I want to design a call signal detector


Joined Sep 24, 2015
Welcome Prakhar997 to AAC. We're a bunch of people who love to HELP others. However, none of us are offering a free engineering service. The best approach is to have an idea about how to go about achieving your goal, then we can offer advice. When posting a question it's best to state what the goal is in the heading. "Need help building a signal detector from a BC548" will get a lot more attention than "Help".

As for my skill level, I doubt I can help. But I like to welcome new members. I was once a new member and I also got a lot of welcomes here. So feel free to ask advice, we love to help.


Joined Oct 2, 2009
We don't design a circuit based on beta.
For example, if we want a gain of 400, we make an assumption that beta is 100 minimum, not 390.
Two amplification stages of gain 20 each will give an overall gain of 400.

Audioguru again

Joined Oct 21, 2019
What makes the call and how much voltage does it produce?
A microphone produces a voltage of about 0.002V if somebody calls it from 20m away.
My land-line phone has 90V when somebody calls it.
A huge difference of 45 thousand times.

If the signal comes from a microphone then any sound (barking dog or a motorcycle) will be detected.


Joined Aug 1, 2013
Don't care. There is nothing magical about a BC548. This circuit meets all of the requirements in post #1: it detects a call signal.

Why must your circuit use a BC548?



Joined Jan 27, 2019
For the record, I am 80% certain that a “call detector“ refers to a circuit that indicates an incoming mobile phone call. In older cellular systems this was a trivial thing. II even had a Motorola promotional pen which had an LED on the top that flashed when a call came in.

Interestingly, it would flash before the phone rang because it was sensing the transmitter in the phone setting up the call with the cell. Anyone who spent a lot of time around computer speakers with an early mobile phone probably learned to interpret the noises they made to know a call was coming in and the phone was about to ring.

sparky 1

Joined Nov 3, 2018
Signals that detect a call are usually radio frequency. Resonance is used, for example 27 MHz will resonate in a tank circuit designed for 27 MHz. It is legal to build a low level transmit and a receive circuit that could use a general-purpose transistor, BC548C having higher HFe than A or B variety.
All will work but some characteristics can be preferred. Beta might be a starting point for amplifier.

The NPN transistor boost the signal. The RF is rectified as DC for indication. The signal can be detected using a diode or diodes.

A receive only circuit can work in legally designated band sometimes there are less restrictions for receive. A walkie talkie is known to handle calls.
A teacher usually will not give an assignment with legal issues like cell phone detector.
If this what you are describing, then the next question might be what radio frequency do you want to detect?
"I want to design an RF detector" ok this video shows a simple passive design for near field detection of RF into light.

Last edited:


Joined Jun 17, 2014
Hey I want to design a call signal detector from bc548 which have beta about 390 so can anyone pls design a circuit for me
Important question:
Is this for a land line phone or a cell phone?
A land line phone is easy to do, you basically create a current limited rectifier circuit that drives the transistor base.
For a cell phone it will be a little harder unless you can sense the cell phone screen dynamics or the speaker.
You can sense the ring of a cell phone by using a microphone near the speaker, but you do have to let the volume be up at least a little. The mic will work with an amplifier circuit (transistors) and provide some sort of output like an LED.