Phone Busy Indicator

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Have you ever been using the modem or fax and someone else picks up the phone, breaking the connection? Well, this simple circuit should put an end to that. It signals that the phone is in use by lighting a red LED. When the phone is not in use, a green LED is lit. It needs no external power and can be connected anywhere on the phone line, even mounted inside the phone.

Schematic

Parts

Part	Total Qty.	Description	Substitutions
R1	1	3.3K 1/4 W Resistor
R2	1	33K 1/4 W Resistor
R3	1	56K 1/4 W Resistor
R4	1	22K 1/4 W Resistor
R5	1	4.7K 1/4 W Resistor
Q1, Q2	2	2N3392 NPN Transistor
BR1	1	1.5 Amp 250 PIV Bridge Rectifier
LED1	1	Red LED
LED2	1	Green LED
MISC	1	Wire, Case, Phone Cord

Notes

This is a very simple circuit and is easily made on a perf board and mounted inside the phone.

LED1 and LED2 flash on and off while the phone is ringing.

Do not worry about mixing up the Tip and Ring connections.

The ring voltage on a phone line is anywhere from 90 to 130 volts. Make sure no one calls while you are making the line connections or you'll know it. :-)

In some countries or states you will have to ask the phone company before you connect this to the line. It might even require an inspection.

If the circuit causes distortion on the phone line, connect a 680 ohm resistor in between one of the incoming line wires and the bridge rectifier.

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Comments

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aisha Phone Busy Indicator Friday, August 15, 2014 2:24:38 AM

plz send me the complete discription and how can i make this circuit.... some one saying in comments "that this circuit didn't work, but after some minor adjustments, it did seem to work just fine. Nice circuit." so plz tell me whats the minor adjusment u done???? plz give me reply as soon as possoble

Fonseka Phone Busy Indicator Sunday, May 18, 2014 12:25:39 AM

i like this cct.that's function as wel. but i want to cct of how to indicated exchange output line correctivity .i know that's very difficult . can you try to drow in cct next send it for me.

PEter D Phone Busy Indicator Friday, February 24, 2012 4:33:42 PM

I am simulating this circuit in LTSpice, and it does not work. The LED drop voltages are not what they are supposed to be. As one comment here says, both LED's are on at the same time. Simulate the circuit with LTSpice and find out that the circuit is bad. Plus, LTSpice shows that circuit draws 2.25 mA when telephone is not in use! This is colossal current! You will have telephone companies contact you thinking that you use power for free for non-telephone needs (lighting, etc). Do simulation before buying and building. Simulation took me only about 1 hr to create.

anonymouslhb Phone Busy Indicator Monday, October 18, 2010 5:30:19 PM

WARNING ! Modern phone lines require a load resistance of ten megohms. The older relay systems would work fine . Your phone will indicate "busy" at all times. You cannot put any load on a new system in the of-line condition. You will have to use a wall-wart transformer to power any of-line system. 10 meg ohms impeadence required to be off line. Basicly , try it , you will find out. .. LHB.

zain Phone Busy Indicator Thursday, March 25, 2010 3:43:41 PM

plz send me the complete discription and how can i make this circuit.... some one saying in comments "that this circuit didn't work, but after some minor adjustments, it did seem to work just fine. Nice circuit." so plz tell me whats the minor adjusment u done???? plz give me reply as soon as possoble

adam Phone Busy Indicator Wednesday, April 01, 2009 12:52:00 PM

I originally posted that this circuit didn't work, but after some minor adjustments, it did seem to work just fine. Nice circuit.

anonymous Phone Busy Indicator Saturday, March 21, 2009 2:09:53 AM

Hi, I have been trying your project but it doesn't seem to be working for me. Mine just has both LED's Stay on constantly. Do different phone providers affect this, or possiby newer phones? Also I did switch the 1.5 Amp 250 PIV Bridge Rectifier because I could not find it with a 1.5 Amp 400 Volt Bridge Rectifier. I dont think this will make a difference but if it does please let me know. Do you know where I could find a 1.5 Amp 250 PIV Bridge Rectifier? Thanks

Germ Phone Busy Indicator Tuesday, March 10, 2009 10:20:54 PM

I have tested that circuit and it works great. LED are dim. use 2N3904 or 2N2222 The only problem I had is that I did not have the right diode for my bridge rectifer. 1N4148 did not work. I then use four 1N4001. It did work, the LED light up nicelly but you could not call!! I measure the voltage and got 48V. I live in Canada. The damn circuit is voltage divider! Do you homework, we will not do it for you. Stop asking questions and build it! It will cost you 1$ in parts!

