NerdKits - electronics education for a digital generation

You are not logged in. [log in]

NEW: Learning electronics? Ask your questions on the new Electronics Questions & Answers site hosted by CircuitLab.

Basic Electronics » Switching Ground with a transistor

June 24, 2011
by dgikuljot
dgikuljot's Avatar

Hey guys, So recently i have been trying to figure out how to use a transsitor to switch on and off something that requires a ground. While doing research on whether i need to use a npn or pnp transistor, there were so many different things out there that i am really confused now. Can someone please explain whether i need to use a npn transistor or pnp transistor to switch ground.

Thanks, Kuljot

June 25, 2011
by dgikuljot
dgikuljot's Avatar

Any transistor pro's on the forum

June 25, 2011
by Noter
Noter's Avatar

You would use a NPN to switch to ground. Here's a link I find helpful whenever I'm unsure of which type of transistor to use -

http://www.kpsec.freeuk.com/trancirc.htm

June 25, 2011
by dgikuljot
dgikuljot's Avatar

Thanks Noter, HAha i have a bunch of pnp transistor but no npn. I wonder if this could be done with the mosfets included in the nerdkit

June 25, 2011
by Noter
Noter's Avatar

Yes, I use the 2N7000 mosfets most of the time as a on/off switch. One advantage is you don't need a current limiting resistor in series to the base.

What are you going to switch?

June 25, 2011
by dgikuljot
dgikuljot's Avatar

So if you remember a little while ago i decided i wanted to make an Rf remote control system to boot the computer. Well the switch pin on the motherboard boots the computer when it recives ground. Although i could just set a mcu pin low and do that, i just wanted to learn how to use a transistor to output ground. So if i wanted to output GND using the mosfet, how would i do it, would i just give ground to the collector and a postive to the base?

June 25, 2011
by dgikuljot
dgikuljot's Avatar

Oops i meant would i ground the source. And pulse the drain with +5 volts when i want to output a ground? Thanks

June 25, 2011
by Noter
Noter's Avatar

The terminology for a MOSFET is a little different. You would connect the source to GND, the drain to the switch pin on the motherboard, and the gate to your control signal which would be +5v to turn on the MOSFET thus short the switch pin to ground and turn on your computer.

June 25, 2011
by mongo
mongo's Avatar

The dirt simple way is an NPN transistor. Emitter to ground, collector through the load to the positive supply. A 10K resistor at the base to limit current and isolate the circuit from the load.

The advantage here is that you can control a 12 or 24 volt load with a 5V positive signal to the base. It does not necessarily have to be the same supply as the rest of the circuit, as long as they share the negative supply (ground).

June 26, 2011
by dgikuljot
dgikuljot's Avatar

Thanks for the replies Noter and Mongo. But Noter i tryed your mosfet method of source to ground, gate to 5v and drain to ground output. But the drain is constantly connected to ground no matter if the gate is receiving 5v or not. In fact the drain only stops outputting ground when gate is grounded. I used a led with the postive conencted directly and the drain connected to the leds negative lead.

June 26, 2011
by Noter
Noter's Avatar

I think that sounds about right. The gate on the MOSFET can't be left to float, it has to be either +5v or GND all the time.

June 28, 2011
by dgikuljot
dgikuljot's Avatar

Ok thanks everyone, the npn transistor works perfecly. I was able to scavage 2 of them from a old hvac thermostat.

Post a Reply

Please log in to post a reply.

Did you know that there are Power MOSFETs for switching big loads on and off? Learn more...