Telephone line in-use indicator design; Release 2

Telephone Line Indicator design

By Kris Heidenstrom (k@heidenstrom.gen.nz)

Release 2, 19 July 1998

Introduction

This document presents a design for an electronic circuit which connects to a telephone line and provides a visual indication of whether or not the line is in use, so that two or more people sharing a line can avoid interrupting each other's voice or modem calls. The circuit has 22 components (see the parts list) and can be built at minimal cost by anyone with some experience in electronic circuit construction.

This document also contains some information on the characteristics of standard telephone lines and equipment.

Before building this circuit, read the legal stuff and the safety advice and also check the caveats for notes on the weaknesses of this specific design and the line voltage check that you should make first to confirm that it will work properly.

Read the legal stuff and the safety advice NOW!

Legal stuff - READ THIS

The information on telephone line characteristics in this document is taken from my personal experience and tests, published standards, and Internet-accessible sources. I cannot and do not guarantee the completeness or accuracy of this information, and I do not claim that it is applicable in all countries, nor to all telephone systems.

Because it draws current from the telephone line, the circuit presented here may not comply with your telephone company's specifications, and connecting it to your telephone line may have legal implications for you. If you are in doubt, check your contract with your telephone company. If you are still in doubt, do not use this design. Although I believe that this circuit is safe and appropriate for connection to a standard telephone line, ultimately any use of the circuit or information in this document is at your own risk. If you do not want to accept that risk, do not use this design and information.

In the safety section I have attempted to warn you of the dangers involved in working with a telephone line, and to suggest safety precautions you should take. However, the responsibility for your safety is yours alone, and I disclaim all responsibility and liability for any injury or damage that may occur as a result of the use of this document and design. If you do not accept full responsibility for your safety, do not continue with this project.

Safety - READ THIS TOO

A standard telephone line can deliver a serious electric shock. In its idle state it carries 48V DC which in certain circumstances can be fatal. During ringing it carries 90V AC which can also be dangerous or fatal. During a lightning storm or a power line fault condition the line can rise to hundreds or thousands of volts above the local earth potential.

Carefully read and follow this safety advice:

Do not touch, or make bodily contact with, the telephone line or anything connected to it.
Always use insulated tools.
While working on any telephone equipment, unplug it from the line.
When connecting equipment to the telephone line, use an appropriate plug and socket arrangement, rather than hard-wiring to the permanent wiring.
Before working on permanently connected telephone line wiring, disconnect the line at the demarcation point.
Do not work on telephone wiring while a thunder or lightning storm is nearby.
Do not work on telephone wiring at all if you have a pacemaker.
Do not put telephone wiring or connections in any conduit, outlet or junction box containing mains wiring or any other electrical wiring.
Do not connect to, remove, or modify any protective or grounding wires placed by your telephone company.

(Note: The last three items are from Phone Man's web page)

If you are not sure you understand these precautions, or if you are not sure you can follow them, then do not continue with this project.

Links to telephone-related documentation

The telephone page of Tomi Engdahl's electronics resource has a huge list of links to good sources of information.

Telephone line characteristics

The parameters (voltages, currents, resistances and frequencies) listed in these sections are typical values only. They may vary from one country to another, or from one exchange to another.

The telephone line you see at your telephone jack point connects to a telephone exchange. This exchange is either a public exchange, or an electrically equivalent private business exchange. These lines are often called POTS (Plain Old Telephone System) lines. Some private exchanges use special or intelligent telephones - these often use non-standard proprietary interfaces which are not equivalent to a POTS line. This document covers POTS lines only.

The exchange provides a line with certain characteristics, and a telephone, fax machine, modem, etc (called an appliance in this document) has electrical characteristics which allow it to perform the normal operations of going off-hook (busy), dialling, monitoring call progress, sending sound to the other party (the callee), receiving sound from the callee, going on-hook (idle), ringing, and receiving calling line identification.

The line to the exchange has two wires, known for historical reasons as "tip" and "ring" ("ring" does not relate to the ringing of the telephone).

