Powering LED via AC Signal

Question

I am looking to power a LED, given an input signal \$v_{in} = A \sin(2\pi ft)\$. The only caveat is that I only want the LED to turn on if a specific frequency is met, but let the LED remained turned off if the frequency is outside my specific range.

If I were to let this filtered signal control the switching of the LED, I have considered using a SPST relay to activate a sub-circuit which the LED would run on, however I have not had much experience with relays, and so I would really appreciate advice on how I might achieve this.

Thanks!

Answer 1

Probably the easiest way is to feed the filtered signal decoupled (i.e. through a capacitor) into the enable of a CC driver. The 1kHz pulsing won't be visible to the human eye, but you might need to drive the LED very close to its maximum current rating.

Answer 2

Decades ago I implemented an anti-skid brake system for a student car project. I used 74LS123, or 74LS121, monostables as pulse-period (cycle-duration) window detectors.

This approach will be much more accurate a detector than bandpass filters. And the logic levels will easily drive LEDs.

Answer 3

Nowadays I would just use a microcontroller. The have integrated timers and counters which make it easy to do something like:

1. count the number of transitions made in a certain time period (like 1ms) on an input pin
2. if that count is within a certain range set an output pin to HIGH else set it to LOW.
3. repeat

This Atmel/Microchip application note describes how to do it with the AVR family of microcontrollers, but it'll be pretty much the same for other mcus.

The components involved can be as simple as the microcontroller plus some input conditioning circuitry. No external crystal is needed if you can live with a 1% error in your timebase.

The only thing left to do is to design some input conditioning so that your signal is detected and your input pin is protected.

Without knowing more about what kind of input signal you want to detect I would just adapt some designs found in other simple frequency counter designs, most notably from the "50 MHz PIC-based Frequency Counter" originally designed by Woldgang Büscher, DL4YHF and copied extensively:

https://www.qsl.net/dl4yhf/freq_counter/freq_counter.html

The input circuitry looks like this:

(Source: http://web.archive.org/web/20111121150450/http://homepage.eircom.net/~ei9gq/counter.html )