I've found plenty of examples of transistor latch circuits that trigger on a positive pulse but I couldn't find one that will trigger on a negative pulse. Is it as simple as inverting the output of the NPN transistor to hold the PNP transistor on as I've done in the attached schematic?

 

Do you want separate ON and OFF push-buttons, or just one alternate-action, ON/OFF push-button?

 

Do you want separate ON and OFF push-buttons, or just one alternate-action, ON/OFF push-button?

Separate On and Off like this

 

So then what's the problem you are trying to solve?
Why do you want it to trigger with a negative pulse?

 

So then what's the problem you are trying to solve?
Why do you want it to trigger with a negative pulse?

I want the On button to turn power on as well as provide a low pulse on the trigger pin of a 555 timer.

 

I want the On button to turn power on as well as provide a low pulse on the trigger pin of a 555 timer.

Then you need a negative going pulse, not a negative polarity pulse.

If both circuits are operating from the same supply voltage, just connect the 555 trigger pin to the top of the ON pushbutton.

 

Then you need a negative going pulse, not a negative polarity pulse.

If both circuits are operating from the same supply voltage, just connect the 555 trigger pin to the top of the ON pushbutton.

Thanks...that was the plan and why I wanted my latch to operate on a low pulse. Basically it turns on and triggers the 555 all from one button. A second button turns everything back off and there shouldn't be any current consumed in the off state besides whatever leaks across the transistor junctions. I just couldn't find an example online of a transistor latch circuit operating on a low pulse....only a high pulse.

 

I just couldn't find an example online of a transistor latch circuit operating on a low pulse....only a high pulse.

I don't understand.
The circuit you show operates on a low pulse (push-button to ground).

 

but I couldn't find one that will trigger on a negative pulse.

Modified circuit in post #3

 

I don't understand.
The circuit you show operates on a low pulse (push-button to ground).

Yes that’s my circuit that I was proposing. It worked.

 

Modified circuit in post #3
View attachment 311883

Oh this looks cool. I’ll try this out! Thanks! What does D1 do?

 

Allows a discharge path for C1 when powered OFF.

 

Allows a discharge path for C1 when powered OFF.

Pardon my ignorance, but is the current path like this?

 

No. C1 charges up through the npn, R5 and the pnp to V+. Discharges through D1, the internal resistance of the 555 and R5 when powered OFF.

 

No. C1 charges up through the npn, R5 and the pnp to V+. Discharges through D1, the internal resistance of the 555 and R5 when powered OFF.

ah gotcha....I didn't think about it discharging through the 555 to gnd.

 

This circuit triggers a 555 timer on the rising edge of a pulse. The pulse in this simulation is 0-5V with 1ms rise time, 1ms fall time and 100ms on time.