Re: transistor base input

lerameur wrote:
Hi all

I have an old analog alarm system. I am hooking up the bell output to
a microcontroller.
Although the output voltage for the bell fluctuates around 6 to 9
volts. I need either a steady 0 or 5v for the input of the
controller.
I made a circuit with 2 2n222 transistor, acting as an inverting
gate. The problem is that the spec *** of these and most transistor
have a Veb of 5 volt. my Ve is at ground therefor my Veb exceeds
this. any ideas on how to get pass this issue.
Thought about a voltage divider at the output of th bell , but I am
scared that if voltage drops a bit below 6v, the inverter will not
pick it up as positive voltage.
thanks

The Veb spec is a limit on how much reverse voltage that junction can stand, not any normal operating voltage. If the emitter is at zero volts, the transistor will switch on when the base voltage is about 0.7 volts more positive than ground, and the base-emitter junction becomes forward biased enough for the base current to rise to about 1/20th of the collector load current.

However, using a transistor as a switch will do nothing to regulate the voltage of the collector load.

It would help us help you if we could see a schematic of your circuit. Either you could post a link to your circuit, draw one with text characters (and a fixed width font, (like Courier) and post it here (this is made easy with a little chunk of software:
http://www.tech-chat.de/ascii-circuits.html )
or post a normal graphic file representation on the alt.binaries.schematics.electronic group, where attachments are allowed.

--
Regards,

John Popelish
.