About the article

Line Switcher

A single phone for two lines

Line Switcher

More and more ISPs, are offering subscribers an extra phone line using Voice over IP (VoIP) on top of their Internet access.

The project described here reduces this inconvenience by letting you connect a single phone to two lines at once, to avoid having to tie knots in the cables! It’s a phone line switcher that routes the ‘correct’ line to the phone automatically.
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Gerber file

CAM/CAD data for the PCB referred to in this article is available as a Gerber file. Elektor GREEN and GOLD members can exclusively download these files for free as part of their membership. Gerber files allow a PCB to be produced on an appropriate device available locally, or through an online PCB manufacturing service.

Elektor recommends its business partner Eurocircuits as the company of choice for its own prototypes and volume production in selected cases.

Component list


R1,R2,R7,R8 = 33k

R3,R11,R18 = 10k

R4,R13,R27 = 1M

R5,R6 = 100

R9,R10 = 100k

R12 = 4MΩ7

R14,15 = 47k

R16,R17,R21,R22,R25,R26 = 1k

R19,R20 = 100k

R23,R24 = 470k


C1–C4 = 100nF (7.5mm pitch MKT)

C5 = 470 nF (7.5mm pitch MKT)

C6,C9 = 6μF8 100 V

C7,C8 = 10μF 100 V

C10,C11 = 220μF 10V non-polarised

C12,C13 = 10nF (5mm pitch Sibatit/ceramic)

C14,C15 = 100nF (5mm pitch Sibatit/ceramic)


D1,D2 = BAT85

D3,D4 = 1N4148

D6,D7 = LED, 3mm, green (low power)*

D5,D8 = LED, 3mm, red (low power)*

T1,T2 = BC557

T3–T7 = BC547

IC1 = 40106BF

IC2 = 4093

IC3–IC7 = TLP620 (Toshiba optocoupler)


K1,K3,K4 = RJ11 6/4 phone socket (Hirose TM5RE1-64)

K2 = 2-way SIL pinheader for power: 9 V NiMH battery

S1,S2 = pushbuttons

RE1 = Bistable 5 V 2-pole c/o relay (Panasonic DS2E-ML2-DC5V or Omron G6AK-234P-ST-US)
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