• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Driving problems.

Not open for further replies.


New Member
I am getting very frustrated with this little problem I have and I just can't seem to work it out. As on one of my previous post's I had the problem in shortning the time of a pulse I got from a digital reciever. I seem to have cleared that up with the 74121 IC,however now I have got the problem,I am using a transistor (2N3904) to drive the switching relay. The "shortned" pulse from the IC I fed into this transistor (base) I just cant seem to bais this transistor on with a voltage divider none of my calculations seem to work. I would also like to know where is the best place to connect the relay VCC to colector or Emitor to ground. Some specs VCC~12V;pulse~2V for 0.33 seconds. I would really apreciate it if I could be put out of my musery and if I could be pointed to a website where I can brush up on my transistor theory or calculations. :(


Well-Known Member
You didn't say how much current the relay coil draws, so I'm going to assume it's 20ma or less, although this will probably work for coil current up to 50ma or more.
Connect the emitter to ground, and the relay coil from +12v to the collector. Connect a 1N4001 across the coil, cathode to +12v.
Connect a 1k resistor between the base and the Q output of your 74121. Connect another 1k resistor from the Q output of your 74121 to the +5v supply that powers the 74121.
The reason for the last resistor (the pullup) is that TTL has very poor current sourcing capability, but very good sinking capability. This resistor supplies most of the base drive when the Q output is high, and the output can sink the 5ma through it when it is low.
I don't know of a tutorial page. Have you done a Google search for "transistor tutorial"?
Not open for further replies.

Latest threads

EE World Online Articles