How can a flip flop alternate between two clocks, turning one off and the other on and repeating?

Question

I am just experimenting with flip flops and am pretty confused about alternating between two clocks. Right, so I want a pulse to be sent to a flip flop first after 18 seconds and then after 2 seconds and then again after 18 seconds.

For this purpose, I have made two 555 timer ICs which generate 18s and 2s pulses respectively. What I have been struggling to do is to, after the 18s timer IC generates a pulse, activate the 2s timer IC and deactivate the 18s timer IC and after the 2s timer IC generates the pulse, I would like to deactivate it and activate the 18s timer IC.

What I want? I want the flip flop to get a pulse after 18seconds, then after 2 seconds, then after 18 seconds and then 2 seconds and so on.

Any help would be greatly appreciated.

Answer 1

Probably, the most simple way to produce the clocking signal you need is to produce a single clock \$Q\$ with a period of 20ms and a 90% duty cycle: by inverting such clock (for example by using a NOT gate or a single fast transistor inverter) you will get another clock \$\overline{Q}\$ with the same period but with a 10% duty cycle. Each of this waveform is shown below:

Then, the original clock signal will provide the 18ms pulse, while the inverted one will provide the 2ms, for example by ORing the outputs of the following simple RCD trigger circuit:

^{simulate this circuit – Schematic created using CircuitLab}

By calling \$t_1\$ the trigger generated by feeding \$Q\$ to the input the of the above circuit and \$t_2\$ the one generated by \$\overline{Q}\$, these signals have waveforms similar to the following ones:

And ORing the two triggers you'll get the following composite one:

A few notes

1. You can use one 555 to generate \$Q\$ and then inverting its output: however the simplest way is perhaps to generate both \$Q\$ and \$\overline{Q}\$ by a single, properly designed, astable multivibrator.
2. The RCD trigger circuit show above is simply an analog differentiator: \$R\$ and \$C\$ control the duration of the trigger pulse, while \$D\$ rules out the negative pulse which would otherwise appear during the falling of the input squared wave signal.
3. This is an hybrid analog/digital solution to your problem: if you need to work with only logic gates, perhaps the solution to which awjlogan alludes is the best.