Electrical – Momentary to Latching Circuit with Zero Off Current

dc motorlatchswitches

I have a project with several physical constraints so that I want to use:
1. A sealed very thin momentary switch as main on/off switch
2. 2 x AAA battery supply = 3V or less
3. Load is a small DC motor pulling max 0.3 amp
4. Momentary to Latching circuit to be as small as possible and with zero current drain in the off state

I have looked a various momentary to latching circuits using transistors, MOSFETS, and Flip Flops but as best I can tell they all have a very small drain current of approximately 10uAmp or more in the off state.

I am correct about the off drain current for these circuits?

If I am not correct can you suggest a circuit that would work for my project?

If I am correct does anyone have a circuit design that would meet true zero drain current for the requirements above?

Many thanks.

Best Answer

Depending on the requirements you could use a latching mosfet circuit as shown below. The first time you push the button M1 is turned on, the second time you push the button it is turned off.

When the circuit is off it does not consume power (other than leakage of the mosfet).

schematic

^{simulate this circuit – Schematic created using CircuitLab}

Related Solutions

Electronic – Resettable mechanical switch

enter image description here

http://digikey.com/product-detail/en/A8GS-S1305/SW1533-ND/4248837

Maybe try one of these. You could have a cap charged to 5V, a depletion-mode fet that is held off when your power is on and dumps the 5V charge through the coil when power is off? This would allow the switch to manually be reset even if power was lost and not purposefully reset.

edit: If it just needs to pop "ON" when the car turns on, have "ON" be the reset position. When the car turns on, have a circuit that momentarily 'resets' the switch to the "ON" position and then the switch will be toggleable.

Electronic – How to restrain this soft latching power switch from eating too much current

The FETs can hog current upto 500 mA. For higher load, it is very easy to find FETs with low Vgs threshold. Before pressing the switch SW1, M1 will be off since Vgs is zero. Once switch is pressed, M3 turns on followed by M1 followed by M2. M2 holds the ground to gate of M1 there onwards. When SW2 is pressed, M2 releases the Gate of M1, making the Vgs of M1 zero again, thereby cutting out 5V_OUT. 7805_5V is regulator output. Needless to say, the switch section will consume significantly less current to be suitable for portable applications. I hope, power dissipation across 7805 is looked upon.

(Vin - 5V)/0.7 Watts