LPT port relay

[Test]

I am trying to use LPT port to control a small 5 V relay (Meisei ps-5). The relay
appears to have 1.75 ohm resistance.

I connected the data pin to the + pin of the relay and ground to the gnd pin. No
click. Voltage drops to about 2V. Then I tried with DC power supply at 4.5V
(1.5A). A nice click is audible from the relay.

(Bear with me: I am a newbie) I conclude that LPT is not suppliying enough
current (live and learn!). After googling for some time I am presented with a new
thing that should: a transistor - as in "transistor radio".

This helpful site has a cirquit that fits the bill:
http://www.epanorama.net/circuits/parallel_output.html#relaycontrol
here is it:

Vcc
|
+------+
| __|__
Relay /^\ Diode 1N4002
Coil /---\
| |
+------+
|
| /
4.7K B |/ C
parallel port >-\/\/\/\/---| NPN Transistor: BC547A or 2N2222A
data pi |\ E
| V
|
parallel port >--------------+
ground pin |
Ground

Above need an extra power supply (Vcc). Is any way to use power (perhaps from
multiple data pins) from the LPT port itself? One pin gives out 0-5V and a few
milliamps. The Meisei relay I have seems to need more.

 

[Test]

"Jon Slaughter" <Jon_Slaughter@Hotmail.com> wrote:

"Test" <test@.nil.invalid.com> wrote in message
news:mf3fd4pqpmoi66r9g453kouh1oa9dqrj4p@4ax.com...
I am trying to use LPT port to control a small 5 V relay (Meisei ps-5). The
relay
appears to have 1.75 ohm resistance.

I connected the data pin to the + pin of the relay and ground to the gnd
pin. No
click. Voltage drops to about 2V. Then I tried with DC power supply at
4.5V
(1.5A). A nice click is audible from the relay.

(Bear with me: I am a newbie) I conclude that LPT is not suppliying enough
current (live and learn!). After googling for some time I am presented
with a new
thing that should: a transistor - as in "transistor radio".

This helpful site has a cirquit that fits the bill:
http://www.epanorama.net/circuits/parallel_output.html#relaycontrol
here is it:

Vcc
|
+------+
| __|__
Relay /^\ Diode 1N4002
Coil /---\
| |
+------+
|
| /
4.7K B |/ C
parallel port >-\/\/\/\/---| NPN Transistor: BC547A or 2N2222A
data pi |\ E
| V
|
parallel port >--------------+
ground pin |
Ground

Above need an extra power supply (Vcc). Is any way to use power (perhaps
from
multiple data pins) from the LPT port itself? One pin gives out 0-5V and a
few
milliamps. The Meisei relay I have seems to need more.


You can't drive the relay directly because, IRC the LPT port can only source
about 20mA and sink about 5 or 10.

This is why they use the above circuit with the external power.

I don't recall how much power the LPT can source total but probably not
enough for your relay.

Sure you can tie all the output pins together and hope for the best. If on
goes low then you got a direct short. You can use diodes to prevent sinking
any current but that will lower your voltage.


LPT output ---- diode ----
LPT output ---- diode ----
LPT output ---- diode ---- All tied together here
LPT output ---- diode ----

The current will basically add up.

So, suppose each output will give a max of 20mA before protection kicks in
and you have 10 pins tied together, then thats a maximum of 200mA that
you'll get. That's assuming the whole chip can output that much.. if not I
imagine you have a real chance of burining up the port.

It's pretty easy to get a wall-wart power suppose and use that with a
transistor as the circuit shows... it's also a lot safer for your parallel
port!

I've seen parallel port relay boxes that don't use an external power. How do they
do it? Use low power relays?

 

[Jasen Betts]

On 2008-09-22, Test <test@> wrote:

I am trying to use LPT port to control a small 5 V relay (Meisei ps-5). The relay
appears to have 1.75 ohm resistance.

1.75 is too low. 175 or 1.75K possibly?

I connected the data pin to the + pin of the relay and ground to the gnd pin. No
click. Voltage drops to about 2V. Then I tried with DC power supply at 4.5V
(1.5A). A nice click is audible from the relay.

(Bear with me: I am a newbie) I conclude that LPT is not suppliying enough
current (live and learn!). After googling for some time I am presented with a new
thing that should: a transistor - as in "transistor radio".

excellent conclusion!

This helpful site has a cirquit that fits the bill:
http://www.epanorama.net/circuits/parallel_output.html#relaycontrol
here is it:

that's the way to di it!

Above need an extra power supply (Vcc). Is any way to use power (perhaps from
multiple data pins) from the LPT port itself? One pin gives out 0-5V and a few
milliamps. The Meisei relay I have seems to need more.

multiple data pins will probably not be enough.

you could try pulling a 5V supply from the keyboard (or mouse) socket
(pinout here: http://pinouts.ws/ps-2-keyboard-pinout.html )

Bye.
Jasen

 

[Jasen Betts]

On 2008-09-22, Test <test@> wrote:

I've seen parallel port relay boxes that don't use an external power. How do they
do it? Use low power relays?

3 possibilities:

latching relays (with extra circuitry)

latchiong relays don't needed energy to stay turned on - a cuircuit
with a capacitor could be used to store up energy to stwitch the relay.

stronger parallel ports (some can put out 12mA on each data line)

this relay would work if driven by 2 data lines:
http://www.mouser.com/Search/ProductDetail.aspx?qs=sEN%2fkO1EG6ZEMuSXIxhRxQ%3d%3d

I did not find any that were able to be powered from a single data line

solid-state relays.

pretty much all solid-state relays only need a few miliamps.
and so can be powered from the even the weakest parallel port.

Bye.
Jasen