> I have a voltage source which is the voltage drop across a dorr
> bell button. The bell is an electronic type requiring a diode
> across the button so what you see, there, is half-wave DC
> ranging from mostly 0 volts up to around 7 with a lighted button
> and this will jump up to about 17 or 24-volts peak when the
> little incandescent lamp burns out in the button which is
> guaranteed at some future date.
>=20
> 	This is nice dirty half-wave DC at power line frequency
> and I want to know when someone presses the button by detecting
> the absence of the signal.
>=20
> 	The plan is to use an opto-isolator with the pulsating
> DC driving the LED through a current regulator so that the LED
> will see about 15 milliamps over as much of the curve as
> is practical.
>=20
> 	One way to keep the parts count down might be to use
> either a 7805 (5-volts) or a LM317 regulator with a couple of
> resistors to set it to around 3 volts and then connect the LED
> through the optocoupler through an appropriate current limiting
> resistor. The circuit will provide a pulsating flat-topped wave
> so the output of the optocoupler will need to be cleaned up but
> my question is whether this pulsating input to the voltage
> regulator will cause unforeseen problems or whether I should
> just build a current regulator out of a single transistor, a LED
> and resistor?

One of the lowest parts count current regulator circuits is an LM317 with o=
ne=20
resistor (circuit in most LM317 data sheets as an application example or se=
arch web=20
images for LM317 current regulator). Resistor from Vout pin to Adj pin, app=
ly Vin to=20
Vin, your LED or optocoupler from Adj pin to GND.

R1 is approx =3D Vref/Iout =3D 1.2V/0.015A =3D 80 Ohms.

How about swapping out the lamp in your door bell switch with an LED (will =
last=20
forever then) and put in series with the optocoupler so it gets regulated c=
urrent as=20
well?



--=20
Brent Brown, Electronic Design Solutions
9 Titoki Place, Pukete,
Hamilton 3200, New Zealand
Ph: +64 7 849 0069
Fax: +64 7 849 0071
Cell: +64 27 433 4069
eMail:  brent.brown@clear.net.nz



--=20
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