# Method for selecting unique resistor values in all combinations

I do controls, and sometimes I want to monitor more dry contacts than I have digital inputs for. I'm cheap, and don't want to pay for more points, so sometimes I'll put some resistors in parallel with the contacts, and put the (resistor||contact) pairs in series, and read them using an RTD input. With three contacts, it's easy to pick three resistor values that give me 8 unique combinations within a comfortable range and with comfortable distance in between. Say 10k, 7.5k, and 15k. Moving on up to six contacts, I've now got 64 states, and picking 6 resistances from the E12 series that can satisfy 64 states may not even be possible.

Is there a succinct way to solve this problem of picking x resistances from a standard series, sticking within one decade? Something I can stick into Excel? Maybe it's very obvious once I restate the problem mathematically in terms of sets or combinatorics, but I worked hard in those classes once so I'd never have to do it again.

Electrically there are obviously many ways to convert a six bit digital value to an analog value, including using resistors and more decades, but anything more complex than single resistors pretty much moves me into "buying a pre-built" solution territory.

• Something like an R 2R ladder making a kind of DAC might work. But that's just off the top of my head, I haven't had time to work out the details. Aug 1 '18 at 3:56
• DAC's usually use dual voltages or currents or SPDT switches. , Can you choose 0.5% R values in binary R sequence to make a DAC with 6 SPST switches? with 1,2,4,8,16,32 [kOhm] to 0V and 64k to 5.00V using an 8 bit ADC? Aug 1 '18 at 4:13
• Try looking up "R-2R ladder". Aug 1 '18 at 13:46

## 1 Answer

With a 8 bit ADC , you can easily gang 6 switches in a binary R ratios.

ADC's normally use SPDT or dual voltages or currents. But you have SPST contacts.

Rather than E10 values which have only 10% tolerance, you can choose the nearest 0.5% values of; 1k,2k,4k,8k,16k,32k

Proof of concept