I haven't had a chance to read everyone else's comments, but my 
recommendation would be to replace the NPN with a 2n7000.

-forrest

James Newton wrote:
> Ok, first, let me just remind everyone that I'm almost totally clueless when
> it comes to analog electronics. Years ago, I took classes in the Navy on
> transistors and I sort of understood it... back then...
> 
> I'm trying to drive a small 5 volt relay from a pin on a PC parallel port. 
> 
> The circuit I'm trying is right out of the PICList archive / FAQ at:
> http://www.piclist.com/techref/io/relays.htm more specifically at
> http://www.piclist.com/techref/io/relay9v.gif but it is for a PIC, not the
> PC parallel port, and certainly not for nStrobe, which is one of the weaker
> lines on the port. This is probably the cause of my troubles, but I don't
> know how to fix it. The attached is the actual circuit.
> 
> RELAY: OMRON G5LE-1 DC5  According to the datasheet:
> https://www.jameco.com/Jameco/Products/ProdDS/187151.pdf it has a coil
> resistance of 63 Ohms. I have a 1N4148 connected backward over the coil to
> prevent kick back.
> 
> PC: Parallel Port pin 1: nStrobe is the line that I need to use to drive
> this relay. It is supposed to be driven, in the parallel port hardware, by
> the equivalent of a 7405 inverting open collector buffer, pulled to +5 volts
> via a 4.7K resistor.
> http://www.piclist.com/techref/io/parallel/signals.htm 
> 
> I have a 2N3904 transistor with the emitter connected to ground, the base to
> the nStrobe signal from the PC parallel port and the collector is connected
> to the relay coil. The other end of the coil is connected to +5VDC.
> 
> When the nStrobe line is not connected to the circuit, using a program on
> the PC to toggle the pin, I can get 0.009 - 5.07 VDC on the pin as read by
> my multi-meter.
> 
> When the circuit is connected, and powered up, I get 0.010 - 0.657 VDC and
> the relay does not close.
> 
> I attached a potentiometer between the base an +5 and turned it until the
> relay just activated, then backed it off and tried toggling the nStrobe
> line. At that point I was able to control the relay from the PC port, but
> removing the pot and measuring its value showed that it was 203Ohms. Not K
> Ohms; Ohms. That ain't going to cut it.
> 
> The parallel port appears undamaged. The relay and transistor do switch and
> the power supply seems to work ok, but 200 Ohms cannot possibly be ok. I
> don't understand how that failed to destroy the port... My old PC must have
> some sort of current limiting built in.
> 
> I had previously been advised (thank you friend) that I might have better
> luck in a different configuration using a BC337-40, but that was a different
> circuit entirely. The point of this new circuit is to avoid the extra cost
> of the more dear transistor. If I have to use it, so be it, but since I'm
> totally clueless, I figured I should ask for help before trying anything
> more. 
> 
> HELP?
> 
> James Newton: PICList webmaster/Admin
> mailto:jamesnewton@piclist.com  1-619-652-0593 phone
> http://www.piclist.com/member/JMN-EFP-786
> PIC/PICList FAQ: http://www.piclist.com
> 
> 
> 
> 
> ------------------------------------------------------------------------
> 

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