Issue converting push switch to toggle action using a D-Type flip-flop

Question

I'd like to convert a momentary switch in a toggle switch using a D-Type flip flop from an SN74HC74AN.

For this reason I've created and simulated a circuit like this:

All works fine on the Multisim Simulation.

Then I've created a real prototype on a breadboard like this:

But, in this case, when I press the momentary switch button nothing happens and the LED remains ON.

I know that the CLK input must be triggered but, on the sn74hc74an datasheet I read:

"Dual D-type positive-edge-triggered flip-flops with clear and preset"

I've also added a 3.9 nF capacitor on the input side but got the same results.

Where am I wrong?

Update.

Eventually I found the right solution.

The issue was related to non triggered signal sent to the 74HC74 inputs pin. Using Schmitt-Trigger Inverters like CD40106B CMOS all works as expeted.

The builded circuit is this and works like a charm.

Answer 1

The device is triggered on the rising edge so I wouold flip your input circuit where the switch pulls the input high. Your preset and clear both need to be tied to +. Then tie the inputs of the other gate to ground or + it doesn't matter. At that point it should work. Your capacitor not shown value may have to be adjusted. TonyM also is giving you a good circuit but remember to terminate the unused flip-flop.

Answer 2

There are some clear design faults in your circuit: floating inputs and no switch debounce. Fixing them will most likely make the circuit work. If not, they will make it possible to debug it further.

Floating inputs

The /1PR and /1CLR inputs on DFF1 are floating. They typically float high but may well not. If they go active, they will stop the DFF responding to the CLK input.

Connect them to the VCC supply rail, preferably through a resistor (e.g. 10K) but a direct connection is fine. Do this for all unused inputs on the 74HC74, including DFF2.

Switch debouncing

The switch contacts will bounce together multiple times when the switch is pressed and released. You won't get the perfect clean action that you might see in simulation, as the example waveform below shows.

Switch debouncing is already well documented on the internet so you can find out about it there and the various solution circuits. Typical debounce periods to allow are 5..10 ms but look at the specs' for your switch.

In short, you can debounce the switch by adding a capacitor in parallel with the switch that, in conjunction with your 10K pull-up, creates an RC circuit that greatly slows the rise time of the DFF1 CLK input. However, this very slow rise time will be outside of the spec' for the 74HC74 CLK input. Although it will work, and has done in such circuits for decades, it's not the ideal design choice.

You also change your switch to a changeover switch and make a debounce circuit using DFF2 in the 74HC74, as shown here.

^{simulate this circuit – Schematic created using CircuitLab}