mgrant@cos.com (Michael Grant) (11/29/88)
Dave Horsfall in Australia was kind enough to air-mail me a schematic
he found in a magazine called Silicone Chip. The design is by John Clark
in the November 1987 issue.
It's totally line powered. When an extension is off hook, it makes an
LED flash. (At least that's what he claims it does on the Australian
phone network! It looks like it'll work in the USA too.)
D1-D4 (4) 1N4004 LED1
+-------------+ +--------+--------+ HB
| | | | R4 > +------->|--+
o +-->|--+-->|--+ | | 10K > 6| |
Phone | | | | > +------+ R5 >
o +--+ +--+ R3 > +--------+---| | 560 >
| | | | | 470K > | | 5| |8 >
| | +-->|--+-->|--+ | > ZD2 ~ C1 |+ | IC1 |--------+
| | | | | 3.3V ^ 220uF --- | LM | C2 |
+-------------+ ZD1 ~ | | 16VW --- | 3909 | 47uF ---
| 12V ^ | | | 4| | 16VW ---
| | | Q2 C+--------+---| | |+
| | | BC550|/ | +------+ |
| R1 > +------| | 2| |
| 470K > Q1 C| B|\ | +-----------+
| > BC549|/ Ev ZD3 ~
| +------| | 56V ^
| R2 > B|\ | |
| 120K > Ev | |
| > | | |
+-------------------+--------+--------+--------+
Parts List:
IC1: LM3909 LED flasher/oscillator
LED1: High Brightness Red LED ---
Q1: BC549 NPN transistor /EBC\
Q2: BC550 NPN transistor -----
ZD1: 12V zener diode Bottom View
ZD2: 3.3V zener diode of Transistor
ZD3: 56V zener diode
D1-D4: 1N4004 silicone diodes
C1: 220uF 16VWW PC electrolytic capacitor
C2: 47uF 16VW PC electrolytic capacitor
R1: 470Kohm 1/4W resistor
R2: 120Kohm 1/4W resistor
R3: 470Kohm 1/4W resistor
R4: 10Kohm 1/4W resistor
R5: 560ohm 1/4W resistor
Notes:
When an extension is off hook, the IC charges and discharges C2
flashing the LED.
The article recommends that no more than 3 of these be used on a
phone line due to current draw. "Current drain of each indicator is
around 200 microamps when all phones are 'on-hook' (ie with phone line
voltage at 50V). With any phone 'off-hook' the current drain of each
indicator ranges from around 700 microamps at 9V to around 200
microamps at 2.5V."
The articles also notes that no substitues should be used in place
of Q2, the BC550 transistor. "This type has been specified to give
high current gain together with a high collector-emitter voltage rating."
I have not tried this yet. As usual, build this device at your own
risk. Thanx to all those who replied to my first posting for info.
-Mike Grant