Line states

If nothing is connected to a telephone line, the exchange will regard the line as on-hook (idle). The exchange provides a DC "idle line" voltage, nominally 48V (typically between 45V and 50V), which appears across the line. You can measure this by connecting a voltmeter across the line. The polarity of this voltage is not defined in relation to the telephone line connector, and might change if lines are re-wired or changes are made at the exchange. For this reason, any device connected to the line should be designed to operate correctly regardless of the polarity of the line voltage.

An appliance, when on-hook (idle), must appear as an open circuit (or nearly an open circuit) to the telephone line. In other words, it must draw no current, or very little current.

As current is drawn from the line, the line voltage drops, until at a certain current, the exchange regards the line as off-hook (in use) and transmits a dial tone down the line. In this state the appliance (telephone, fax etc) has a resistance of a few hundred ohms, and draws about 25 to 40 milliamps (up to 80 milliamps in unusual cases) from the line, dragging the line voltage (measured at the appliance) down to typically 5 to 10 volts. The appliance can then place a call, using pulse dialling (also called "loop disconnect" or "LD" dialling) or DTMF (dual tone multiple frequency) dialling (also called "touch-tone" dialling) to inform the exchange of the number to be dialled. When the appliance goes back on-hook, the exchange regards the line as being in the idle state again, and terminates any call that was in progress.

A ring signal is generated by the exchange to indicate an incoming call. It consists of bursts of low frequency AC voltage across the line, typically 90 volts RMS at 20Hz. The normal idle line voltage (48V DC) may change, disappear, or reverse polarity during ringing (I'm not sure which; also this may vary from country to country). While AC ring voltage is present, the audible ringer (bell or warbler) on the appliance will emit its noise. The timing and pattern of the bursts of AC is known as the "ring cadence" and varies from country to country and also if the "distinctive ring" feature is used. Regardless of the ring cadence, the presence of a large AC voltage on the line can be interpreted as a ring signal of some kind. The exchange will only send a ring signal if the appliance is currently on-hook (idle).

The above description applies to standard, single subscriber lines. In unusual arrangements such as party lines, there may be other special characteristics involved.

Monitoring a Line

There are two ways to monitor whether a telephone line is in use - detect the loop current, or detect the line voltage change. Each method has advantages and disadvantages, which are summarised in the tables below.

Detecting Loop Current

Detecting loop current involves breaking the circuit to the appliance(s) on the line, and inserting a device in the line, upstream of all appliances, to detect the current drawn by any appliance when it goes off-hook. The current-detecting device (usually a special relay or an optocoupler) will usually signal some external circuitry, which requires a separate power supply. This circuitry may drive one or more LEDs or light bulbs, at one or more locations, to indicate the line state.

If you want to investigate this method, check out Erg Components. They make a range of relays specifically designed for this purpose. The BT55/4 from their Type 55 range is probably the best choice. It has two coils with very low resistance (3 ohms each) and low capacitance between the two coils. You break both lines at some point upstream of all the appliances you want to monitor (usually near where the line enters the house) and connect one coil into each break in the line. When any appliance goes off-hook, the current flow in the line causes the relay to activate. The contact can directly drive LEDs or small light bulbs, or can be connected to another relay to boost the switching capability. A power supply is required for the indicator circuit (this type of circuit would not normally be line-powered).

Detecting Line Voltage Change

Detecting the line voltage change involves monitoring the voltage across the line, and detecting when an appliance is off-hook (in use) by detecting the drop in the line voltage (from 48V to 5-10V). This is the approach used in the design presented here.

Detecting Loop Current Method

Advantages Disadvantages

Adds no on-hook leakage current
Cannot be detected from exchange
Works well regardless of line and appliance characteristics
Circuit can be made very simple
Requires modification to fixed phone wiring
Requires additional wiring to indicators at other locations (if wanted)
Cannot be line-powered - requires a power supply
Cannot be simply plugged into an outlet to monitor line state
Could have legal implications

Monitoring Line Voltage Method

Advantages Disadvantages

Does not require modifications to fixed wiring
Requires no additional wiring to other locations (the line can be monitored separately at each location)
Can be made line-powered (see caveats)
Can be simply plugged into any phone outlet to monitor the line state
Adds small leakage current when on-hook (see caveats)
Can be detected by telephone company
Doesn't work if any appliance pulls the line voltage below about 5V (also see caveats)
Draws a small amount of extra current from the line when off-hook (in use)
Could have legal implications

