Digital Pot Help

[fiveten]

I am trying to build a digital pot but I am not sure exactly what components to use. I am trying to build a circuit that has a varying resistance on two, two output circuit leads.

I have a 555 I can use for a clock pulse
I have 2 resistor network 16pin dips isolated
I also have misc ics 4011, 4081, 4017, 4001
Resistors, diodes, caps and stuff.
I may have some relays too
I also have a basic stamp but if I don’t have to use it I will prefer not to.

I want it to switch automatically when I turn it on. I can add a reset too.

I think I got everything I need, I am just not sure how to make the resistors switch together.


Thanks for any help

 

[akg]

u can use 4066 bilateral switch for making a variable digital resistor .but this has discreet values.

or if u can use pwm , a switched capacitor can be used as a smooth pot

 

[mstechca]

you may want to explore what I am exploring right now, an R/2R ladder.

Basically, you take a counter, and determine how many bits you want to use. Let's call this number X. Now take x 100K resistors and x 200K resistors, and wire them up according to an easy R/2R ladder schematic you can find on the internet.

As soon as the count is adjusted, the output voltage is adjusted. Be careful if you use too many bits, because you could lose linearity when adjusting the count. Resistors have tolerances too that must be taken into account.

 

[Roff]

I am trying to build a digital pot but I am not sure exactly what components to use. I am trying to build a circuit that has a varying resistance on two, two output circuit leads.

I have a 555 I can use for a clock pulse
I have 2 resistor network 16pin dips isolated
I also have misc ics 4011, 4081, 4017, 4001
Resistors, diodes, caps and stuff.
I may have some relays too
I also have a basic stamp but if I don’t have to use it I will prefer not to.

I want it to switch automatically when I turn it on. I can add a reset too.

I think I got everything I need, I am just not sure how to make the resistors switch together.

Thanks for any help

You could get better advice if you tell us your application. Do you need a variable voltage, or a variable resistance, or...? How is this "digital pot" used?

 

[fiveten]

Thanks for any help[/quote]
You could get better advice if you tell us your application. Do you need a variable voltage, or a variable resistance, or...? How is this "digital pot" used?[/quote]

I am using it for a automotive application. I am looking for a resistance value. The car's computer supplies voltage and ground. The values are from 500 ohms up to 16k ohms in about 5-10 steps.

I guess I could buy a digipot. can someone recomend a manual one. I don't think I know how to program one.

Thanks, fiveten

 

[ljcox]

You can use an analogue switch IC such as the 4016, 4051, 4052, 4053, etc.

You will need a binary counter (possibly an up/down one). The 4017 is not suitable as it has decoded outputs.

In order to help further, we need more information.

 

[fiveten]

The resistance values will substitute temp values to the computer. The computer sens a 5v ref. and calculates a voltage drop accross the signal to determine a voltage. an 80 deg value will be about 500 ohms and a 190 deg value will be around 1.6k ohms. There are two temp sensors, hence the two, two lead outputs. Basically I am simulating a cold start from 80 deg to 190 deg. I am trying to get the computer to perform an operation that only takes place after a cold start. Because the computer supplies the voltage and ground I will need a variable resistance value.

Thanks, fiveten

 

[ljcox]

What is going to drive the digital pot? Will the computer output an address to indicate the resistance value you want? We are not mind readers.

 

[fiveten]

Did you read the whole post? I have a 555 as a timer that will switch a controller. I am supplying a resistance value to the computer. The the computer does nothing but read the value I give it throught the digipot I am trying to make.

My post was: I am trying to make a digipot that has a variable resistance. I can run it off a battery or what ever I choose. I just need an idea of a ic that can take a signal from my 555 to step a resistor network. The values will be what I choose.

If you need more info, ask what you need, I can't read your mind. What are you looking for?

 

[Roff]

As I understand it, this is a two-terminal digitally controlled variable resistor - not a pot, technically speaking. Is this correct?
What is the input resistance of the port that's reading the programmable resistance?

 

[ljcox]

Yes, I did read your post, but it is a bit vague. If you read the replies by the others, no one seems sure as to what you want. You need to specify what you want in more detail.

What I think you want is a counter driven by a 555 acting as an oscillator. This means that the resistors will be chosen in sequence at the rate determined by the oscillator frequency.

Or do you want to increment it by hand, ie. by a push button?

The 74HC4051 has 8 switches, selected by a 3 bit binary code and an inhibit.

So you can choose one of 8 resistors. If this is what you want, I'll post a circuit for you tomorrow. If fact, It will be possible to have 9 steps.