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4PDT to SPDT trick

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Hi guys.

I have a circuit that uses a 4PDT switch to flick 4 seperate parts from one circuit to another (if that makes sense).

Is there a way to use a SPDT switch in conjuction with some electronic switches to do the same thing?

I thought I could use the MAX333CPP quad SPDT analogue switch, but have had no luck with it.

Any ideas.



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Sorry guys, okay here we go. This should make things clearer.


The "forward and reverse limit" connections go straight to the + input of a LM4558 dual op amp. The "limit in" connection will be around 0.1v and no more than 1mA.

The "forward and reverse value" connections should have no more than 1mA, and as you can see is +5v in.

The "forward and reverse signal" connections would have a max of +12v and 12mA. The PWM signal in again has a max of +12v.

What I find is that the switch does not seem to change the state of the SPDT sections. I have yet to exam is closely to figure out if they are staying in the correct state (in terms of "normally closed")

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Yes, it does work now (with the 10k resistor), with only one small problem.

The number 1 SPDT and the number 4 SPDT in the chip are connected to latching comparators that stay on once a mode is selected (by the switch), and indicate through an LED. To turn the comparator off when the mode is switched (and turn on the other one), they are grounded through the MAX333CPP. Problem is that (I assume due to the internal resistance of the chip) even when PIN 3 is connected to PIN 2 (ie grounding it), there is about 0.12v on PIN 3 which keeps the indicator LED on, regardless of the actual mode selected.

Is there anyway to overcome this so that when in the appropriate mode (via the switch) that PIN 3 and PIN 18 ground properly through their respective connection?

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Its a bit hard to describe, so heres the setup


As you can see, when the switch is open, and as long as there isnt to much voltage on the load signal feed, the forward comparator will latch on, turning on LED2.

When the switch is closed, it SHOULD ground the forward comparator (thus turning it off) and turn on the reverse comparator (assuming that again, there is not too much load signal) and latch on, thus turning on LED1 (and turning off LED2).

However, the MAX333CPP doesnt seem to ground PIN 3 or 18 very well, and the comparators do not turn off, and so the LED's stay on.

Is there some way to get a good grounding through the chip for those forward and reverse limit feeds to the comparators?

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Hi again

Maybee there is an another solution after all, i didn't thought of it at first!
The smitt-trigger- action resistors, do they need to be that big? Is it possible
to rise them to, lets say, 100K or so? Than the current throu the switches are
reduced, and also the voltage. Also an another possibillity come to my mind,
that we take later.


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I'll keep it in mind  :)

But I had another idea.

I changed PIN's 2 and 17 to connect to -5v, rather than ground. That way I figured it would drag the comparators + inputs to a negative voltage (and turn them off).

However, it didnt seem to work. When I changed the switch, the LED's would flick off, then come straight back on. On checking the outputs of the comparators, I found that when the switch was changed, they would drop voltage for a moment, then go back up again, in effect turning off for a split second, then back on again.

Why would this be happening if there was a negative voltage on the + input to a comparator and 0v or very slightly positive on the - input?

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I have hooked the negative 9 volt rail to the MAX333CPP.

The voltage on PIN 3 when it is supposidly grounding is now 0.08v. Lower, but not low enough to turn off the comparator.

What is this other solution of yours?

Try putting a voltage divider with a pot on the other input to the comparator. Adjust the pot to cancel the small voltage you are seeing on the other input. You could even go a little further so you need a high voltage to switch the comparator.
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Got it sorted now. I had changed the comparator to the -5v rail, and I figure that the -5v through the max333cpp was too close to V-.

I changed it back to -9v for the comparator and it works fine. I also put in the reverse diodes to protect the transistors bases.

Staigen - the -5v comes from a regulator elsewhere in the circuit (part that wasnt important to the original question, so I didnt bother showing the whole thing).

Works a treat now - thanks guys.

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