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electronic switching of signals without voltage loss

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jeandeau

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Proposed project:
I wish to build a circuit board which selects from four or more signal sources and sends the signal (0 to 3 Volt) to another circuit for display. I considered a momentary switch pulsing a ripple counter with the output of the counter activating 5V mechanical relays. This seems expensive (cost of relays) if I build many of these. therefore, I proposed to myself replacing the set of mechanical relays with a single chip which would act in a similar fashion as the mechanical relays.

Problem:
It seems such solid state devices have 30 to 300 Ohms resistance across the switched electrical circuit.

Proposed solution:
Use a quad opamp, an analogue signal switch (eg MC14066B), and a decade counter. Input each signal to it's own OpAmp plus input. Each opamp output goes to its own solid state switch path, all switch outputs are tied and a single return goes to all opamp minus inputs, the single return is read for voltage. Each time the momentary switch is pressed, the clock input on the counter recieves the voltage pulse and indexes the counter output to the next output, which in turn selects the next channel on the analogue switch. All opamp outputs connected to open switches will go to maximum voltage (the rail). The one opamp connected to a closed switch path will set it's output voltage to a level that balances the output of the switch with the plus input signal on the opamp. The result is, the resistance of the switch is compensated, regardless of temperature or other factors affecting resistance within certain limits. The reset of the counter could be set to four or five to offer immediate resequencing or a zero state where a zero is displayed.

Question:
Would this work? Am I missing something obvious because I have little experience. would the opamps be damaged or draw a lot of current sitting on the rail?. Would the LM358 Opamp be a good choice? Is it better to connect the signal to the minus input with return to the plus input? Is there a sturdy -12V source other than a battery if I must use a split power supply?

Primary function is a multiple input automotive thermometer using analogue devices such as diodes or lm35 ICs. Perhaps I am on the wrong track altogether and a better approach is recommended.

Thanks in advance to all who read this post and offer their suggestions. :)
 
I think you need to consider what it is that the multiplexer is feeding.

If the display device has an input resistance of 1M ohm, the 300ohm ON resistance of the multiplexer will be in series with the signal path to the display and there will be a voltage divider effect.
The voltage division will be 1,000,000/(300+1,000,000) = 0.9997 not a lot, infact a loss of accuracy of 0.03%.
In which case I dont think that you need to worry.

It all depends on the input resistance of the thing that the multiplexer is feeding.

JimB
 
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