Phone Call Detector

My mother lives a great distance from me. She is old and can hardly hear. When I call her at the phone, she rarely hears it. To improve things, I decided to design a device that gives light signals when the phone rings. I usually call her in the evening when she's watching TV. This device located next to the TV would attract attention and the probability that she will find that the phone is ringing is higher. In this instructable is described the design of this device.

The schematic of the device is based oh the 555 timer in monovibrator configuration. The time is set to around half of minute. A preamplifier done by NPN small signal transistor is put in the front of the timer. The inductance with value ~ 100uH detects higher activity of the phone when ringing and triggers the timer. It seems that two physical principles works with this device:

• When the phone is called it communicates with the main station or the GSM cell switching his transmitter on higher power and these signals are detected by the coil, converted to AC current and amplified by the preamplifier.
• When called the phone speaker starts to produce sound. The coil of the microspeaker creates electromagnetic waves which are caught by the coil and converted to electrical signal

The coil is embedded on the PCB.

The only adjustment of the device is to put initially a potentiometer instead the R3 resistor and to adjust it to the triggering point of the timer. The timer must remain off, but small turning of the axis of the potentiometer have to trigger it on. After that the current value of the potentiometer is measured with ohmmeter and it is replaced with normal resistor. The standard value of the resistor should be closest but lower than the potentiometers measured value.

At the output of the timer I added four flashing LEDs : green, blue, yellow and red. Throw each of them I pass 20mA current. On the table above are given the needed voltage drops over the diodes. Based on this table the values of each limiting resistor is calculated - because the device is supplied by 5V (USB charger) and the output of the 555 timer is also 5V, the voltage drops over resistors are calculated so : 5V - Vled, where Vled is taken from the table and depends on the LED color. The resistor value then in kOhm is (5V - Vled) / 20mA.

To be able to charge the phone additional USB cable is soldered to the board.

The PCBs for the project contain as mentioned above the sensing coil.

They were produced by the PCBway fab in a short time with high quality.

All gerber files can be downloaded here. There you can order also the PCBs for the incredible price of 5 USD per 10 pieces..

The PCB supports few input configurations. It seems that the best results are achieved by the configuration shown in the given schematic. That means - not all devices have to be soldered on the PCB. On the place of the input diode a ceramic or poly capacitor is soldered.

The housing was printed using PLA filament. The stl files are attached here

Small LED reflectors were glued on the top cover, Such reflectors I bought here. For that purpose the LEDs were desoldered, connected with wires and inserted in the reflectors.

The housing was closed using screws for plastics.

After proper soldering and resistor adjustment the device start to work immediately. Sometimes is possible false triggering, when the phone connects to main station. In the case the device triggers by the phone speaker , this does not happen.

I have checked few times how the device works and always I had response from my mother. It seems that the device is fulfilling its purpose

Of course, this device can be used also if you do not want to be disturbing by the annoying phone ringing when working, but you have the possibility to keep an eye on the call detector constantly.