this? Mar 3, 2018 at 18:12