Tape-recorder as rotary sensor

Forum 2005-2010 (read only) Hardware Interfacing
1

Recently i scrapped a numerical printer from an old printing calculator, similar to this one:

(sorry had to remove image, since this is my first post :(, but google for "printing calculator", and you'll get the idea )

I got the driver chip running (a real sweet transistor array, up to 50V, 500mA) and everything, it has a button switch that is bumped each revolution.

But there is also a recording head, similar to the ones found in tape recorders/players, pulsing 39 times per revolution (some small metal pieces on a rotating gear passes by). I need to read that to get the printer fully functional.

Now my question is how to do that, without damaging the Arduino? The output voltage of the head is very varying, depending on the rotation speed. So I'll need some kind of limiter, right?

2

A comparator amp is the typical solution, although a darlington transistor might be able to work OK.

Lefty

3

Wouldn't a zenner diode do it too or?
-but yeah tought about the op-amp too, i'll have a look at it. Thanks :slight_smile:

4

No a zener didoe won't cut it. The read head output is basically a low level analog (10s of millivolts maybe) pulse signal that needs amplification and squaring up to be compatable for a digital input 0 & 5vdc levels, so a op amp type comparator would be the best choice.

Lefty

5

Well my problem is kind of the other way around...

At my fathers advise i tried hooking it up to one of the analog pins, trough first a 10kOhm and then a 220Ohm resistor. And then sending the data to my laptop, while manually turning the gear...

I got readings of around 2,5 volts, and that was just at a slow turning speed, so my problem is that i don't want to burn off my Arduino, if the voltage should rise over 5 volts.

6

Reading of the pulses via analog input pins will not reliable give you the precise timing needed, you are bound to miss pulses.

7

Ok thanks I'll try it with an op-amp then :slight_smile: Do I have to look for a specific one, or will any op-amp do it?

And won't I still burn it off if the input voltage extends the supplied?

8

Any normal op amp can be configured to work as a comparator or you can use a comparator chip. If the comparator or op amp is powered with +5vdc then it's output can not go higher then that value, so that shouldn't be a problem. The input to the op amp shouldn't get a very high voltage from magnetic head, they are pretty low level devices.

Lefty

9

Yeah, well as mentioned before i got readings of 2,5volts, trough a 200Ohm resistor.

I don't own an oscilloscope, and my multimeter isn't fast enough, so i don't know the exact output voltage of the head; but it's passing by a lot faster than tape normally does; about 220 cm per sec, or 7,9 km/h. (gear is ca. 1,2 cm in diameter, and it's got ca 58 rps's)
So i do think it could exceed 5 volts, if i removed the resistor :s

10

I got it working with a LM324N-op-amp, thank you very much retrolefty :slight_smile: