What Is Wrong With Me ??

Discussion in 'Electronic Basics' started by ahonda55, Oct 17, 2005.

1. ahonda55Guest

Hi all , i have made this circuit ..
http://img313.imageshack.us/img313/7242/speedswitch8xc.jp

it is a speed switch using LM290
all of the circuit supply is 12
And i had used a 100K variable resistor for R and adjusted it to abou
80
C is .074uF and .01uF for the other capacitor
and i have used a red LED as loa
and i provided a very high frequency as Fin , so the LED is O
but,, when i remove the Fin it always on
what is wrong ?? what is wrong what is wrong ???? lo

i want to let the switch On at about 120Hz , what are R and C value
should i use ??
and what is wrong with my work
Plz help

Thanx a lot :

2. ehsjrGuest

You have not mentioned a resistor in series with the LED load,
and your diagram does not show one. You need to limit the
current through the LED with that resistor. Assuming a 12V
supply, a 1K resistor will limit it to 12 mA, which is fine -
you'll get a healthy glow from the LED. You could use a smaller
resistor for more current - up to say 30 mA, but there is no
need. In any even, it may be too late, now. The output
transistor in the chip could already be shorted, which would
give you the condition you have - LED always on.

Rule of thumb: Unless you have a good reason not to, always
use a current limiter with LEDs.

Ed

3. DMGuest

I don't believe you've hurt the output transistor since a shorted output
would permanantly short the LED causeing the LED to blow.

Make sure you are pulling down the input pin to ground through a proper size
resistor (eg >1K). Otherwise noise and/or a dc voltage on that high
resistance pin could be interpreted as a high frequency input.

Also the "other" capacitor, in most cases, should be larger than the timing
capacitor since its performing an integrating function.

Let me know how it goes.

Dorian

4. ahonda55Guest

Hi all
first , there was a problem and i have fixed it, the ground connectio
in pin 8 was not connected , now it is o
second , i forgot to tell you that i already use a 1K resistor wit
the LE

The LED now is always OFF , whatever i change R1 , it is Off

the IC is working good, i know that because i test it every short tim
in my LED tachometer circuit

The LED is off, but the LED itself is working good , i tested it

note , i use the 14 pin IC see it here.
http://img343.imageshack.us/img343/6222/290729073ms.jp

What do you think friend

5. ahonda55Guest

Hi , the circuit now is alive !
i was connecting the ground to pin 11 and 12, now i separated th
ground and connected it to pin 12 only (GND

but the circuit acts like it is mirrored , i mean , the LED is OFF bu
when the frequency goes low the LED becomes ON .
as i read in the data sheet (load is energized if Fin => 1/2RC) bu
now something strange happens

any ideas ?

Than