View Full Version : Multiple power supplies: newbie

03-02-2005, 11:25 PM
Hello smart people, I'm somewhat new to this crazy world of forum posting, so please bear with me. I've been assigned to construct a device that, when triggered by a push button, starts a series of lights, each with different DC voltages, that will stay on for a few seconds, and then turn off. Sounds easy, right? Well... I've read enough to know that I need to run each light off of a transistor- relay coupling. My question is: the pushbutton is running active high from VDD (5V)- is there enough power in the stamps 5V output to run the 4 relays I need to trigger from the VDD as well? I'm assuming that I would hook up these four relays in parallel, so that the voltage remains 5V at each transistor- relay point. The tricky thing is that each relay will fire in series- relay one goes on, stays on, then relay two goes on.. etc. Does this mean that the power consumed will be increasing in increments, slowly killing the stamp? I guess the other option for me is having a isolated power source for the relays- but I've already got like 5 or so isolated power source for the lights the relays will be running. It 's getting ridiculous with all these crazy power sources!! Please help. Me no smart.

03-03-2005, 12:10 AM
The BS2 can only supply about 50 mA TOTAL out its pins, and the relay's take way more than that, so you'll need a high-current switching device between the two.

Not a problem, this question is asked all the time, look in the forums for it.

03-03-2005, 12:15 AM

nick bernard
03-03-2005, 03:15 AM
the uln2803 is a dandy IC that can help with your relay driving needs. it even has counter emf diodes built right in. The pin designations are strikingly convienient. with this IC you can drive the relays at a more convient voltage.


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Post Edited (nick bernard) : 3/2/2005 8:41:41 PM GMT

03-03-2005, 10:12 PM
Thanks for the schematic! I'm going to order afew LN2803s to play around with. It's wonderful to know that there is such a great pool of helpful people to draw from when help is needed.·http://forums.parallax.com/images/smilies/smile.gif