Detecting Loop Current Method
Advantages		Disadvantages
Adds no on-hook leakage current Cannot be detected from exchange Works well regardless of line and appliance characteristics Circuit can be made very simple		Requires modification to fixed phone wiring Requires additional wiring to indicators at other locations (if wanted) Cannot be line-powered - requires a power supply Cannot be simply plugged into an outlet to monitor line state Could have legal implications

Monitoring Line Voltage Method
Advantages		Disadvantages
Does not require modifications to fixed wiring Requires no additional wiring to other locations (the line can be monitored separately at each location) Can be made line-powered (see caveats) Can be simply plugged into any phone outlet to monitor the line state		Adds small leakage current when on-hook (see caveats) Can be detected by telephone company Doesn't work if any appliance pulls the line voltage below about 5V (also see caveats) Draws a small amount of extra current from the line when off-hook (in use) Could have legal implications

Caveats

In the on-hook (idle) state, an appliance is not supposed to draw any current from the line. The telephone company specifies this as a maximum leakage current and/or a minimum resistance. Any device that connects across the line to monitor its state will draw some current. The design presented here has an on-hook resistance of about 20 megohms, giving a leakage current of about 2.4 microamps (assuming 48V on-hook line voltage). If two of these circuits are used, the resistance will be about 10 megohms and the leakage current will be about five microamps. This small amount of leakage current should cause no practical problems for the telephone company, but they may be able to detect it. Please make sure you have read and understood the legal stuff section.

The design presented here requires an off-hook line voltage of about 5V or more in order to operate properly. In rare cases, the off-hook line voltage may be too low. Before building this circuit, check your line voltage by measuring the voltage across the line (remember to observe the safety precautions!) with a multimeter, and taking each appliance off-hook in turn. If the voltage drops below 5V in any case, this circuit is not suitable and you must use some other design, such as the line current method (not described in this document).

Schematic and circuit description

Diodes D1-4 are connected as a bridge rectifier, ensuring that the top rail is always positive regardless of the line polarity. This means that the circuit can be connected to the telephone line either way round, and need not be reversed if the line polarity is reversed by the telephone company.

Transistor Q1 is a darlington device. Resistors R1-4 give a total resistance of 40 megohms, which forms a voltage divider in conjunction with R9. The voltage divider provides a certain fraction of the line voltage to the base of Q1. When the line voltage is high (i.e. the line is idle), this voltage is high enough to forward-bias Q1. In this state, Q1 pulls its collector low, removing base bias from Q2, Q3 and Q4. This turns Q2-4 and the LED off.

When any appliance on the line goes off-hook (in-use), the line voltage drops. The voltage on the base of Q1 drops, and Q1 turns off, allowing the voltage on the base of Q2 to rise, due to the current flow through R5-8. Q2 conducts and supplies bias to Q3, which in turn supplies bias current to Q4. The tiny base current from R5-8 is amplified in each transistor, so that Q4 is able to pass enough current to illuminate the LED. Resistors R10 and R11 limit the maximum current at each stage.

Q2-4 must withstand the full line voltage (which peaks at over 200V during ringing) while the LED is off, therefore they must be specified to withstand at least 250V, preferably more.

Q5 limits the current through Q4 to give a constant LED brightness regardless of the line voltage. It operates by drawing off Q4's base current when the voltage across R12 rises above about 0.6V. Q4 and Q5 reach a state of equilibrium, with the voltage across R12 being about 0.6V. As R12 is 560 ohms, Q4's emitter current (which is approximately equal to the LED current) is about 1mA.

During ringing, the LED blinks dimly.

Stripboard layout and Construction

The quickest and easiest construction style is stripboard (14 tracks by 13 holes). The layout is shown to the right. The drawing shows the view from above, looking down on the components. The tracks, shown in grey, are on the underside of the board. The layout is arranged much like the schematic, with the LED (D5) on the right.

