Output from chip to transitor to switch motor. Please help!

Discussion in 'The Projects Forum' started by thomast, Oct 30, 2006.

  1. thomast

    Thread Starter New Member

    Oct 30, 2006
    5
    0
    Hi - i hope someone can help as i have to get this done by thursday!

    Here goes: I want to control 3 motors using touch tones sounds from my mobile phone. I've built the tt7 'touch tone decoder' kit from ramsey electronics to decode the touch tone sounds. The output from this circuit is 'active low' - when it detects the tone for say the 3 key, the output on pin 3 goes from +3.5V to 0V. When you release the 3 key and stop playing the touch tone the voltage from the 3rd output pin goes back up to 3.5V. the maximum current output from the chip is 20mA.

    I want to use the output from this touch tone decoder to drive some motors, so that when i push a certain key on my phone a motor starts and when i release it the motor stops. I think i need to use a PNP transistor for this - when the output from the tt7 circuit is high (no phone key pressed), no current flows from emmitter to collector, but when the voltage from the tt7 drops to 0V the larger current flows from emmitter to collector. I think this is right but i'm not sure!

    My problem is that i don't know which transister to use for this purpose. I read that you just need to make sure that the minimum current gain and the minimum current for the load (my motors) are high enough. The motors I want to use are fairly standard small motors, stall current of about 600mA, operating current of about 200mA i think.

    I've got some PNP transistors, but they're not behaving as expected. One lot (code 2N2907 from www.maplin.co.uk) has an Ic of -600mA and an Hfe(min) of 100. My other lot (code BC640 from www.maplin.co.uk) have an Ic of -1000 and an Hfe(min) of 40. In a testing circuit with a 9v battery and resistors, the first set seem to be behaving like NPN transitors (when the base current is high the LED lamp is bright, when i remove the base current it's dim), and the second set don't seem to work at all (the lamp is bright no matter if the base current is on or not).

    Could anyone tell me where i'm going wrong? Should i be using relays? It would be great if someone could suggest a transistor that i could buy that would do this job.

    Ok sorry for long post, any help appreciated as i've got to make this work by thursday!
     
  2. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Even small relays require more current than your chip will source, so a transistor is the right call.

    Is it possible that you were sold a set of 2N2906 instead of 2N2907? They would've been right next to each other in the warehouse. Can you make out the lable on the device?
     
  3. thomast

    Thread Starter New Member

    Oct 30, 2006
    5
    0
    I've just checked and it is a 2N2097. I think i'm a bit confused - should a pnp transistor allow current to flow from emmitter to collector if there's no current through the base? Then does that mean the base could be disconnected entirely and current would still flow from emmitter to collector?
     
  4. n9352527

    AAC Fanatic!

    Oct 14, 2005
    1,198
    4
    Could you post your test circuit please?
     
  5. thomast

    Thread Starter New Member

    Oct 30, 2006
    5
    0
    Hi everyone - many thanks for your help but i've worked it out! Not entirely sure what i had wrong before but i started from scratch and i've got the whole thing working. :) yay!

    I have a new problem however - i want to make it so the direction the motor spins in can be reversed; so you press the 2 key and you go forward and you press the 8 key and you backwards. So the tt7 touch tone decoder kit i have has outputs for each touch tone on the phone. So what i was going to do was just hook up 2 transistors to each motor, but with the bases wired to different outputs on my touch tone decoder, and the collectors of the transistors wired to opposite terminals on the motor.

    However I've read that when you use motors with transistors, you have to use a protection diode wired 'in reverse' across the motor (which i have done in my test circuit) to divert current generated when the load is switched off (??). The problem i think i have is that if i wire 2 protection diodes both 'in reverse' across my oppositely connected motor, then i'll have a diode wired across my motor in each direction - short circuiting it(?). So if anyone could make a suggestion as to how to get round this problem, (if it is actually a problem?) then i think i'll be home and dry, and i can fly my helium balloon with my telephone.
     
  6. n9352527

    AAC Fanatic!

    Oct 14, 2005
    1,198
    4
    Have a look at transistors bridge.
     
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