acme fixer Phone Busy Indicator Tuesday, March 10, 2009 12:29:09 PM

This circuit may work well, but it draws too much current from the phone line when it is idle, causing the central office to think that there is leakage on the line. I strongly advise you to not use it, instead use the one at Bowden's hobby circuits (google for it). Bowden's draws much, much less current when idle.

prince Phone Busy Indicator Wednesday, February 11, 2009 12:51:21 PM

tell me that how i make the major project with combine this project to the other like that ringer, busy indicator and more in telephones. and plz tell me the combined circuit to make the major project and inform me about the components used

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aisha	Phone Busy Indicator	Friday, August 15, 2014 2:24:38 AM
plz send me the complete discription and how can i make this circuit.... some one saying in comments "that this circuit didn't work, but after some minor adjustments, it did seem to work just fine. Nice circuit." so plz tell me whats the minor adjusment u done???? plz give me reply as soon as possoble
Fonseka	Phone Busy Indicator	Sunday, May 18, 2014 12:25:39 AM
i like this cct.that's function as wel. but i want to cct of how to indicated exchange output line correctivity .i know that's very difficult . can you try to drow in cct next send it for me.
PEter D	Phone Busy Indicator	Friday, February 24, 2012 4:33:42 PM
I am simulating this circuit in LTSpice, and it does not work. The LED drop voltages are not what they are supposed to be. As one comment here says, both LED's are on at the same time. Simulate the circuit with LTSpice and find out that the circuit is bad. Plus, LTSpice shows that circuit draws 2.25 mA when telephone is not in use! This is colossal current! You will have telephone companies contact you thinking that you use power for free for non-telephone needs (lighting, etc). Do simulation before buying and building. Simulation took me only about 1 hr to create.
anonymouslhb	Phone Busy Indicator	Monday, October 18, 2010 5:30:19 PM
WARNING ! Modern phone lines require a load resistance of ten megohms. The older relay systems would work fine . Your phone will indicate "busy" at all times. You cannot put any load on a new system in the of-line condition. You will have to use a wall-wart transformer to power any of-line system. 10 meg ohms impeadence required to be off line. Basicly , try it , you will find out. .. LHB.
zain	Phone Busy Indicator	Thursday, March 25, 2010 3:43:41 PM
plz send me the complete discription and how can i make this circuit.... some one saying in comments "that this circuit didn't work, but after some minor adjustments, it did seem to work just fine. Nice circuit." so plz tell me whats the minor adjusment u done???? plz give me reply as soon as possoble
adam	Phone Busy Indicator	Wednesday, April 01, 2009 12:52:00 PM
I originally posted that this circuit didn't work, but after some minor adjustments, it did seem to work just fine. Nice circuit.
anonymous	Phone Busy Indicator	Saturday, March 21, 2009 2:09:53 AM
Hi, I have been trying your project but it doesn't seem to be working for me. Mine just has both LED's Stay on constantly. Do different phone providers affect this, or possiby newer phones? Also I did switch the 1.5 Amp 250 PIV Bridge Rectifier because I could not find it with a 1.5 Amp 400 Volt Bridge Rectifier. I dont think this will make a difference but if it does please let me know. Do you know where I could find a 1.5 Amp 250 PIV Bridge Rectifier? Thanks
Germ	Phone Busy Indicator	Tuesday, March 10, 2009 10:20:54 PM
I have tested that circuit and it works great. LED are dim. use 2N3904 or 2N2222 The only problem I had is that I did not have the right diode for my bridge rectifer. 1N4148 did not work. I then use four 1N4001. It did work, the LED light up nicelly but you could not call!! I measure the voltage and got 48V. I live in Canada. The damn circuit is voltage divider! Do you homework, we will not do it for you. Stop asking questions and build it! It will cost you 1$ in parts!
acme fixer	Phone Busy Indicator	Tuesday, March 10, 2009 12:29:09 PM
This circuit may work well, but it draws too much current from the phone line when it is idle, causing the central office to think that there is leakage on the line. I strongly advise you to not use it, instead use the one at Bowden's hobby circuits (google for it). Bowden's draws much, much less current when idle.
prince	Phone Busy Indicator	Wednesday, February 11, 2009 12:51:21 PM
tell me that how i make the major project with combine this project to the other like that ringer, busy indicator and more in telephones. and plz tell me the combined circuit to make the major project and inform me about the components used