Simple DIY tachometer made from old serial ball mouse

[[email protected]]

I am trying to create a simple DIY tachometer using parts to a Radio
Shack serial ball mouse I have. The idea is to use the shaft encoder
(disk with equi-spaced slots) and the (what seems to be) infrared
emiter and reciever to obtain an electric pulse. That is, as the disk
spins, the IR signal will repeatidly be broken causing the reciever to
see a pulse of IR. My problem is the circuit building. Does anyone
have any idea on how to get this project to work? Any help would be
great. THanks. Lucas.

 

[Rich Grise]

I am trying to create a simple DIY tachometer using parts to a Radio
Shack serial ball mouse I have. The idea is to use the shaft encoder
(disk with equi-spaced slots) and the (what seems to be) infrared
emiter and reciever to obtain an electric pulse. That is, as the disk
spins, the IR signal will repeatidly be broken causing the reciever to
see a pulse of IR. My problem is the circuit building. Does anyone
have any idea on how to get this project to work? Any help would be
great. THanks. Lucas.

Once you've got the mouse apart, look at the component values around the
optical interrupter. The LED current is probably something on the order
of 20 mA, since that's fairly standard for LEDs, but it would be
worthwhile to see what resistor they're using in the mouse, and use the
same value to drive the LED. The output device is just a phototransistor
or photodiode - ground its emitter (or whatever is supposed to be the
negative pole - I don't remember how to use PHDs), run a 4.7K or so
resistor from the collector to VCC, and you should get a pulse train.

It's a trivially simple circuit, to us self-appointed "experts" - what
I'm wondering is how you're going to manage the mechanical part.

Good Luck!
Rich

 

[[email protected]]

Thanks for the advice, there is one problem, the phototransistor/diode
has three leads to it. Im not sure what it is exactly. Do you have
any idea? I mean, i have played around with the IR
phototransistor/diodes sold at radio shack that have two leads (look
very similar to a LED), but this is different. Anyway, if anyone knows
what exactly im dealing with, I guess thats what I need to know. thanks
a lot, Lucas.

 

[Rich Grise]

Thanks for the advice, there is one problem, the phototransistor/diode
has three leads to it. Im not sure what it is exactly. Do you have
any idea? I mean, i have played around with the IR
phototransistor/diodes sold at radio shack that have two leads (look
very similar to a LED), but this is different. Anyway, if anyone knows
what exactly im dealing with, I guess thats what I need to know. thanks
a lot, Lucas.

Sounds like one lead is ground, and the other two are the + of the LED
and PHT. Do some experiments - use an ohmmeter to see which is the diode:
only two of the leads should behave that way - that's the LED and ground;
the other lead is the collector of the PHT. If you read two diodes, one of
them should be "upside down" - that would be the photodiode if that's
what's in your device.

This is just a semi-educated guess, of course - is there any way to trace
out the circuit it came out of?

Good Luck!
Rich

 

[Rich Webb]

Thanks for the advice, there is one problem, the phototransistor/diode
has three leads to it. Im not sure what it is exactly. Do you have
any idea? I mean, i have played around with the IR
phototransistor/diodes sold at radio shack that have two leads (look
very similar to a LED), but this is different. Anyway, if anyone knows
what exactly im dealing with, I guess thats what I need to know. thanks
a lot, Lucas.

Google is your friend:
http://www.seattlerobotics.org/encoder/200311/johnson/Mousebot.html

 

[Jasen Betts]

Thanks for the advice, there is one problem, the phototransistor/diode
has three leads to it. Im not sure what it is exactly. Do you have
any idea? I mean, i have played around with the IR
phototransistor/diodes sold at radio shack that have two leads (look
very similar to a LED), but this is different. Anyway, if anyone knows
what exactly im dealing with, I guess thats what I need to know. thanks
a lot, Lucas.

stick it back in the mouse, power it up, and make some voltage measurements.

 

[[email protected]]

Thanks rich for the great link, that pretty much answered most of my
questions.