Thursday, September 27, 2007

Tape speed control via pwm

As a follow on from my previous post, i applied the PWM from the Arduino to cassette player. And... success! Well, sort of. I can hear a frequency of around 400Hz when the PWM is too weak to drive the motor. This is annoying, and I am wondering if there is a way to minimise this sound.

This is the same setup as last night, the only difference being that pins 1 and 2 of the ULN 2803 were tied together as well as pins 18 and 17. In the breadboard picture, these new connections are the small, red jumpers placed in line with the relevant pins on the ULN2803 IC (the only IC in the photo).

The reason for this tying together of pins is that i don't have any data for the motor inside of the tape machine. Thus, it might such more juice out of a single set of darlington transistors than they can deliver. Thus, more power can potentially be achieved by chaining multiple sets of the array together. This is a technique taken from page six of the Picaxe Manual (Part Three).

Below you can see a little video (27 seconds) of the thing in action.

For the record, the music on the tape in the video is an untitled number by Drowning Goldfish.


MikMo said...

I tried to do the same, with a super cheap walkman. I have the same anoying ca. 400 Hz tone.

It's there as soon as the program starts to run on the Arduino board. I was listening with headphones, and even if the PWM pin on the Arduino board was not connected to the walkman (ground was connected) i could hear the tone faintly in the background.

I tried powering the Arduini board with an external ps. and disconnect the USB cable, no difference.


Sebastian Tomczak said...

I find that the best results are had by sending data to the motor from between 100 and 255. Any lower, and the noise becomes too unbearable. But with the motor i was using, the data value of 100 represents a stopped state anyways. Yep.

It would be different for different motors.


Anonymous said...

this is something that I have wanted to do for a while!! BTW your stuff is amazing, thanks for inspiration. Though, I wanted to do it more on the analog side, small tape loop scaled to x # of keys with envelopes etc.. Poor mans Mellotron.. haha!! Again you rock!!

Mattias Valenca said...

Maybe some sort of notchfilter circuit could solve the problem?