Would T-junction help low current situation

T

Test

Guest
I have a doorbell that uses 18V. I'd like to put a relay between so that I can
record when doorbell rang (the relay is wired to a computer).

Unfortunately the relay seem to introduce too much resistance as the doorbell
won't ring:


(not working)
+-------+
| |
| +-------------->| Relay coil |------> gnd
Bell | |
| |
| |
| +--------- --------------------< Vcc
| | /
+-------+ / Switch
/

Would this help?
+-------+
| |
| +------+------>| Relay coil |------> gnd
Bell | | |
| | +---------------------------> gnd |
|
| +--------- --------------------< Vcc
| | /
+-------+ / Switch
/
 
"Test" <test@.nil.invalid.com> wrote in message
news:r9mhe4ttjio0371epdbmhqe4ohs7r1cosu@4ax.com...
I have a doorbell that uses 18V. I'd like to put a relay between so that I
can
record when doorbell rang (the relay is wired to a computer).

Unfortunately the relay seem to introduce too much resistance as the
doorbell
won't ring:


(not working)
+-------+
| |
| +-------------->| Relay coil |------> gnd
Bell | |
| |
| |
| +--------- --------------------< Vcc
| | /
+-------+ / Switch
/

Would this help?
+-------+
| |
| +------+------>| Relay coil |------> gnd
Bell | | |
| | +---------------------------> gnd
|
|
| +--------- --------------------< Vcc
| | /
+-------+ / Switch
/
Click to expand...
Your second approach won't work either (you may have just drawn it
incorrectly).

You need to only connect the relay coil across (in parallel with) the bell.
The relay will get power when the bell does.


BELL
|----|
| |----------------------GND
| |----RELAY------|
|----| |
|--------SWITCH-----VCC




Bob
--
== All google group posts are automatically deleted due to spam ==
 
Test wrote:
I have a doorbell that uses 18V. I'd like to put a relay between so that I can
record when doorbell rang (the relay is wired to a computer).

Unfortunately the relay seem to introduce too much resistance as the doorbell
won't ring:


(not working)
+-------+
| |
| +-------------->| Relay coil |------> gnd
Bell | |
| |
| |
| +--------- --------------------< Vcc
| | /
+-------+ / Switch
/

Would this help?
+-------+
| |
| +------+------>| Relay coil |------> gnd
Bell | | |
| | +---------------------------> gnd
| |
| +--------- --------------------< Vcc
| | /
+-------+ / Switch
/
Click to expand...
How does a relay coil with ground on each end of it ever get
energized? If the relay is rated for something close to
18VAC, connect it across (in parallel with) the bell. If it
is an AC relay rated for a lower voltage, you may need to
add a series resistor to burn up the extra voltage. If it
is a DC relay, you might use a bridge rectifier to convert
AC to DC.

--
Regards,

John Popelish
 
"BobW" <nimby_GIMME_SOME_SPAM@roadrunner.com> wrote:

"Test" <test@.nil.invalid.com> wrote in message
news:r9mhe4ttjio0371epdbmhqe4ohs7r1cosu@4ax.com...
I have a doorbell that uses 18V. I'd like to put a relay between so that I
can
record when doorbell rang (the relay is wired to a computer).

Unfortunately the relay seem to introduce too much resistance as the
doorbell
won't ring:


(not working)
+-------+
| |
| +-------------->| Relay coil |------> gnd
Bell | |
| |
| |
| +--------- --------------------< Vcc
| | /
+-------+ / Switch
/

Would this help?
+-------+
| |
| +------+------>| Relay coil |------> gnd
Bell | | |
| | +---------------------------> gnd
|
|
| +--------- --------------------< Vcc
| | /
+-------+ / Switch
/

Your second approach won't work either (you may have just drawn it
incorrectly).

You need to only connect the relay coil across (in parallel with) the bell.
The relay will get power when the bell does.


BELL
|----|
| |----------------------GND
| |----RELAY------|
|----| |
|--------SWITCH-----VCC




Bob
Click to expand...

The problem is that I can't get to the Vcc wire between the switch and bell. It
is inside the wall.



+-------+
| | (wall ends)
| +-------------->| Relay coil |------> gnd
Bell | |
| |
| | (wall starts)
| +--------- --------------------< Vcc
| | /
+-------+ / Switch
/
 
"Test" <test@.nil.invalid.com> wrote in message
news:9lrhe4loaf61vo19h0kgq48iosa8vujkra@4ax.com...
"BobW" <nimby_GIMME_SOME_SPAM@roadrunner.com> wrote:


"Test" <test@.nil.invalid.com> wrote in message
news:r9mhe4ttjio0371epdbmhqe4ohs7r1cosu@4ax.com...
I have a doorbell that uses 18V. I'd like to put a relay between so that
I
can
record when doorbell rang (the relay is wired to a computer).

