switching a bjt transistor with .5v changing

[koroba74]

i want to switch on and off a 1 volt output on a 5v circuit by using a .5v feed. when the voltage drops to less than .5v i want the circuit to cut off the 1volt output. i am trying with a darlington npn mpsa14 .. i am not having any luck.. i have tried several ways.. i need a someone to chat with aobut this... perhaps on yahoo messenger email.. koroba74@yahoo.com
thanks

 

[crutschow]

Sounds like you need a comparator, such as an LM339, to detect the 0.5v signal. You connect one comparator input to 0.5v (as from a resistive divider). The other input goes to your signal. The comparator will then change state depending on whether you input is above or below 0.5v. The comparator output can then drive whatever other circuit you need.

 

[koroba74]

thanks for thehelp with that transistor circuit

thanks i think that answers a question.... i am sure it does in fact.. now if i can figure how to use this darned lm3914.. I have six of the things.....
that way i can get several varied responses to the input signal no matter...the lm3914 has 10 cmparators although I only need about 5 of them... i am reading how to set it up now...

 

[crutschow]

The LM3914 has a 1.25V reference driving ten 1k ohm resistors in series going to the comparators. Each comparator's reference is therfore .125V above the one below, thus the first will trip at .125V, the second at .25V, the third at .375V, etc.

 

[Externet]

Use a germanium transistor.

 

[Willbe]

i want to switch on and off a 1 volt output on a 5v circuit by using a .5v feed. when the voltage drops to less than .5v

I think one silicon transistor will work if the collector resistor has a high-enough value. 0.5 v b-e is the turn-on threshold, with 1.0 v for sure full on.

What current can the input to your circuit input supply (source or sink); what current does your circuit output need to supply (source or sink)?

 

[Muhammad89]

i want to switch on and off a 1 volt output on a 5v circuit by using a .5v feed. when the voltage drops to less than .5v i want the circuit to cut off the 1volt output. i am trying with a darlington npn mpsa14 .. i am not having any luck.. i have tried several ways.. i need a someone to chat with aobut this... perhaps on yahoo messenger email.. koroba74@yahoo.com
thanks

SCRs can also be used for such application, C9013 e.g, however, you will be in need of using either a resistor to short the Thyristor, which in turn will off your circuit or a capacitor, the one mostly used. which is capable to drop the holding current of the SCR to less than it needs to keep running.

Muhammad89