"Flip-Flop" in a 555 Timer demo board

Question

I'm looking to put together a 555 timer on a PCB as a learning aid (that is, put three 5k resistors, a couple comparators, a flip-flop, transistor, and an inverter on a PCB to represent the internal workings of a 555).

All of the components are pretty straightforward to find except the flip-flop.

Since the flip-flop inside the 555 is a level-triggered flip-flop (as opposed to an clock-enabled flip-flop typically available - as described in this comment), I'm having trouble finding an appropriate IC for this.

Does anyone have any recommendations and/or advice?

Answer 1

This Q covers many things 555: Is the NE555 the IC I need, and if not, what do I replace it with?

One of the answers explores the internals of the 555. You can use that as a guide to construct your homebrew 555 from the following:

• two comparators
• two 3-input NOR gates
• one inverter (for RESET) or use another NOR gate
• one NPN or n-FET transistor (for DISCHARGE)
• three 5K ohm resistors

The two NOR gates form an R-S flip-flop that's triggered by the comparators.

If you want to model a device with an extended voltage range like the 555 itself, your best choice is to use a CMOS gate like the CD4025B 3-input NOR. For the comparators you could use LM393 (but don't forget to add the pull-ups on the outputs) or a push-pull type like the TLC3702.

Answer 2

Any flip-flop with a S and R input will work.

Better yet, make your own with a couple of NOR gates (controlled by a '1' at the input).

SR Flip-Flop: NOR or NAND?

However, use a 3-input NOR gate for the reset gate so it the flip-flop can be reset from either the "Threshold" comparator or the "Reset" input of the 555.