need electronic circuit help

F

Thread Starter

freq

I have two encoders and need to switch from using one to the other with a fast electronic switch. Unfortunatley, my knowledge of component level electronics is very limited. Can someone give me some advice on building this circuit, or a site with a circuit drawing I can copy. This is something I need right away so furthering my education is not practical at this time. I was thinking of using trasistors or zener diodes, but I may be way off. Help please.
 
W
Freq,

Use a 74xx157 quad 2 input multiplexer. Hook the encoder channels (A, B, and Index) to three sections of the MUX, then use the select line to choose which one you want to be output. Be sure to hook the unused inputs to ground to avoid problems with latchup. Depending on the logic levels used, you can use an LS, HC, HCT etc. part for the task.

Regards,

Willy Smith
Numatics, Inc.
Costa Rica
 
C

Curt Wuollet

What you want is already in IC form as a digital multiplexer or mux. I assume you need to switch four or possibly 5 lines. This should be
available for a dollar or two at ttl levels. If you need higher voltage levels there are CMOS muxes that will work. I'll suggest a few when I get to work.

Regards

cww
 
R
this will work if the shaft encoders are stopped when you switch them, and you have a chance to initialize the count in whatever you're using to
monitor them. you may get a false step when you switch, since the shaft encoder signals are basically quadrature square waves. If both shaft
encoders are moving when you try to switch from one to the other you could get fairly messed up with false counts. This is the synchronization problem I alluded to earlier.

Rick Daniel

www.instrument.com
 
G

Gabriele Corrieri

Can you be more precise?

What is the maximum frequency of encoder?
You can calc by multiply the maximum speed and the encoder pulses.

What is the output voltage?
E.g. 5v 24v, push pull, single ended, open collector, driver

Which signal act a switcher by one to other encoder?

If you reply I can draw a basic circuit.
If you want contact me directly

Gabriele Corrieri
[email protected]

 
C

Curt Wuollet

Look at the data sheet for a 74HC157. This is the ttl equivalent of a 4 pole 2 position switch. When enabled, a high on S selects one set of four inputs and a low on s selects the other set of inputs. The "common" are the "y" outputs. If the encoders can't operate at 5 v logic levels, you simply use a voltage divider (example 3.3k and 680 ohm) to get the ttl levels and do your switching, then you can use a DM7407 hex high voltage open collector buffer to restore the 24 v logic levels. This setup will switch in something less than 75 nanoseconds which should be fast enough. The whole setup should be less than $3.00. If you need to switch more inputs you just double the setup. Let me know if you need a
drawing. This goes on my list of "Gizmos that would be handy to make little boards for".

Regards

cww
 
Hello List

Please take into account that you may get false counts or even quadrature errors when you switch encoders this way.
 
W
Rick,

True, but in many applications (e. g. motor tachometers, or conveyor speed & direction display) there shouldn't be any problem, even if you switch encoders while running. If it's for absolute position controls in precision
machinery, you're right: more care should be taken. We don't know much about the application, but it sounded like a Q&D requirement to me.

Regards,

Willy Smith
Numatics, Inc.
Costa Rica
 
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