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:
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.