LED light based on AC not DC

[Dr Mitch]

How can I make an LED light only if the AC voltage is a certain level,
and completely ignore the DC voltage?? I have cap in series with it,
but that doesn't seem to do the trick.

 

[Fred McKenzie]

In article <1186617446.792096.165950@d30g2000prg.googlegroups.com>,
Dr Mitch <doctormike@gmail.com> wrote:

How can I make an LED light only if the AC voltage is a certain level,
and completely ignore the DC voltage?? I have cap in series with it,
but that doesn't seem to do the trick.

Dr. Mitch-

An LED is a diode, which will rectify the AC, causing the series
capacitor to quickly charge up to a peak value. After that, no further
current will flow.

Try connecting a diode (or a second LED) back-to-back with the LED, with
a suitable resistor to limit current. Since the second diode will
conduct when the LED is reverse-biased, the capacitor will only charge
to the steady DC voltage.

This just lights the LED. It doesn't determine whether the AC voltage
has reached some particular level. Assuming the AC voltage is high
enough to overcome the diodes' forward voltage drops, only the LED
brightness would change as AC voltage changed. Perhaps an LED level
detector would work in place of a simple LED.

Fred