Turning on a transistor abruptly ?

[Externet]

Hi all.
Instead of using a comparator, decided to try if I could turn on a white led abruptly by using a zener, from a smoothly rising voltage divider. Did not do it.
Then I decided to use two forward biased diodes instead of a 1.4V zener

I supposed that the transistor would turn on when the potentiometer wiper was at over the 2V of the forward drop sum.

It does not. Advancing the potentiometer from zero volts, base voltage rises gradually from zero and turns the led slowly on from 0.5V .
Do not want to use a mosfet; the curiosity was about a transistor and 'kee' behavior switching implementation.

Any suggestions on how to do it abruptly ? On or off, no betweens.... or what is wrong in the circuit.

When led starts glowing:
Wiper voltage = 1.2
Between diodes = 0.9
Base = 0.6
-Each diode is not dropping 0.7V, but 0.3V-

A red led instead of the two Si diodes does it better, but not abruptly yet.

[gerty]

Have you considered using a gate from a Schmidt trigger such as a 4093b?

[Externet]

Thanks, gerty.
No, I just tried to do it simplistic to find out if somehow would work with a zener instead of other triggering devices.

[gerty]

I guess I totally missed your objective !

[Robert Reed]

Zener would probably be better. They have very fast breakdown time. But remember that to convert a very slow rising voltage to a very fast step response takes a LOT of gain!!!!

[sghioto]

But remember that to convert a very slow rising voltage to a very fast step response takes a LOT of gain!!!!

Roberts correct, which is the heart of this circuit but does require two transistors.

Steve G

[Bob Scott]

Hi Externet,

I don't know if you are aware, but with the circuit that you drew, adjusting the potentiometer to full 5V on the wiper will cause excessive current to flow through the diodes and transistor base.

ie: whisps of smoke may arise and the transistor will "pop" audibly. (You probably knew this.) B-)

[Externet]

Thanks, Bob
Yes, at every power up the wiper is at zero, then advanced only to the point where evaluation readings are taken and never beyond. Smoke is still kept tightly packed in all components