# Need help for a Dflop implementation in LTspice

I'm trying to implement an analog debouncing filter which uses a 555 timer and a D-flop. Here is the recommended circuit.

But when I simulate this in LTspice as below, the output is 1V constant:

Above red V2 represents the 555 timer clock, green V1 represents the noisy input signal; and blue Vout represents the output.

Why is the debounced output not the same as in the page I follow? How can I correct it?

• Your input is only getting to about 1V minimum. This may be above $V_{IL}$ of the flip flop. But based on the 1V constant output I think something is not grounded properly (FF output should be either VCC or GND) – jbord39 Oct 31 '16 at 1:38
• Is the D flop a model of a particular technology; LS, HC, or HCT; or is it just a generic model of LTspice's on making? – owg60 Oct 31 '16 at 2:18
• There's no need to ground unused pins for A-devices, they are internally connected to the 8th pin, which is ground. You can see this if you view the netlist. Also, R1 is completely useless because voltage sources have (machine) zero internal resistance. For your circuit to work, add a DC voltage source in series with V1, that has -1V. Be sure to connect it such that you subtract it from the original signal, not add it. – a concerned citizen Oct 31 '16 at 6:39