Soft Latching Power Switch Circuit

Im currently working on a Wire cutter/stripper loosely based on this one: Arduino Automatic Wire Cutter and Stripper

I would like for the unit to shut itself off after it has finished a job and I found several examples to follow but they are all for battery powered systems. I am hoping to add a capacitor to power the circuit until the power supply comes online.

I chose this circuit(EEVblog #262 - World's Simplest Soft Latching Power Switch Circuit) to start with making only 2 changes so far.

  1. move q3 to the other side of switch S1 and eliminated the R/C combo to allow the arduino to shut things down.( I just realized I will have to add those components back in to allow the switch to also shut off power DOH! )

  2. Added a capacitor to initially power the circuit.

I'm guessing it's not that simple and I'll need to do more to get this to work. What do ya'll think?

The only fault with the circuit as-is, is the missing resistor in the base of Q3 from the Arduino.

I'm not sure Q3 is necessary at all. If you put a diode between R2 and the base of Q2, and connected the Arduino pin to the junction of the diode and R2, then S1 would not attempt to back-feed the Arduino. Setting the pin to OUTPUT (but never HIGH) will shut off the circuit.

The 100nF capacitor across S1 would auto-start on the first application of power to the whole circuit.

The only fault with the circuit as-is, is the missing resistor in the base of Q3 from the Arduino.

Sounds promising 8)

I'm not sure Q3 is necessary at all.  If you put a diode between R2 and the base of Q2, and connected the Arduino pin to the junction of the diode and R2, then S1 would not attempt to back-feed the Arduino.  Setting the pin to OUTPUT (but never HIGH) will shut off the circuit.

A little above my pay grade but I will noodle it out. :wink:

The 100nF capacitor across S1 would auto-start on the first application of power to the whole circuit.

Ok you got me on this one.
Are you saying I should add a cap. across S1?

The plan is for first application of power to come from C1 which should last until the phone charger comes online to take over. The datasheet for the SSR says it presents a nominal 1k load so I'm thinking C1 should be somewhere between 47 - 470µF depending on how long it takes the phone charger to take over.

I see the value of a hi-Z state now, Thanks Paul!

I redrew the circuit to reflect the change and added the components back in so S1 can turn the device both on and off. I also clarified how power was to be supplied, and I think I see the problem now.

first application of power

Doh! Without a charge on C1 this thing is dead in the water. Correct?
This seems to be a big problem. I could "jump start" it the first time, but how long would that last if not turned on for a long period.

The only solution I can think of is to move the phone charger to full time power and use the circuit to switch everything else. Unless someone knows a better option?


EDIT: I forgot to add a ground connection at the junction of C1 and Q2

I have no idea what Q3 is supposed to do in this version. The Arduino connection is correct, just make sure it never gets set to output a HIGH - or put a second diode in series with this wire; cathode to Arduino.

If you want it to self-start, put a capacitor across S1.