Latching Relay vs FlipFlop Circuit

[Rocketman46]

Hi all,

I need some analog design advice. I am designing a circuit that every time a switch is pressed the output latches to the opposite state. I will repeat this circuit 10 times for 10 switches.

In regard to reliability, what is the most reliable circuit, the relay or the flipflop latch design. Also could you point me to what is considered the best most reliable configuration and components.

Thanks,

Rocketman46

 

[crutschow]

The solid-state circuit is always more reliable than the mechanical switch, assuming it's used well within its ratings.

How much voltage and current does the load require from the relay/FF?

 

[Rocketman46]

Hi Crutschow,

The parameters are below:

* pressed switch voltage = 5v

* latched output = 5v

* current = ~500ma

Thanks

Rocketman46

 

[KlausST]

Hi,

Designing electronics is always combined with specifications. Mostly values with units.
The only values we see is "I will repeat this circuit 10 times for 10 switches." but this has nothing to do with reliability.

Therefore it's impossible to answer your question.

For sure the wear out of a soloid state relay may be less than a mechanical relay ... but this quickly turns into the reverse if there is inductive load causing high voltage spikes...

I think reading the datasheets is the only way to get reliable informations.

Klaus

 

[Rocketman46]

Hi,

I have checked the circuit through, and no inductive voltage spikes.

So could you recommend a good reliable circuit?

Thanks,

Rocketman46

 

[schmitt trigger]

Each circuit has its own set of pros and cons.

Solid state (flip flop) latching circuits will be cheaper for sure, although they will require a small board and a few other components.
But if you lose power, you also lose the state it was in.
Latching relays will retain its state even with no power applied.

 

[crutschow]

If you tell us more about the exact application for this circuit, we can better recommend a proper solution.

 

[dick_freebird]

You have to start with "reliable, to what?".

Relays are rated to 100K cycles. You might get more if the
contact stress is low. Integrated circuits can (not to say,
will, under abuse condition) deliver billions of cycles.

But if you're going to flip it twice a day, and that's it,
fuggedaboudit.

Now the latching relay has a nice feature, in that it
will "remember" what it was set to, across power loss.
A flip-flop will reset ("to something"). That's a whole
different dimension of reliability. Maybe resetting at
power-on is a feature though. We can't make that call.

Relays can take abuse that ICs can't, and walk (or limp)
away. But if this isn't part of the picture, then so what?

 

[Rocketman46]

Hi all,

Thank you for your inputs.

This flip flop circuit is in between (in series) between a 5v push button control board, and an ecu. The ecu requires latching operations to control functions. After reading further I can reduce the current supply to the ecu to 10ma.

Updated parameters are below:

* pressed switch voltage = 5v

* latched output = 5v

* current = 10ma

The circuit I have chosen is the 3rd circuit down (setting the start state) on the below link, and I have replaced the output motor with a resistor. Also in this default design, when the circuit is switched on, the output will always be logic zero 0.

https://www.pcsilencioso.com/cpemma/flipflop.html

Look forward to your reply.

Rocketman46

 

[crutschow]

That circuit should do what you want logically.
But if you want a 5V output, you will need to add a 5V regulator, such as a 7805 (plus required decoupling caps at input and output), to power the circuit, and if you need 5V out, 10mA to ground, you will need to add a PNP buffer transistor.

 

[Rocketman46]

Hi,

Just to confirm my thoughts before I redesign the circuit:

1) I can change the 12v input to a 5v input and the flipflop circuit will run correctly if I change the output FET to a 5v friendly version?

2) The output PNP after the FET is used to limit the current to 10mA?

If the above is correct I will draft a new circuit up.

Thanks,

Rocketman46

 

[KlausST]

Hi,

What circuit are you referring to?

especially this:

2) The output PNP after the FET is used to limit the current to 10mA?

No schematic at the given link of post#9 includes a PNP. Or am I blind?

--> show us your circuit.

Klaus

 

[Rocketman46]

Hi,

So do i need to remove the output FET and replace with an NPN or, add an NPN in addtion to the output FET?

Thanks,

Rocketman46

 

[KlausST]

Hi,

impossible to answer as long as you don´t show a circuit.

Klaus

 

[Rocketman46]

Hi,

I have uploaded an image of my circuit. I believe this should give 5V 10mA on each latched switch press.

Can you confirm if my circuit is correct and any feedback welcome.

Thanks,

Rocketman46

View attachment FF.pdf

 

[KlausST]

Hi,

Your "output" is just 5V. Always. It is not switched.
On key press it toggles. One time it consumes about 10mA from the 5V supply, on the next toggle it consumes about nothing.
Is this what you want?

About your circuit:
There is an RC at the clock input. It's time constant is 100us .... and only in falling direction. The 100us are too low for a true debouncing. This is about useless. Adding a series resistor and re-calculating the values may improve funtion.

Your true debouncing circuit is in the feedback path (D input). It generates a tau of 22ms in both directions. This makes sense.

Klaus

 

[Rocketman46]

Hi KlausST,

Thanks for getting back.

My desired output after system startup is as follow:

1) Button press 1, output = 5V, 10mA

2) Button press 2, output = 0V, 0A

3) Button press 3, output = 5V, 10mA

4) Button press 4, output = 0V, 0A

5) etc etc

So I can go back and redesign the circuit, is it best to stick with an n-fet or change to an npn transistor for this application?

Many thanks,

Rocketman46

 

[KlausST]

Hi,

You can´t define "output = 5V, 10mA" and "output = 0V, 0A"
* Either it is 0V/5V voltage controlled,
* or it is 0mA/10mA current controlled.

Voltage and current depend on load resistance.

--> Tell us what´s the use of your "output". Where does it go to? What is it connected to and how?

Answer this question before we do the next step.

Klaus

 

[Rocketman46]

Hi,

The module on the output of this ff circuit requires a latched 5v output, from 5v to 0v. The module requires 10mA to operate. So I guess the correct wording is a latched 5v output that can supply 10mA - my bad explanation sorry.

Thanks,

Rocketman46

 

[KlausST]

Hi,

I don't know a device that needs 10mA on a logic input. Therefore I doubt it.
It's a pity that your module is so secret that you can't tell what it is.

I really want to help, but it's too boring for me to ask again...

Klaus