Begin by cutting the stripboard to size and making the 31 cuts in the tracks (12 cuts at each end, and 7 cuts in the circuit area), as indicated by the curved gaps in the grey strips on the diagram. Remember the diagram shows the non-copper side, so mirror it for a copper side view. Use a drill bit (about 4-5 mm or 3/16 inch diameter) for this. Cut just deep enough to remove the copper. Carefully check for copper slivers around the edges of the cuts, as these could short to adjacent tracks.

Next, mount the wire link (next to the bottom wire from Q1) and resistors. R1-8 at the top and R12 at the bottom are mounted vertically. R9-11 are mounted flat against the board.

The remaining components are polarised. Mount diodes D1-4 with their cathode indicator bands pointing upwards, as shown in the drawing. Next, mount Q1-5. These are all oriented the same way, with the collector (C) at the top, base (B) in the middle, and emitter (E) at the bottom. Bend the collector and emitter wires outwards slightly to suit the stripboard hole pitch. Q4 and Q5 connect to three adjacent tracks, but Q1, Q2 and Q3 all have their emitters bent outwards more than normal, so that the emitter misses one track and connects to the next one. Push the transistor down so its bottom is about five millimetres above the stripboard, then solder it.

Next, connect the two wires on the left. These need to connect to a telephone plug, which will be plugged into the line (using a Y-adapter if necessary). The American telephone system uses the two centre pins of an RJ-11 (aka Western Electric) plug for the tip and ring lines.

Finally, connect the LED (D5). Its polarity is normally indicated by a longer lead - this is the anode (A) which goes towards the top. If you can't tell the polarity this way, measure the LED with a digital multimeter set to the diode test range - with the probes one way round, the LED should light, telling you that the red probe is connected to the anode (A). Warning! Don't use an analogue meter (moving pointer type) for this test, as it may apply 9V to the LED which can damage it if applied in the reverse direction. Before mounting the LED into the board, slide a length of insulation (taken from some hookup wire) onto each lead. If you want to mount the LED elsewhere, you can use hookup wire from the board to the LED, but keep the leads short (10 to 20 centimetres).

Before testing the circuit, use a screwdriver to scrape along between the copper strips to remove any left-over flux, then use an old hard-grade toothbrush and some solvent (isopropyl alcohol is excellent for this, but you can use methylated spirits) to clean very thoroughly between the strips.

Parts list

Quantity	Reference(s)	Value	Description and comments
8	R1-8	10M	Resistor, 10 megohms, 0.25 watt, 5% (see notes)
1	R9	1M5	Resistor, 1.5 megohms, 0.25 watt, 5%
2	R10,11	10K	Resistor, 10 kilohms, 0.25 watt, 5%
1	R12	560R	Resistor, 560 ohms, 0.25 watt, 5%
4	D1-4	1N4007	Silicon diode, 1000V, 1A
1	Q1	MPSA14	Silicon transistor, darlington (see notes)
3	Q2-4	MPSA42	Silicon transistor, 300V or 250V VCE rated (see notes)
1	Q5	2N3904	Silicon transistor, general purpose
1	D5	LED	High-efficiency red LED, 5mm or 3mm (see notes)

Component notes

Resistor values are not critical - a tolerance of 5% is fine. Carbon resistors will work, but use metal film resistors such as the Philips SFR25 series if possible, especially for R1-8. The two groups of four 10M resistors in series (R1-4 and R5-8) are specified this way because 10M is the highest value readily available from hobbyist shops. If you can find 39M resistors, you can replace each group with a single resistor and save some space. If you can find 22M resistors, you can replace each group with two 22M resistors in series.

The transistors are more critical. Because of the very small currents used in this circuit, the transistor parameters (the DC current gain and the leakage current) are important, especially for Q1 and Q2, so avoid old stock and don't use recycled transistors.

Q1 can be any small-signal NPN silicon darlington transistor such as 2N6426/7, BC372/3, BC517, BC618, MPSA13/14/25/26/27/28/29. If you have a choice, choose the device with the lowest leakage current. You can use a non-darlington transistor if you reduce R9 to about 680 kilohms and use a high-gain transistor such as a BC548C. The layout assumes an MPSA14 (American pinout), so check the pinouts if you use something else.

