relay elimination

japer

hello
i'm totally confused again
Working on a series circuit that stays high 4.75 volts max current 20 ma
that can go low .610 volts max current 20 ma.
The idea is to switch a transistor on the low signal to ground
With an analog mind a NC relay that pulls open high and makes connection on low or
loss voltage.
I do not want to use a relay
space limitaion,small current,and even with diode protection just to noisy
also i do not want to use the stamp>Just to easy.Plus a have something special
planned for the stamp.
Should i look at Voltage Follower CMOS,TTL,Shift Registers,Bias diodes?
Could some point me toward a good. search ,tutorial, book
that would pertain to the above circuit design
i have exhausted my search ideas and lost a lot of sleep
thanks for any infomation

japer

Chris Savage

To be honest, it's not clear to me exactly what you need or are trying to do.

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Chris Savage
Parallax Tech Support
csavage@parallax.com

japer

hello Chris Savage

yea thats the problem i'm having
i have a signal generator that does not have clock. has a high voltage of 4.75 volts a low signal of .61 volts,
the frequency and duty cycle can vary
is their a way to use this low signal to control a transistor to ground ?
money is an issue
can i spike a transitor ?
the confution lies with the wasted signal the circuit is normally high.
would dual transisor with reverse bias diodes and resitors type of circuit work ?

japer

japer

hello
ok
it finally came to me.
the solution was to raise the low output
of the sensor to .69 volts.and using a parralle cicuit and 2 transistors
latter transisor only come on when the 1st transistor is low
voltage pulldown
is their a formula for voltage pull down?
my head finally soften up after it hit the wall

japer

Beau Schwabe

japer,

It sounds as though you need a "buffer" of sorts... The Low (.61V) output of your
signal generator would work if it could drive more current?

Here is a solution that could be the answer for you. The configuration might be a
little different than what you would expect, but this circuit does work well. The +5V
at the "LOAD" can be a voltage other than +5V.

Although this link below describes selecting the proper resistor to drive the base of a
transistor switching a motor, the same principle applies to other devices connected
to a transistor. In the BUFFER schematic that I provided "R" would be the 1K resistor.
...And don't forget to add an additional .6V drop across the PNP transistor.

http://forums.parallax.com/attachment.php?attachmentid=37701

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Post Edited (Beau Schwabe (Parallax)) : 12/3/2005 7:59:00 AM GMT

japer

hello Beau Schwabe
thanks for your input
i was thinking about this type of circuit
this circuit answers the question i need

japer

Beau Schwabe

The possible problem with this circuit (using 2 NPN's) is that your low voltage of .61V may be just enough to keep the first
transistor from turning completely off. Suppose you could add a diode to the input to drop the voltage by .6V to increase
the threshold level.

2 NPN's - Your "trigger" will be when the signal from your generator is much closer to GND ( just about 0.6V )

1 NPN and 1 PNP - Your "trigger" will be when the signal from your generator falls just below +5V ( about 4.4V )

Depending on your needs, the difference between the two circuits should be considered.


BTW) With a diode on the input of the 2 NPN version the "trigger" will be closer to 1.2V rather than 0.6V

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

japer

hello Beau Schwabe

so i should lower voltage below .61 volts ?
ouch
the neat thing about this type of setup is it could loop itself

japer

Beau Schwabe

japer,

The input diode will automatically take care of lowering the input voltage below 0.6V

It's just that the B-E junction threshold of the transistor is very close to your "LOW"
oltage value which might still keep the transistor "on" when you want it off.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Post Edited (Beau Schwabe (Parallax)) : 12/4/2005 11:26:56 PM GMT

japer

hello Beau Schwabe

yes i see were this may be a problem maybe a slight chance
in some case were #2 npn may not turn on if #1npn goes low
the ideal switch would be :
IF #1npn IN=0 Then 'flip back #2npn to low on high #1npn input signal
#2npn IN=1
store this high bit until recalled later

this type of circuit will have to wait until until a better
understanding of this style of circuit can be understood.
assembling this trigger today testing reliability

japer