Printer Port relay control

[Sieg]

Hi,
Who can help me figure out whats wrong with my relay controller?
I got this schematic somewhere off this forum and built it exactly the same. It doesn't work however, the relay wont switch

I took some measurements of the voltages in key places. All relative to ground.



Can anyone tell me whats wrong?

[NC Cams]

Printer ports do a good job of SINKING current - they don't always SOURCE eurrent well. Thus, you may not be driving the fet hard enough to turn it on.

Try "pulling UP" the fet gate to see if it is "turning on" fully. IF that works, you may need a better bufffer between your LPT and your fet driver.

[Al_The_Man]

The 2N7000 'Fetlington' (darlington/FET) was specifically designed to interface with TTL & CMOS technology, the turn on threshold is 3v, so you should not have any problem what so ever with turning it on with a printer port.
The fact that a FET is a voltage operating device, does not require any current from the source in this application.
IOW as soon as you have 3v+ on the gate WRT the source it should turn on.
What I suspect is you do not have the 5vdc and 12vdc commons connected.
Al.

[pminmo]

Your measurements indicate something amiss between you port gnd and printer port output pin. That voltage should be no higher than 5V. Make sure you use one of pins 18 thru 25 for the gnd side.

[Sieg]

I think I had the wrong diode. I went to the local electronics store and showed the guy the schematic I was building. Figuring he knew his stuff, I asked him to get me the right parts needed.

The diode he gave me was a 1N5408. After looking at some other schematics I swapped it with an 1N4002. Now the relay switched on, but wouldn't switch off anymore. Next I swapped the transistor for a BC547B and presto, it worked!

I still have 1 question tho.
I use the 12v line from a pc molex. (yellow wire). Do I need to connect the source of the transistor to the black wire of that molex AND to the printerport's ground, or do I only connect it to the printerport's ground? It works either way, but I'm not sure which is the best and safest way?

[Al_The_Man]

As long as you had the diode reverse biased, it did not even come into play until you switch off the relay, the diode is not a super critical component as to value, I believe as Pminmo said you had some issue with the grounding commons in order to get the voltages you did.
I have used the 2N7000 for about 20yrs now, & so far have not seen any problems.
The port common and the 12v common are the same, but I would use both to the buffer as this ensures the 12v relay common current goes directly back to the PS instead of through the port/PC board common.
There is no reason it should not have worked with the 2n7000 if wired correctly.
Al.

[Sieg]

I guess we'll never know what was wrong, but I'm sure you're right.

It works, so I'm happy.

Thanks guys!