The transistor type used for Q2-4 must be a silicon NPN transistor (not a darlington) and must have a collector-emitter voltage (VCEmax) rating of at least 250V. It should have as much gain as possible, and its leakage current must be very low. This means that small, low-current devices (like those used as video output transistors in TVs and monitors) are the most suitable. The MPSA42 is readily available, and works acceptably, but there are much better parts available, as shown in the following table. The layout assumes MPSA42 (American pinout), so check the pinouts if you use something else.

The following table lists transistors that may be suitable for Q2-4. See the pinouts section for details of the pin allocation for the TO92-packaged devices. The best device in the table is the Philips BF487. Unfortunately, it has a Japanese pinout, so you would need to cross the top and middle wires over, to make it fit with this layout.

Suggested transistors for Q2-4

Type Package Manufacturer VCE IC HFE min HFE typ Leakage current Notes

2N3439 TO39 Philips 350 1A 30@IC=2mA Not specified ICEO<1µA@300V

2N3440 TO39 Philips 250 1A 40@IC=20mA Not specified ICEO<1µA@200V

2N6431 TO18 Motorola 300 50mA 25@IC=1mA Not specified ICEO<0.1µA@200V

2SC2610 TO92 Jap. Hitachi 300 0.1A 30 60@IC=1mA ICEO<1µA@250V

2SC3380 UPAK (SMD) Hitachi 300 0.1A 30 60@IC=1mA ICEO<1µA@250V

2SC4647 TO92 Jap. Hitachi 300 0.1A 30 60@IC=1mA ICBO<1µA@250V

2SC4702 MPAK (SMD) Hitachi 300 50mA 60@IC=2mA 100@IC=0.1mA ICBO<0.1µA@250V VG

BF393 TO92 USA Motorola 300 0.5A 25@IC=1mA 50@IC=1mA ICBO<0.1µA@200V

BF420 TO92 Jap. Motorola 300 0.5A 50@IC=25mA 50@IC=1mA ICBO<10nA@200V

Philips 300 50mA 50@IC=25mA 65@IC=1mA ICBO<10nA@200V

BF422 TO92 Jap. Motorola 250 0.5A 50@IC=25mA 50@IC=1mA ICBO<10nA@200V

Philips 250 50mA 50@IC=25mA 65@IC=1mA ICBO<10nA@200V

BF483 TO92 Jap. Philips 250 50mA 50@IC=25mA 100@IC=0.1mA ICBO<20nA@300V VG

BF485 TO92 Jap. Philips 300 50mA 50@IC=25mA 100@IC=0.1mA ICBO<20nA@300V VG

BF487 TO92 Jap. Philips 350 50mA 50@IC=25mA 100@IC=0.1mA ICBO<20nA@300V VVG

BF844 TO92 USA Motorola 400 0.3A 40@IC=1mA 80@IC=1mA ICES<0.5µA@400V G

MPSA42 TO92 USA Motorola 300 0.5A 25@IC=1mA 50@IC=1mA ICBO<0.1µA@200V

Philips 300 0.5A 25@IC=1mA Not specified ICBO<0.1µA@200V

MPSA44 TO92 USA Motorola 400 0.3A 40@IC=1mA 80@IC=1mA ICES<0.5µA@400V G

MPSW42 TO92 USA Motorola 300 0.5A 25@IC=1mA 60@IC=1mA ICBO<0.1µA@200V

PBF259 TO92 USA Motorola 300 0.5A 25@IC=1mA 50@IC=1mA ICBO<50nA@250V

PN3439 TO92 USA Philips 350 1A 30@IC=2mA Not specified ICEO<1µA@360V

PN3440 TO92 USA Philips 250 1A 40@IC=2mA Not specified ICEO<1µA@250V

Suggested transistors for Q2-4
Type	Package	Manufacturer	VCE	IC	HFE min	HFE typ	Leakage current	Notes
2N3439	TO39	Philips	350	1A	30@IC=2mA	Not specified	ICEO<1µA@300V
