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Latching Toggle Relay with powerfail remembrance

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would this really be cheaper than $6.07 ? (which is the price for the 4 relays?)
Those are really cheap relays.
Are they reliable devices?

But after some conjuring, I came up with a fairly simple circuit to convert a single, dual coil, DPDT latching relay into a toggle flip-flop that will remember the setting when the power is removed without a backup battery.
It also requires only about 120μA of quiescent operating current.

The circuit is shown below:
Momentarily pressing the PB switch (S_PB) causes the circuitry to turn on one of the MOSFETs, enabling current through the respective relay coil, and causing it to change stages.
The coil signal pulse-width is determined primarily by the time-constants R3C1 and R4C2.
The PB signal is routed through the closed relay contact to energize the opposite state relay coil, flipping the relay state.
This leaves one SPDT contact to use as desired.

One important characteristic of this circuit is that the coil pulse width is independent of the trigger pulse width (as long as it's sufficient to start the relay contacts opening), so even through the PB pulse is routed through one of the contacts, the contact changing states doesn't affect the coil pulse width and the relay is always reliably switched.
This is shown in the LTspice simulation below where the coil pulse-widths (red and green traces) are longer (≈350ms) than the input 100ms pulse width (blue trace).

The simulation shows the relay alternating states (yellow trace) for each push of the button.

I didn't calculate the parts cost, but all the parts are quite inexpensive and I'm quite sure would cost less than 4 relays.

upload_2018-6-18_9-10-29.png
 
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Just to throw in another circuit. From "Ideas for Design" in the Electronic Design May/June 2018:
https://www.electronicdesign.com/an...lip-flop-remembers-state-during-power-failure
Like that circuit.
Obviously about as simple as you can make a toggle circuit with one latching relay.

Below is the LTspice simulation of the circuit.
I had to change the design slightly, since the latching-relay model I have has a positive common connection for the two coils, thus the circuit polarities had to be reversed, with the capacitors connected to V+ instead of common.

upload_2018-6-18_16-0-12.png
 
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ccurtis.

I posted that in #22, and Crutschow posted a modified version of it in #23.

Ken
 
Ok, so i made this scheme and it worked.

However, i wanted to use the same 12V power supply to create a variable Voltage to be switched.

I noted that the load was 1kOhm resistance (according to documentation), and i bought a variable rotary resistance that can handle 0.5W with resistance 50MOhm.

I tuned the variable resistance to get 6V which all worked nicely...

until i connected the load, suddenly, I got 10V, and turning it much less was difficult, because it could get to 8V then to 4V, and then to < 1V ...

It seems like the load is interfering with the voltage divider in some way...

Does anyone have an idea how i can fix this?

also, when i tried to switch it off with the relays, i got suddenly 12V instead of 0V like it was when the load was not connected...
 
Does anyone have an idea how i can fix this?
You can't, not with what you have. When you measure the voltage out with a high impedance voltmeter there is really very little load on your source resistance. When an actual load is applied the divider out resistance will change since your load is in parallel with half of your divider. If I place a 10K pot across a 12 volt source and adjust my pot for exactly mid range I will measure about 6 volts on the wiper. What happens when I place for example a 5K load on that wiper? What will happen to the voltage out I previously had?

Ron
 
You need a voltage regulator to generate a variable voltage such as an LM317.
That will give a constant voltage independent of load (up to its current limit).
 
Right, I educated guessed; when resistance is a big difference (x100) in parallel, it's the highest resistance that doesn't matter much; ...

In effect, per load (i might have more in parallel), i'll need a constant resistor per load, I assume my 12V voltage generator will keep it's voltage as long as i don't go over the max power.

This means, that if i have a resistor(1k rated for 6W) in series for each load (1k); i'll have 6V over each load, which is satisfactory for me.
 
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