Unfortunately the relay seem to introduce too much resistance as the
doorbell
won't ring:


(not working)
+-------+
| |
| +-------------->| Relay coil |------> gnd
Bell | |
| |
| |
| +--------- --------------------< Vcc
| | /
+-------+ / Switch
/

Would this help?
+-------+
| |
| +------+------>| Relay coil |------> gnd
Bell | | |
| | +---------------------------> gnd
|
|
| +--------- --------------------< Vcc
| | /
+-------+ / Switch
/

Your second approach won't work either (you may have just drawn it
incorrectly).

You need to only connect the relay coil across (in parallel with) the
bell.
The relay will get power when the bell does.


BELL
|----|
| |----------------------GND
| |----RELAY------|
|----| |
|--------SWITCH-----VCC




Bob


The problem is that I can't get to the Vcc wire between the switch and
bell. It
is inside the wall.



+-------+
| | (wall ends)
| +-------------->| Relay coil |------> gnd
Bell | |
| |
| | (wall starts)
| +--------- --------------------< Vcc
| | /
+-------+ / Switch
/
Click to expand...

Well, then you'll have to go back to your original idea and either use a
relay with a very low coil resistance or an opto-coupler. This way, you'll
only lose a couple of volts across the bell.

One other idea comes to mind -- get a Sawz'all.

Bob
--
== All google group posts are automatically deleted due to spam ==
 
"Test" <test@.nil.invalid.com> wrote in message
news:9lrhe4loaf61vo19h0kgq48iosa8vujkra@4ax.com...
"BobW" <nimby_GIMME_SOME_SPAM@roadrunner.com> wrote:


"Test" <test@.nil.invalid.com> wrote in message
news:r9mhe4ttjio0371epdbmhqe4ohs7r1cosu@4ax.com...
I have a doorbell that uses 18V. I'd like to put a relay between so that
I
can
record when doorbell rang (the relay is wired to a computer).

Unfortunately the relay seem to introduce too much resistance as the
doorbell
won't ring:


(not working)
+-------+
| |
| +-------------->| Relay coil |------> gnd
Bell | |
| |
| |
| +--------- --------------------< Vcc
| | /
+-------+ / Switch
/

Would this help?
+-------+
| |
| +------+------>| Relay coil |------> gnd
Bell | | |
| | +---------------------------> gnd
|
|
| +--------- --------------------< Vcc
| | /
+-------+ / Switch
/

Your second approach won't work either (you may have just drawn it
incorrectly).

You need to only connect the relay coil across (in parallel with) the
bell.
The relay will get power when the bell does.


BELL
|----|
| |----------------------GND
| |----RELAY------|
|----| |
|--------SWITCH-----VCC




Bob


The problem is that I can't get to the Vcc wire between the switch and
bell. It
is inside the wall.



+-------+
| | (wall ends)
| +-------------->| Relay coil |------> gnd
Bell | |
| |
| | (wall starts)
| +--------- -----------B[PNP]E---< Vcc
| | / C
+-------+ / Switch |
/ +--[Relay Coil]---<gnd
Click to expand...



Try sensing the bell current with a transistor. See above:

You might need a resistor from B-E to shunt some of the bell current, but
if you use something like an MJE170 or other medium size power transistor
it should be able to handle about 1 amp. Don't forget to add a diode across
the relay coil and also perhaps one across B-E. If you have an AC bell then
a similar method will work, but it would need some extra components. You
might also be able to use a SSR or optoisolator, with a bridge rectifier if
AC, and an appropriate current shunt resistor.

Paul
 
On Sun, 05 Oct 2008 19:48:09 +0300, Test <test@.nil.invalid.com> wrote:


+-------+
| | (wall ends)
| +-------------->| Relay coil |------> gnd
Bell | |
| |
| | (wall starts)
| +--------- --------------------< Vcc
| | /
+-------+ / Switch
/
Click to expand...
---
First things first.

Is the bell AC or DC, how much current does it draw, and what wiring can
you access from your side of the wall?


JF
 
On Oct 5, 8:21 am, Test <test@.nil.invalid.com> wrote:
I have a doorbell that uses 18V. I'd like to put a relay between so that I can
record when doorbell rang (the relay is wired to a computer).
Click to expand...
Or, you can wire the doorbell button straight to the relay coil,
so that (when the button is pressed) it activates the relay
and one pair of contacts rings the bell, while another pair
of contacts informs your computer.

If you ground the 'common' terminal of an SPDT relay, the
one branch can have a pullup resistor and your computer,
the other can complete the bell circuit. With multipole relays,
it's even easier.

This way, you're sensing the doorbell switch position, which
is a simple task, rather than the high current of an operating
doorbell.
 
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