2N3440	TO39	Philips	250	1A	40@IC=20mA	Not specified	ICEO<1µA@200V
2N6431	TO18	Motorola	300	50mA	25@IC=1mA	Not specified	ICEO<0.1µA@200V
2SC2610	TO92 Jap.	Hitachi	300	0.1A	30	60@IC=1mA	ICEO<1µA@250V
2SC3380	UPAK (SMD)	Hitachi	300	0.1A	30	60@IC=1mA	ICEO<1µA@250V
2SC4647	TO92 Jap.	Hitachi	300	0.1A	30	60@IC=1mA	ICBO<1µA@250V
2SC4702	MPAK (SMD)	Hitachi	300	50mA	60@IC=2mA	100@IC=0.1mA	ICBO<0.1µA@250V	VG
BF393	TO92 USA	Motorola	300	0.5A	25@IC=1mA	50@IC=1mA	ICBO<0.1µA@200V
BF420	TO92 Jap.	Motorola	300	0.5A	50@IC=25mA	50@IC=1mA	ICBO<10nA@200V
Philips	300	50mA	50@IC=25mA	65@IC=1mA	ICBO<10nA@200V
BF422	TO92 Jap.	Motorola	250	0.5A	50@IC=25mA	50@IC=1mA	ICBO<10nA@200V
Philips	250	50mA	50@IC=25mA	65@IC=1mA	ICBO<10nA@200V
BF483	TO92 Jap.	Philips	250	50mA	50@IC=25mA	100@IC=0.1mA	ICBO<20nA@300V	VG
BF485	TO92 Jap.	Philips	300	50mA	50@IC=25mA	100@IC=0.1mA	ICBO<20nA@300V	VG
BF487	TO92 Jap.	Philips	350	50mA	50@IC=25mA	100@IC=0.1mA	ICBO<20nA@300V	VVG
BF844	TO92 USA	Motorola	400	0.3A	40@IC=1mA	80@IC=1mA	ICES<0.5µA@400V	G
MPSA42	TO92 USA	Motorola	300	0.5A	25@IC=1mA	50@IC=1mA	ICBO<0.1µA@200V
Philips	300	0.5A	25@IC=1mA	Not specified	ICBO<0.1µA@200V
MPSA44	TO92 USA	Motorola	400	0.3A	40@IC=1mA	80@IC=1mA	ICES<0.5µA@400V	G
MPSW42	TO92 USA	Motorola	300	0.5A	25@IC=1mA	60@IC=1mA	ICBO<0.1µA@200V
PBF259	TO92 USA	Motorola	300	0.5A	25@IC=1mA	50@IC=1mA	ICBO<50nA@250V
PN3439	TO92 USA	Philips	350	1A	30@IC=2mA	Not specified	ICEO<1µA@360V
PN3440	TO92 USA	Philips	250	1A	40@IC=2mA	Not specified	ICEO<1µA@250V

Q5 is specified as 2N3904, but can be any general purpose small-signal silicon NPN transistor (not a darlington). You could use a BC547B, a BC548C or even an MPSA42 or any of the types suggested for Q2-4 above. The layout assumes a 2N3904 (American pinout), so check the pinouts if you use something else.

D5 should be a high-efficiency LED (light-emitting diode), preferably red (as red LEDs are usually most efficient, and have the lowest forward voltage), and designed to produce high light output at low current, such as any of the LEDs listed in the following table.

The "lens type" field in the following table indicates the type of lens in the LED. A clear lens usually has a narrower viewing angle (is more directional) with a piercing light, whereas a diffused lens has a wider viewing angle with a softer light, which I think is better for this application. The values in the "intensity" column are in mcd (millicandelas) and are typical values, not guaranteed minimums. I have sorted the table by size, colour and lens type.

Suggested LEDs for D5

Size Colour Lens type Manufacturer Part number Viewing angle Intensity
3mm Red Diffused Hewlett Packard HLMP-1700 50 degrees 1.8 at 2 mA

Hewlett Packard HLMP-K150 60 degrees 2.0 at 1 mA

Kingbright L-934LSRD 60 degrees ?

