Need help with transistor switch (on at open switch)

[baxterdmutt]

Hi,
This is probably a simple question and been answered before but I can't seem to find the solution or figure out how to do it.
I need to use a transistor to switch on a low amperage circuit (or a small relay.) That's not a problem except I need the current to flow when my external switch is off, not on. To clarify I have a motion sensor that I am using to have a circuit turn on and do some stuff. The motion sensor is normally closed (N/C) and becomes OPEN when something passed in front of it. That's when I need the transistor to activate a relay, when the switch is open. I don't have a lot of electronic know how. I manage most times but this one has me stumped. If anyone has an answer then please help. If you can scribble a quick schematic so that I know what you mean, I would appreciate that. Thank you very much for your help!

 

[MaxHeadRoom78]

Simple with a 2n7000 Fetlington mosfet, connect a 10k resistor from Gate to +ve, from the gate pin wire your NC (corrected) switch to COM
Connect mosfet Source pin also to COM, connect drain pin to relay, up to 60vdc.
Connect reverse biased 1n4007 across relay.

Max.

 

[AnalogKid]

What voltage and current are the transistor going to switch? The solution is two transistor's in series. The first one's collector goes to a pullup resistor instead of the relay coil. It also connects to the base resistor of the second transistor.

ak

 

[audioguru]

Connect the relay coil from the collector of the NPN transistor to the positive supply. Add a diode parallel with the coil to quench the high voltage spike.
A resistor from the base of the transistor to the positive supply turns on the transistor and relay when the motion detector output is not conducting. Connect the output of the motion detector to the base of the transistor and the emitter of the transistor.
When the output of the motion detector conducts when it is activated by someone then the transistor and relay turn off.

 

[baxterdmutt]

Simple with a 2n7000 Fetlington mosfet, connect a 10k resistor from Gate to +ve, from the gate pin wire your NO switch to COM
Connect mosfet Source pin also to COM, connect drain pin to relay, up to 60vdc.
Connect reverse biased 1n4007 across relay.
Max.

My switch is NC.

 

[baxterdmutt]

Connect the relay coil from the collector of the NPN transistor to the positive supply. Add a diode parallel with the coil to quench the high voltage spike.
A resistor from the base of the transistor to the positive supply turns on the transistor and relay when the motion detector output is not conducting. Connect the output of the motion detector to the base of the transistor and the emitter of the transistor.
When the output of the motion detector conducts when it is activated by someone then the transistor and relay turn off.

I must have explained wrong, because my sensor conducts when it is off. That's the problem that I was having.

 

[MaxHeadRoom78]

I meant NC, the switch will hold off the 2n7000/relay until activated.
Max.

 

[baxterdmutt]

I think I just figured out a solution. The solution would be to use an NPN transistor as a NOT gate and then go from there.

 

[baxterdmutt]

I meant NC, the switch will hold off the 2n7000/relay until activated.
Max.

Thanks, That would work!

 

[MaxHeadRoom78]

I think I just figured out a solution. The solution would be to use an NPN transistor as a NOT gate and then go from there.

What do you think post #2 is?
Max.

 

[baxterdmutt]

What do you think post #2 is?
Max.

Sure, but if I can do it with an NPN instead of a Mosfet, I figured, why not (because I have a ton of them)?
I also posted that before I read the responses. I should have refreshed my browser first

 

[MaxHeadRoom78]

Is the sensor contact-closure or solid state?
Max.

 

[audioguru]

I do not know why some people talk about opening and closing a door or window instead of talking about a transistor or circuit conducting or not conducting.
I think they are the same people who talk about a NOT gate instead of a logic inverter.

 

[baxterdmutt]

Is the sensor contact-closure or solid state?
Max.

Contact closure.

 

[baxterdmutt]

I do not know why some people talk about opening and closing a door or window instead of talking about a transistor or circuit conducting or not conducting.
I think they are the same people who talk about a NOT gate instead of a logic inverter.

I am not sure what you meant by that, but I'm pretty sure 'NOT gate' is a valid term as is an Inverter and I didn't say anything about an open or closed transistor.

 

[MaxHeadRoom78]

I do not know why some people talk about opening and closing a door or window instead of talking about a transistor or circuit conducting or not conducting.
I think they are the same people who talk about a NOT gate instead of a logic inverter.

Probabally those of us that work with Boolean logic!
Max.

 

[baxterdmutt]

Thanks to all who helped me with this. I should be able to get this simple project done now.

 

[Tony Stewart]

Keep in mind all open collector or open drain drivers are inverting and use reverse clamp diode across relay coil, if being used.

 

[baxterdmutt]

Keep in mind all open collector or open drain drivers are inverting and use reverse clamp diode across relay coil, if being used.

Yes. Thanks

 

[Les Jones]

Yet another alternative would be to drive the relay directly from the sensor but use a normally closed contact on the relay to drive the load. This has the disadvantage the the relay will be energised for most of the time thus consuming some power. Here is an explanation for baterdmutt's benefit. The normally closed contact on the relay will allow current to pass when there is no current passing through the relay coil. As the contact on your sensor is closed when it is not activated the relay coil will have current passing through it. This mean that the normally closed contacts on the relay will be in the open state so no current will pass to the load. When the sensor is activated its contacts open causing the relay coil to be de energised which means that the relay's normally closed contact will now be closed. The relay is behaving as the NOT gate you wanted to use.

Les.