LPT port relay

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:

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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.

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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!

Reply to
Jon Slaughter

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

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Theres tones of info on using the lpt port for various applications. I'm sure if you search google you'll find the exact info your looking for.

Reply to
Jon Slaughter


1.75 is too low. 175 or 1.75K possibly?



excellent conclusion!

that's the way to di it!

multiple data pins will probably not be enough.

you could try pulling a 5V supply from the keyboard (or mouse) socket (pinout here:

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Bye. Jasen

Reply to
Jasen Betts


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:

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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

Reply to
Jasen Betts

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