Transistor self-latching circuit

[crapalanche]

Here's the scenario: one of my friends has decided that he would like to add a timer to shut off his reading light (clip on variety, DC) after some period of time. To that effect he has procured a digital kitchen timer with a buzzer. The plan is to turn on (and keep on) the light with a momentary switch. When the timer goes off, something needs to be triggered to shut things down (probably use a enhancement P type MOSFET with the source and drain hooked up to the power rail. Drive the gate high off the buzzer output to disengage the current, I don't think I need to even rectify the wave.)

Well, I was poking around with circuits that take a momentary switch input and stay on. I'm looking at sequential logic, of the bistable multivibrator variety. I see it can be done with SR latches. I've got some 555's handy personally, nothing else on me for SR latches, but I also have a boat load of BJT's. So I figured I'd play around with the latter.

I've got a working circuit, see I think I have a decent understanding of this, but maybe not in light of what I constructed myself and didn't just find on the Internet. I also noticed that I can turn this bad boy on with just my body capacitance. I presume this is due to a positive feedback loop between the two transistors? It's sorely lacking of any ESD protection, however.

I first designed this with a looped-back NPN transistor. I'm posting because I was hoping someone here would be so kind as to tell me why this circuit does not latch. I'm driving it at 5 volts, everything is pretty standard small signal components. I also tried this circuit which is a darlington pair with the emitter looped back to the base.

 

[aljamri]

Two important things may get you better answers.

1- put your circuit as attachments so it can be enlarged.
2- put name, numbers and values for your components.

Thanks

 

[aljamri]

In the second circuit, the transistor seemed not doing any job, even if you removed it, lamp will be off until you press the push button, it will be ON.

The same thing in the third circuit.

 

[NFI]

Your circuit won't latch because by tying the base and emitter together, they're at the same potential, hence turning off the transistor. For the transistor to conduct, you need a Vbe potential difference of about 0.7V to bias the base-emitter junction - does the transistor actually switch at all in your circuit? If you want a simple latching circuit with a single component, try a thyristor...

 

[BrianG]

I recently made a NiMH charger that uses transistors that latch. Maybe you could adapt it for your usage? Anyway, here is what I'm talking about: https://www.electro-tech-online.com/threads/just-a-sanity-check-on-transformers.104746/#post860784

 

[Ubergeek63]

I've got a working circuit, see View attachment 43084 I think I have a decent understanding of this, but maybe not in light of what I constructed myself and didn't just find on the Internet. I also noticed that I can turn this bad boy on with just my body capacitance. I presume this is due to a positive feedback loop between the two transistors? It's sorely lacking of any ESD protection, however.

add a base resistor, a base emitter resistor and a base emitter capacitor to the lower transistor to shunt the static around the base emitter junction