Siemens LS3366-PS 70 degrees 3.6 at 1 mA

Temic TLLR4401 50 degrees 2.0 at 2 mA

Clear Hewlett Packard HLMP-K155 45 degrees 3.0 at 1 mA

Siemens LS3336-QT 40 degrees 5.6 at 1 mA

Green Diffused Hewlett Packard HLMP-1790 50 degrees 1.6 at 2 mA

Temic TLLG4401 50 degrees 2.0 at 2 mA

Yellow Diffused Hewlett Packard HLMP-1719 50 degrees 1.6 at 2 mA

Siemens LY3366-PS 70 degrees 3.6 at 1 mA

Temic TLLY4401 50 degrees 2.0 at 2 mA

Clear Siemens LY3336-RU 40 degrees 9.0 at 1 mA

5mm Red Diffused Hewlett Packard HLMP-D150 65 degrees 3.0 at 1 mA

Hewlett Packard HLMP-4700 50 degrees 2.0 at 2 mA

Kingbright L-53LSRD 60 degrees ?

Temic TLLR5401 50 degrees 2.0 at 2 mA

Clear Hewlett Packard HLMP-D155 24 degrees 10 at 1 mA

Green Diffused Hewlett Packard HLMP-4740 50 degrees 1.8 at 2 mA

Temic TLLG5401 50 degrees 2.0 at 2 mA

Yellow Diffused Hewlett Packard HLMP-4719 50 degrees 1.8 at 2 mA

Temic TLLY5401 50 degrees 2.0 at 2 mA

Suggested LEDs for D5
Size	Colour	Lens type	Manufacturer	Part number	Viewing angle	Intensity
3mm	Red	Diffused	Hewlett Packard	HLMP-1700	50 degrees	1.8 at 2 mA
Hewlett Packard	HLMP-K150	60 degrees	2.0 at 1 mA
Kingbright	L-934LSRD	60 degrees	?
Siemens	LS3366-PS	70 degrees	3.6 at 1 mA
Temic	TLLR4401	50 degrees	2.0 at 2 mA
Clear	Hewlett Packard	HLMP-K155	45 degrees	3.0 at 1 mA
Siemens	LS3336-QT	40 degrees	5.6 at 1 mA
Green	Diffused	Hewlett Packard	HLMP-1790	50 degrees	1.6 at 2 mA
Temic	TLLG4401	50 degrees	2.0 at 2 mA
Yellow	Diffused	Hewlett Packard	HLMP-1719	50 degrees	1.6 at 2 mA
Siemens	LY3366-PS	70 degrees	3.6 at 1 mA
Temic	TLLY4401	50 degrees	2.0 at 2 mA
Clear	Siemens	LY3336-RU	40 degrees	9.0 at 1 mA
5mm	Red	Diffused	Hewlett Packard	HLMP-D150	65 degrees	3.0 at 1 mA
Hewlett Packard	HLMP-4700	50 degrees	2.0 at 2 mA
Kingbright	L-53LSRD	60 degrees	?
Temic	TLLR5401	50 degrees	2.0 at 2 mA
Clear	Hewlett Packard	HLMP-D155	24 degrees	10 at 1 mA
Green	Diffused	Hewlett Packard	HLMP-4740	50 degrees	1.8 at 2 mA
Temic	TLLG5401	50 degrees	2.0 at 2 mA
Yellow	Diffused	Hewlett Packard	HLMP-4719	50 degrees	1.8 at 2 mA
Temic	TLLY5401	50 degrees	2.0 at 2 mA

Component Pinouts

Most of the transistors mentioned in this document and listed in the table above use the "TO92" package. Because the parameters of most of the transistors used in this design are somewhat critical, you may wish to substitute better or more readily available devices. This section should help by showing the pinouts for all TO92-packaged transistor types mentioned in this document. There are three common pin arrangements for the TO92 package - the American, Japanese and European pinouts - as shown in these illustrations. Thanks to Motorola for the stylish component drawing.

TO-92 American
pinout, 2K
American
2N3904
2N6426,7
BC372,3
BF393
BF844
MPSA13,14,25-29,42,44
MPSW42
PBF259
PN3439,40

TO-92 Japanese
pinout, 2K
Japanese
2SC2610
2SC4647
BF420,2
BF483,5,7

TO-92 European
pinout, 2K
European
BC517
BC547B
BC548C
BC618

Testing

Connect the circuit to a 9V battery. Connect the battery one way, then the other way. The LED should light in both cases. With the battery connected, check for about 0.6V across R12. If the circuit passes these basic functional tests, check its operation on a telephone line.

