http://img297.imageshack.us/img297/5800/epiclolcircuitqs5.jpg

Basically I want the light to turn on when a signal is sent along d0. I am using a SPST reed relay that is 5vdc, coil resistance=250 ohms, contact rating .5A, nominal current 20ma. Does it matter which diode i use to protect my printer port from high voltage spikes (im using a random orange/black one i cant read the writing)? This is my first circuit with a relay, I'm wondering if it would be safe to plug this in yet. I built the circuit exactly how it is drawn.

 

http://img297.imageshack.us/img297/5800/epiclolcircuitqs5.jpg

Basically I want the light to turn on when a signal is sent along d0. I am using a SPST reed relay that is 5vdc, coil resistance=250 ohms, contact rating .5A, nominal current 20ma. Does it matter which diode i use to protect my printer port from high voltage spikes (im using a random orange/black one i cant read the writing)? This is my first circuit with a relay, I'm wondering if it would be safe to plug this in yet. I built the circuit exactly how it is drawn.

It would be safer to use an optocoupler to drive the relay. But if you want to use the diode use a IN4007 diode and put two of them in parallel in case one fails.

 

Is your printer port really able to source that much current?

 

It only needs 20ma right?

 

Hmm - that diode sounds like a Zener. Zeners won't work well for a reverse EMF protection diode.

Try connecting it in series with a 1K to 10K resistor across 12VDC. If it's a Zener, you'll measure a steady low voltage (around 0.6v-0.7v) one way, and another higher voltage the other way.

A standard diode will drop perhaps 0.6V-0.7V one way, and the full 12V the other way.

If you're desperate for some standard rectifier diodes (1A 1N400x series) Radio Shack sells an assortment of them for a buck or so.

 

It only needs 20ma right?

"Only" is a dangerous word in this context. Unless things have changed in the last ten years, your printer port cannot source that whopping huge 20ma of current. Sink, perhaps, but not source. It's a communications interface, not a power supply.

 

I'm trying to build this http://discolitez.com/pmwiki/pmwiki.php?n=Hardware.SSR. Where should I shop for parts if I am not desperate and can wait for shipping? (5 dollars for a relay at radio shack makes me want to start stealing things from their store for ripping people off).

 

Indeed, the executives of RatShack have earned the ire of many a hobbyist.

Try these:

http://www.goldmine-elec.com/

http://www.allelectronics.com/

http://www.cascadesurplus.com/

http://www.mpja.com/

http://www.skycraftsurplus.com/index.asp

(shoot each of the above an eMail if they don't list your need - the often have stuff on hand not listed online)

http://www.jameco.com/

http://dkc1.digikey.com/us/digihome.html

and of course http://www.ebay.com/