# Help with circuit design -switching IC pin between a resitor and a pot

Discussion in 'General Electronics Chat' started by mattbrailsford, Nov 1, 2015.

1. ### mattbrailsford Thread Starter New Member

Oct 31, 2015
13
0
Hi,

I was wondering if someone could help me with the design of a circuit for the following scenario.

I have a momentary switch connected to a 3v signal. In addition, I have an IC with a pin that needs to be connected to ground via a resistor (it reads the resistance value, so the resistance of the line needs to be known). Now, when the switch is open, I need the IC pin to connect to ground via a potentiometer, however when the switch is closed, I need the IC pin to connect to ground via a fixed value resistor.

Many thanks

Matt

2. ### Alec_t AAC Fanatic!

Sep 17, 2013
5,973
1,135
An analogue multiplexer/gate (eg. a CD4066) could be used, provided its resistance is stable or a lot less than that of your resistor/pot. Or logic-level MOSFETs with a low Rds(on) might be suitable as switches.
What values do the pot and resistor have?
Is it the pot wiper or end terminal which goes to the IC?

3. ### mattbrailsford Thread Starter New Member

Oct 31, 2015
13
0
The resistor is 100k, with the pot being 200k and it's the POT wiper that goes to the IC.

4. ### dl324 Distinguished Member

Mar 30, 2015
3,378
651
The OP has omitted some details that he gave in his other related thread.

The circuit is operating from 3V and he may have LEDs in series with the resistances to be measured (didn't get far enough along in that thread to clear up requirements).

He was trying to accomplish this with a SPST momentary switch. I was trying to make him understand that he was probably going to require a DPST switch. He's talking of measuring resistances, when in actuality he'll need to measure voltage. Having different "resistances" being switched complicates matters.

I would have guided him to CD4066 also as his original circuit was using a single P channel MOSFET. 3V is the minimum operating voltage for CD4066, switch resistance will be at it's highest and switching speed will be at it's lowest. On resistance variation isn't characterized below 5V operation.

5. ### mattbrailsford Thread Starter New Member

Oct 31, 2015
13
0
Yes, there will be LED's, but these are on the 3v rail that is connected to the momentary switch, so shouldn't affect the resistances to be measured as the resistors are connected between the IC pin and ground, not the 3v rail.

I understood I needed a different type of switch but I said I had to use a SPST button hence my question was asking how I can make it into a DPST using a p-channel mosfet, as I can't change the SPST switch (granted, I fluffed my terminology, but I did explain the behavior I needed).

So yes, these questions are for the same project, but I've asked the questions in isolation so that I can understand them based on the problem being posed, and yes, I have omitted elements so that I can focus on the single element.

6. ### dl324 Distinguished Member

Mar 30, 2015
3,378
651
When ohmmeters are used to measure resistance, they usually apply a known current and measure voltage. So your application will actually be measuring a voltage drop. Because of that, using different resistors with different currents will be problematic.
Personally, I prefer to know as many details as possible up front. That avoids confusion and frustration when requirements are changed or previously unmentioned complications are introduced.

Last edited: Nov 1, 2015
7. ### Alec_t AAC Fanatic!

Sep 17, 2013
5,973
1,135
Is this 'resistance measuring' being done with an ADC? If so, don't ADCs generally require a source resistance less than ~10k?