NE555 Flip Flop Question

[wattsup]

I've breadboarded an NE555 flip flop with a dpdt relay to make a true bypass switch. It works great but when I power it down then back on again, it defaults to the off state. Is there a way to have it remember the last state it was in when I power it off then on again? Or at least start in the on state?

Schematic I used is from this page:

https://diy.thcustom.com/switching-relay-true-bypass-circuits-not-using-a-microcontroller/

[Rob Strand]

I've breadboarded an NE555 flip flop with a dpdt relay to make a true bypass switch. It works great but when I power it down then back on again, it defaults to the off state. Is there a way to have it remember the last state it was in when I power it off then on again? Or at least start in the on state?

There's no way to rememeber the state unless you keep the NE555 permanently powered.

The power-on state depends on a whole lot of factors.   Some circuit aren't set-up correctly and they kick the NE555 into the wrong state at power up.  This can arise in many ways depending on the specifics of the circuit.   There's two ways out.  Arrange for the circuit to power up on the state you want, or, add a power on reset circuit to force the NE555 to the desired power-up state.

I think you will find this problem has been discussed in the past.   Search through the forum archives.

[PRR]

You need non-volatile memory. Data does not evaporate easily.

In the 1950s, magnetic donuts held their state through power loss but needed a heap of ancillary circuits to write and read.

CMOS can hold state for a year++ on a small battery. See CMOS Cookbook by Don Lancaster. (10MB PDF) Pg 261

Flash Memory, also Disk Drives, will hold state but again need a heap of ancillary circuits to write and read.

A latching relay will hold state through loss of power and modest physical abuse; also uses no power except when switching, and is probably the function you want.

(A now-obscure technology was the ratchet relay.)