Guest 77710 Posted July 21, 2005 Report Share Posted July 21, 2005 I have another circuit that has an output of 5VDC (the input on the schematic) I want to use transistors to control LED's, one on when the other is off and vice versa will this work? (ignore the orientation of the LED's I could not remember correctly when I drew this) I would also like to hook up a small relay, either mechanical or solid state, so it closes when the first LED (one on the left) is on. can anyone tell me if this will work, or if i have the schematic compleatly wrong.Thanks Quote Link to comment Share on other sites More sharing options...
audioguru Posted July 21, 2005 Report Share Posted July 21, 2005 Hi Sevens and a ten,Welcome to our forum. ;DNice try, but you missed a few important things.The circuit has two inverting transistors for the 2nd LED so both LEDs light at the same time.Try again. ;D Quote Link to comment Share on other sites More sharing options...
Guest 77710 Posted July 21, 2005 Report Share Posted July 21, 2005 Thanks for the help. So as drawn the two LED's will light opposite of one another? Also what values would be good for the resistors in the upper right side? Quote Link to comment Share on other sites More sharing options...
audioguru Posted July 21, 2005 Report Share Posted July 21, 2005 So as drawn the two LED's will light opposite of one another?No, you have two inverting transistors driving the 2nd LED so both LEDs light at the same time. Get rid of the PNP transistor and drive the 2nd LED with just the NPN transistor for the LEDs to light opposite of one another.Also what values would be good for the resistors in the upper right side?You don't need the PNP transistor and its resistors so concentrate on selecting a resistor value for R2.If the collector current of the NPN transistor is limited to 20mA with a 150 ohm current-limiting resistor in series with the LED that will be at its collector, how much current gain will the NPN transistor have?Most little transistors have a current gain of at least 100, so how much base current will it need?Hint, R2 will have 5.0V minus the transistor's 0.7V base-emitter voltage across it.You should also add a resistor from the base of the NPN transistor to ground to turn it off. 100k will be fine. ;D Quote Link to comment Share on other sites More sharing options...
Guest 77710 Posted July 21, 2005 Report Share Posted July 21, 2005 Okay I think I have done it tell me what you think of this revision I am still a little unsure of what you mean by just using the one NPN. Also did I get R2 right? Thanks Quote Link to comment Share on other sites More sharing options...
audioguru Posted July 21, 2005 Report Share Posted July 21, 2005 Hi Sevens plus a ten,[move]Perfect! Perfect! Perfect! [/move][move] ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D [/move] Quote Link to comment Share on other sites More sharing options...
audioguru Posted July 21, 2005 Report Share Posted July 21, 2005 OOOps,Boy, did I ever get it wrong, a second NPN transistor is needed to turn-off the LED driver transistor so that the LEDs will light opposite of one another. Quote Link to comment Share on other sites More sharing options...
Guest 77710 Posted July 21, 2005 Report Share Posted July 21, 2005 Thanks for the hepl again. I tryed the schematic you gave me on the breadboard and it almost works, the LED's alternate but the LED in the upper right is very very dim, on the meter it is only getting 1.5V and the other is getting 2.9V is there any way to fix this? i tryed a smaller resistor on the anode of the led but nothing. I guess the second transistor's base is not getting enough current. ??? Quote Link to comment Share on other sites More sharing options...
Guest 77710 Posted July 21, 2005 Report Share Posted July 21, 2005 Well never mind, just out of curiosity i put the transistor in my little 4$ radioshack multi-component checker and sure enough the one transistor was bad. So I popped a new one in and it now works great. Thanks for all the help audioguru. Now I have just one more question ;D The output from the other circuit is really just a logic circuit, about the only thing it can drive is an LED I wanted to hook up a relay with the first LED (simple right ;D) well long story short i have 12V relays, to use them will i need to add another NPN with the collector going to 12V through the relay? Quote Link to comment Share on other sites More sharing options...
audioguru Posted July 21, 2005 Report Share Posted July 21, 2005 one transistor was bad.Bad transistors don't work very well, do they? will i need to add another NPN with the collector going to 12V through the relay?Yes, as long as the transistor is rated for more than the current of the relay's coil. The transistor will need its own base resistor.Also, the relay's coil will need a diode across it to arrest the very high voltage created by the coil when its magnetic field collapses when its coil's current is stopped. Maybe that's how your bad transistor got fried. ;D Quote Link to comment Share on other sites More sharing options...
Guest 77710 Posted July 21, 2005 Report Share Posted July 21, 2005 awesome audioguru I really appreciate all the help today :). When I get the whole thing put together I will have to post it, right now it is in quite a few piles of parts lying around my room. Quote Link to comment Share on other sites More sharing options...
login
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.