2 Protection diodes needed in different directions across motor = short circuit?

Thread Starter


Joined Oct 30, 2006
Hi everyone - ok so the story so far...

I want to control (forward, reverse and off) 3 motors using touch tone sounds from my mobile phone. I've built the tt7 'touch tone decoder' kit from ramsey electronics to decode the touch tone sounds. It decodes every touch tone to a different output. The outputs from this circuit are '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.

I want to use the output from this touch tone decoder to drive some motors, so that when i push a certain key, say 2 on my phone a motor spins forward, and when i release it the motor stops, and when i push say 5, the same motor spins in reverse.

I've got the circuit working with a single motor going in one direction (yay!) - (when i push 2 it spins) - using a PNP transistor with the base wired to my decoder kit. But now i want to be able to make it go in reverse. 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 :D .

Ok any help with this problem greatly appreciated!