Hi Folks. Thanks for including me.
My first time here so please be patient and gentle
. I'm trying to create a latching circuit using transistors and its driving me nuts! The idea of the circuit is to replace the electronics in a kettle. This latch is intended to supply power to a thermo switch using the original thermistor to turn off a comparator latch using a 555 timer. The kettle needs power after the 555 turns off as I'll be using pin 7 to drive a beeper to indicate when the kettle has boiled and turned off. The latch will drop out as soon as the kettle is lifted off its base and loses power. The 555 will power the original circuit's relay via an existing transistor (the led is just for development purposes. Driving the led, the 555 draws 22mA). I've got the 555 part sorted and works well, it's just this latch that's giving me strife, I'm this close to saying 'damn it', and using a dpdt relay to make the latch. As per the attached drawing, it's a really simple circuit, but the led lights as soon as power is applied and stays lit until it is removed. I even tried using a cap to ensure that the base of BD140 is initially high, insuring the tyranny is turned off. On my first try, I used 2 x 557's with no luck, so I tried using the 140 , thinking that a tranny with less gain might fix the issue. I've seen variations of this circuit and they seem to work. Can someone please point me in the right direction, and help me regain my sanity.
Many thanks.
P.S. The resistor I'm driving the led with is actually 1k.