steve said:
Hey thanks. I will give it a try. Hate to toss out something Ive
already put together.
By the way Im not exactly sure how this works. If anyone is able to
give a quick overview of how its suppose to work.Im a little confused
(novice) on what the first transister does connected to the phone line.
thanks.
When the phone is on hook, the base of the first transistor
is a little over +1 volt, so the transistor is biased on.
That places ground on the emitter of the second transistor,
which is a PNP, so it is biased off. With the second transistor
off, the cap can't charge and the 3rd transistor is not turned
on. When the phone is lifted, the first transistor is off,
which removes the ground from the 2nd transistor emitter.
Current from the battery through the led and the 1meg resistor
charges the cap - when it rises to about .6V higher than the
base of transistor 2, the transistor is biased on, which in
turn biases the last transistor on. That provides a path
for ~ 40 mA current to light the LED. The cap discharges
into the base of the 3rd transistor, biasing the second
transistor off and the cycle repeats.
There are two high resistance paths that drain the battery
when the phone is on hook. The combination of the 1 meg and
2.2 meg resistors connect to the base of transistor 2 is
one path. The second is through the other 1 meg resistor
and the first transistor, which is biased on by the phone
being on hook. You need a different circuit if you want
to avoid draining the battery. This one will draw around
10 uA with the phone on hook - and earlier experiments
showed you can see the glow at only about 3 uA.
(I'm still scratching my head over that. You must have
good eyes!)
Do you have a DMM that has a uA scale? It would be a
good idea to measure the draw from the battery with the
phone on hook. If it is only ~ 10 uA, it's really not
worth going to a different circuit. A typical 9V battery
should last a long time if you don't use the phone.
Where this circuit really uses current is when the phone
is off hook. In rough numbers, a 1 minute phone call
burns up as much battery life as 33 *hours* of the phone
being on hook. You ought to be able to see the LED easily
at ~2 mA instead of 40 mA. That would reduce the off hook
battery drain by a factor of 20 - which is well worth
doing. To do that, change the 180 ohm resistor to a 3.3K
and see if it glows brightly enough for you with the phone
off hook.
Ed
