If I understand you correctly, you have done all the schematic at
http://rbanks.customer.netspace.net.au/pport.htm for except that there
is no relay. In that case it is normal that you are not getting any
signal on the collector of the transistor. Your circuit can be modelled
/^\ Diode 1N4002
parallel port >-\/\/\/\/ B |
data pi |E
parallel port >---------------+
ground pin |
That is, the transistor is acting just like a wire, and the relay isn't
As you can see, in this circuit you should be reading 0V on the
collector of the transistor, but the transistor is ready to conduct
current coming through your relay.
Quoted from Peter on Thu, Aug 14, 2003 at 12:05:12AM +1000.
> I am trying to build a very basic parallel port interface to control a relay. (I've copied the basic schematic, that I found elsewhere on the web, to: http://rbanks.customer.netspace.net.au/pport.htm for your reference.)
> The problem I'm having is simple: it just won't work! : )
> I've connected the 5v+ pin of my parallel port to the 4.7k resistor. (It doesn't matter which direction the resistor goes does it?) I've then connected the resistor to the base pin (B) of my transistor (an NPN 2N2222A). I've connected the ground pin of my parallel port to the emitter pin (E) of my transistor. This is as much as I've built for now as I'm not even getting a signal from my transistor! (The signal from my transistor is supposed to trigger the relay.)
> I then send 5v+ out my parallel port (or just under 5 volts, maybe 4.8 volts). I've measured it with my multimeter. Then I put the red cable of my multimeter on the collector pin (C) of my transistor and the black cable of my multimeter on the emitter pin of my transistor and I get nothing! However, when I put the red cable of my mulitmeter on the other side of the resistor (after resistance) I get between 2 and 3 volts (I think, that's from memory). Then if I put the red cable of my multimeter on before the resistor I get just under 5volts (probably about 4.8 volts, but my multimeter is one of those analogue ones with the needle so I can't be sure, it may be a little more or a little less).
> It seems everything works fine until the transistor. I would really appreciate any help... as you can probably see from my I-put-the-red-cable-there language, I'm very new when it comes to electronics, but I am a great java programmer : D
> Kind regards,
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