A circuit that outputs 3V when there was a pulse input

How long a pulse? 3v at how much current? Does there need to be a reset feature to turn off the 3v?

We need more detail.

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I want to flash an LED when there is an incoming phone call.

Well a one-shot or something like that. 74HC123 or a 555 thing.

George H.

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Thank you for the tip! :)

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A land line telephone has a 20 HZ 90 volt ring signal imposed on the line. It should be a piece of cake to run a led on that. I'd use a cap to isolate the AC and dropping resistor and clamps to protect the led.

Bound to be something on line already.

Google: telephone light flashing circuit

About 418,000 results

Does it have to be an LED?

eg: wire a NE2 neon indicator and a 100K resistor in series across the phone line.

There is indeed a neon bulb in the handset of the telephone I wanna modified. It flashes when there is an incoming call. And that's what I want to replace with a modern, brighter LED.

Tell us more.

are you hard of hearing? telephone in a noisy location? etc..

My old man already added a led to his telephone so he can "see" a call when he's watching TV. Dad's in his 90's and not very lucid or I'd ask him how he did it.

I had LED's working 24/7 on the phone line's "on hook" current (you can steal about 5 milliamps at 48 VDC without affecting the line) just to make a night light and battery charger that wasn't affected by power outages.

But there's already so much on line that you don't have to understand electronics to make something that works.

If you're on a telephone line that works like a North American POTS line then the ring signal is a 100V, 20Hz sinusoid. Think capacitor, resistor, and a string of LEDs. You probably want two strings in opposite directions, because LEDs don't like being reverse-biased by much. Or a bridge rectifier feeding the LEDs.

You're limited in the current you can use (check your local specifications for a ringer load equivalent), but you have lots of voltage to play with. So more LEDs in series = more light. With modern LEDs you can probably have people screaming and covering their eyes when the phone rings.

There's phone line active circuits online and in dozens of beginner books.

The Vf of a white LED is nearer 3.4V - don't try to regulate the voltage, its supply must be current limited.

Only if the phone is plugged-in. For battery-run phones, I've heard that NOT plugging a Samsung Galaxy phone/tablet into the wall while using its features causes battery problems. Using it with the USB wall-charger plugged-in helps to ease battery problems. (I don't know about capacitors).

I think this circuit can be very simple indeed. A suitable resistor will limit the current during the ring signal, around 4.7 kohms or larger. No point in the capacitor really. There is a DC voltage on the phone line of around 48 volts which you don't want to light the LED, so wire it up with a polarity that reverse biases the LED from the DC. Then only the AC will make it light up. If your home was not wired professionally the installer may not have preserved tip and ring polarity when installed. In that case you might need the capacitor.