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!
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!