Because the performance of this circuit depends very much on the characteristics of the transistors, here are two tests you can perform, to gauge how well the circuit is working. They are the dropout voltage test and the idle leakage current test.

Dropout voltage test

Connect the circuit to a 9V battery, with a 10K potentiometer (rheostat) inserted into one line, and connect a digital multimeter, set to measure voltage, across R12. With the potentiometer at minimum resistance, the meter should indicate about 0.6V. Record the value, multiply it by 0.8, then increase the potentiometer resistance until the meter reading drops to the value you calculated. Disconnect the multimeter from R12 and connect it across the circuit input terminals. This voltage is the dropout voltage of the circuit.

The dropout voltage should be less than about 5V with a red LED, and less than about 5.5V with a green or yellow LED. If it is higher, there may be a problem with the circuit. My prototype (with a red LED) has a dropout voltage of about 4V. If the dropout voltage is unusually high, see the fault finding section.

Idle leakage current test

This test requires the use of a telephone line. Remember to observe the safety precautions, except for the rule about not working on anything that is connected to the line (since the circuit must be connected to the line during the tests). Be very careful not to touch the line connections or any part of the circuit while it is connected. Use insulated tools.

Connect the circuit to the telephone line (or a 48VDC voltage source) with a 100 kilohm resistor inserted into one line. Connect a digital multimeter (an analogue meter will not work), set to measure voltage, across the resistor. Make sure the line is idle (all appliances on-hook). If the voltage indicated on the multimeter is high (more than 40 volts), briefly short the resistor, and check that the voltage returns to a low value when you remove the short.

Note the voltage (in volts) indicated on the meter, and multiply it by ten. The result gives the idle leakage current drawn by the circuit, in microamps. It should be less than 2.5 microamps. If it is higher, see the fault finding section.

Mounting

You can mount the unit in a small plastic box. Be very careful to ensure that no conductive parts of the circuit can be touched, as the ring voltage and other line voltages can be dangerous.

A tidier mounting method would be to incorporate the unit into an existing telephone, fax machine or computer, if there is enough room. You can mount the LED through the case wall. Be careful to keep telephone line wiring well away from any other wiring or metal. Make sure that things cannot move around and allow any part of the circuit to make contact with anything else.

Fault-finding

I hope you won't have any trouble with the circuit. If you do, here are a few things to check before you email me or ask a friend for help. Problems are most likely to be caused by one or more of the following:

Tracks not cut correctly

Whiskers of copper shorting tracks near track cuts

Solder bridges between tracks

Wrong component values

Transistors or diodes inserted the wrong way

Flux or dirt between tracks (be sure to scrape and scrub the board as described in the construction section)

If you have a problem with the circuit, first re-check your work against the design and layout given here, carefully checking for each of the possible errors just listed. If you discover any of these problems after you have applied power to the circuit, you may have damaged one or more of the transistors, or the LED, and you could save time later if you replace them before continuing.

Subtler problems could be caused by variations in transistor parameters, as some of these parameters are quite important in this design. Manufacturers specify limits for these parameters, but the limits usually represent worst possible cases, and most components perform much better than these specifications, but there are still variations between components, between batches, and between manufacturers. If you have problems with low gain, or high leakage, you may need to try the same device from a different batch, or a different manufacturer, or try a different device.

If you change the combined resistance of the R1-4 group, the line voltage threshold may change, and you may need to change R9 to compensate. If the LED remains on even when the line is free, increase R9. If the LED remains off even when the line is in use, decrease R9.

Always disconnect the circuit from the line before working on it.

If you have checked all the items listed above and would like some guidance, you can email me a clear description of the symptoms of the problem and I will try to help.

If you have problems with the design, especially due to transistor parameter variations, please tell me about them, so I can consider improving the design or specifying different components.

Summary

The design presented here provides a fairly straightforward way to monitor a telephone line that is shared by several people and/or modems. I believe the design is appropriate for use with most telephone systems and will not cause problems, however, connecting the circuit to the telephone line could have legal implications - if in doubt, check your contract with your telephone company.

Please send any comments, suggestions, problem reports etc to k@heidenstrom.gen.nz.