« Home

Mobile Phone Incoming Call Detector

Topics: Telephone

Detect incoming mobile phone calls with this schematic for a small electronic circuit that flashes an LED when it detects an incoming coil. Requires only 1.5 Volts DC.

Screenshot: Mobile Phone Incoming Call Detector

The most important part of this circuit is a 130-150 turn coil, which acts as a pick-up antenna for a signal that is emitted by your mobile phone whenever it has an incoming coil. The signal induced in this coil/inductor is amplified through a transistor, which is then used to drive a monostable 555 Timer. The output voltage of the IC is used to drive a ultra-bright LED.

There is a total component count of 12, which includes the 1.5 VDC cell (which can be an AA or AAA battery). The amplification transistor is the very common BC547 NPN.

Parts List:
  • R1 - 100K 1/4W Resistor
  • R2 - 3K9 1/4W Resistor
  • R3 - 1M 1/4W Resistor
  • ----
  • C1,C2 - 100nF 63V Polyester Capacitors
  • C3 - 220μF 25V Electrolytic Capacitor
  • ----
  • D1 - LED Red 10mm. Ultra-bright (see Notes)
  • D2 - 1N5819 40V 1A Schottky-barrier Diode (see Notes)
  • Q1 - BC547 45V 100mA NPN Transistor
  • IC1 - 7555 or TS555CN CMos Timer IC
  • ----
  • L1 - Sensor coil (see Notes)
  • B1 - 1.5V Battery (AA or AAA cell etc.)
Alternate References:
warren says: (February 12 2008, 10:57pm)

great marketing idea. keep em coming

yong says: (February 23 2008, 3:16am)

Looks like the detection circuit only works when the mobile phone is placed a few centimetres away from it.Anyway to increase the range to few metres? how can we modify the circuit to achieve that?

yong says: (February 23 2008, 3:17am)

Anyway to increase the pick up range from few cm to few meters??

ElectroKits says: (February 23 2008, 4:01am)


Transistor Q1 is responsible for flipping the trigger pin on the 555 timer (pin 2). The transistor does the flipping when the pickup circuit comprising of L1 and C1 receives a signal from the cell phone.

So, the objective would be to make this part of the circuit more sensitive to the signals. I can think of 3 potential ways to do this...

1: You can play around with coil L1 to see if you can create a more sensitive pick-up. Try rotating the plane of the coil (vertical/horizontal) when it rests on the table. If that doesn't work, you can try playing about with the number of turns. Try putting on a few more turns... if that makes it more sensitive, then keep on going until you find that it's at the maximum sensitivity. If adding more turns made it less sensitive, then try removing turns until you find maximum sensitivity.

2. The biasing of Q1 is provided by R1. Just the biasing current/voltage alone is not enough to trigger the 555 timer, however when the received signal from the phone is coupled into the circuit, the two combined are enough to trigger it. So, one option might be to have the bias current in Q1 working closer to the trigger point, meaning that an even weaker signal should still do the triggering. Try replacing R1 with an 82k resistor in SERIES with a 22k variable resistor. Initially, set the variable resistor to it's maximum value. Then, make a call to your cell phone that's placed a meter or so away, and slowly decrease the value of the variable resistor until the LED (hopefully) illuminates. You need to make sure that it's only being triggered while the phone is ringing. If the LED illuminates even when the phone is not ringing, then the biasing current has gone too high, and the trigger point has been passed. If this happens, then you may need to go to step 3...

3. If neither of the above worked for you, then you'll probably need to either a) replace Q1 with something that has a higher ac Beta value, but of the same or very similar dc Beta value; or b) modify the circuit to include some kind of "pre amplifier" for the signal that's picked up by the L1 C1 circuit.

I hope this is of some help - and I'm sure that some of the others viewing this page will have some of their own ideas about making this work. I hope they provide their input!

Either way.... please come back and let us know how you get on!

nightbird_11 says: (April 20 2008, 3:10pm)

i have only avaliable NE555 timer IC so plz tell me about the circuit using NE555 plz

Login or Register to Comment...

* Never displayed or shared!
Note: Comments by new registrants are reviewed prior to being shown.
We're looking for high quality posts! Please allow time for approval!