The output of the flip-flop is too heavily loaded in your latest circuit. Simulation shows this prevents toggling.
Here's a variant, which uses a NMOS FET to do most of the donkey-work.

 

Thank you for the drawing. How would you trigger the circuit?



Are you saying my least Drawing in post #51 doesn't work? But swapping the 2n3904 to 2N3906 will make it work?

TONY

2N3906 is a PNP.

 

The output of the flip-flop is too heavily loaded in your latest circuit. Simulation shows this prevents toggling.
Here's a variant, which uses a NMOS FET to do most of the donkey-work.
View attachment 100491

Will a 1N4148 across the coil's terminals be fast enough to protect the fet?
How about placing a tvs between source and drain for added protection?

 

Good info here. Love IT!!! Keep it coming.

Thank you all.

TONY

 

Thank you for the drawing. How would you trigger the circuit?

With the flip flop in your most recent schematic.

 

Right... I was confusing mosfet's behavior with that of bjt's.... thanks for clarifying

No problem. If the OP had wanted to make the LED indicator part of the flip flop, I would have suggested N channel MOSFETs instead of BJTs.

 

Will a 1N4148 across the coil's terminals be fast enough

Reverse recovery speed is only important when a diode is switching from conducting to nonconducting.

 

With the flip flop in your most recent schematic.

I understand now ... Where you indicate 'FF OUT' on your drawing. I thought that was YOUR output, not mine. So I was wondering where YOUR input was. Dummy me. Sorry.

By looking at your drawing and seeing the Relay coil, Lamp, Etc that I wanted. That should've told me everything.

So by combining your drawing with mine. That should just about to do it? ... I'm not even sure if the values I put in my drawing are right.

Thanks

TONY

 

Hi guys,

I am at a complete loss. I have googled Latching transistor circuits and have only found diagrams using 12v Positive triggers, not a ground trigger. I am looking to build a latching circuit using NPN PNP transistors when pulsed by a N.O. switch to Ground.

Requirements:
Has to work with 12v DC to be used in my car.
Circuit Latches when pulse to ground by N.O. switch. Unlatched when SAME N.O. switch is pressed to ground.
Output from transistors need to offer Full Ground when active. Output will activate an 'On Board' 12v SPST Relay And send a Ground signal to illuminate a 12v Lamp 'OFF board'.
No reset switch needed or will be used.
Would like to have LED on board to light when circuit when circuit is active/Latched.

I'm not looking for someone to the work for me. Hoping Someone here can point me in the right direction for answers or send a link that have answers. Doesn't even have use NPN PNP transistors. That just what I am thinking need to be used.

Any feedback will be great and appreciated.

TONY

Hi. Just use the existing latching circuit and precede it by a simple transistor voltage amplifier, which will invert the signal. Use a pull up resistor on the base. When you ground the base, voila!

 

Reverse recovery speed is only important when a diode is switching from conducting to nonconducting.

That, I didn't know... I've learned something new today. Thanks!

 

So by combining your drawing with mine. That should just about to do it? ... I'm not even sure if the values I put in my drawing are right.

The resistor values aren't that critical. If you're going to keep R10, you should put a matching resistor on the other transistor.

You're showing NPN transistors, but the part number is for PNP; just make sure you use NPN.

 

The output of the flip-flop is too heavily loaded in your latest circuit. Simulation shows this prevents toggling.
Here's a variant, which uses a NMOS FET to do most of the donkey-work.
View attachment 100491

Better. It toggles but it doesn't latch. Did I wire something wrong?

 

Hi guys,

I am at a complete loss. I have googled Latching transistor circuits and have only found diagrams using 12v Positive triggers, not a ground trigger. I am looking to build a latching circuit using NPN PNP transistors when pulsed by a N.O. switch to Ground.

Requirements:
Has to work with 12v DC to be used in my car.
Circuit Latches when pulse to ground by N.O. switch. Unlatched when SAME N.O. switch is pressed to ground.
Output from transistors need to offer Full Ground when active. Output will activate an 'On Board' 12v SPST Relay And send a Ground signal to illuminate a 12v Lamp 'OFF board'.
No reset switch needed or will be used.
Would like to have LED on board to light when circuit when circuit is active/Latched.

I'm not looking for someone to the work for me. Hoping Someone here can point me in the right direction for answers or send a link that have answers. Doesn't even have use NPN PNP transistors. That just what I am thinking need to be used.

Any feedback will be great and appreciated.

TONY

And now for the most complicated solution I could come up with. I left the LEDs in so if you do decide to build it you could see it work. The LEDs can be removed and the relay and all added later.
It is a J-K flip flop design done in transistors. A one IC version for 12 V operation? Use a CD4027.

 

I modified it for my application. See attached.


I've used a 1k resistor in another circuit. LED did not illuminate. I kept change resistor out til LED came on with 330.



What MrAI stated, definitely seems to be "lot more circuitry".



That is VERY detailed. But, I do not even know where to begin on that drawing. Is there an "Over my Head" emoji for your response.



I didn't included those resistors in my newer drawing.

------

Thank you for all your responses. You guys are great help. I did create a newer drawing based in the TIM17 drawing from post #22.

I need it to be GND trigger pulse Push ON/Push OFF. Have GND Out to illuminate 12v Bulb. Drive a LED when circuit is Active. Latch a 12v relay.


TONY

Hi,

Yes, more circuitry so it depends how far you want to go with this.

How about:
δ

 

It toggles but it doesn't latch. Did I wire something wrong?

In the sim it latches ok. But then, sims and reality don't always tally . It may be a timing issue, a transistor gain issue, or dependent on whatever you're using as a trigger. Try tweaking a few of the component values.

 

In the sim it latches ok. But then, sims and reality don't always tally . It may be a timing issue, a transistor gain issue, or dependent on whatever you're using as a trigger. Try tweaking a few of the component values.

What Sim are you all using? ... I have Altium designer 13. ... is there a Sim on that?

Tony

 

I sim with LTspice (a free download from Linear Technology).
Btw, I've just noted you were using a N.O. switch as the trigger. Did you have a pull-up resistor to a positive voltage? There needs to be at least a 1V swing for the toggling to work reliably. Contact bounce of that switch might have been the reason you got erratic behaviour.

 

Btw, I've just noted you were using a N.O. switch as the trigger. Did you have a pull-up resistor to a positive voltage?

There is a resistor in parallel with the N.O trigger. I think it could be considered a pull up resistor.

 

But since your switch goes to ground, isn't that a pull-down resistor?

 

Total current in the power transistor is about 300mA so loading on the flip flop is too high.

I suggest you add a PNP transistor to drive the BD139 and use a LOW output from the flip flop to turn everything on.

I have combined your diagram with the Flip Flop I posted earlier and attached it to this post. Is this what you had in mind?

I'd also reduce the current in the LED. It's just an indicator, so 5-10mA is probably sufficient.

So increase the resistor value to the LED from 330 to 1K. Correct? If so, I didn't show that on the drawing.

But since your switch goes to ground, isn't that a pull-down resistor?

Are you thinking I need resistor to 12v after the N.O. push button in my drawing?

Thank you for all the input here guys.

TONY