what transistor to use?

[taylorclark]

For my most recent project, i've been working on a remote control airsoft gun. I've already got the gun all ready, and working, i just need a way to control it using a small, cheap rc car. My plan is to connect the wire that would normally go to the motor, to a transistor. But, i don't know what transistor to use, or exactly how to use it. I need it to control 6 volts, from 4 AAA batteries, and i'd like to be able to switch it on using 2-3 volts. the less, the better! If someone could point me to a transistor(preferably from radioshack's website), that would be fantastic, and tell me how to set it up. Thanks!

[802Chives]

hmm...... this application may be a little more involved then you would initially suspect.  Here are my thoughts: the RC car controller uses analog type controlls, which means the further you push the controller knob, the faster the car will go. This is usually achieved using Pulse Width Modulation(PWM) which is typically a signal that operates at a constant frequency with varying pulse width depending on how far the lever on the controller is pushed. for example when the controller is pushed all the way the signal will be on 100% of the time giving 100% of the motor speed. If the controller is only pushed %50 of the way the signal will only be on %50 of the time thus giving you %50 of the motor speed. 

As far as using a transitor to controll a trigger signal may or may not work.  One problem I can forsee is if the RC controller is pushed in the reverse direction, it may damage your circuit as that would reverse the polarity of your signals on the transistor.(as it would to get reverse direction out of ur motor)  Without being able to see the circuit you are intending to use I couldnt begin to guess how it will act if you set it up the way you want to.

To this I suggest a mechanical solution which may look ugly but will work rather well I think.  Mount your RC motor with a cam that presses the trigger as it spins. It will probably require the gearbox from the car to slow the motor down to the correct speed.  When implemented correctly it would fire at different rates depending on how far you press on the RC controller.

If you still want to play with transistors tho, the 2n3904 is the most common type, and try googling "transistor as a switch" for help with using them.  Tinkering on a circuit like this may result in better results then what I would come up with by evaluating the design theoretically.  Remember the Base is what controls the transistor, the Emmitter usually goes the more negative side of the switch and the Collector to the more positive side. Try a 1.5k resistor in series with the base that connects to the motor signal. the Emmitter and Collector will be connected accross the existing trigger switch which will cause a virtual connection when a signal is applied to the base.  Be warned tho, "tinkering" can result in smoking parts and zapped fingers, so make sure you can afford to lose the stuff your working on if you go this route.