Designing a variable resistor logic for an input module

Question

I'm currently trying to design a circuit which controls the input of a receiving module which interprets the current flowing through itself as StateA, StateB or StateC. What the input module does with the states is irrelevant in this matter, since I'm focusing on making these currentStates flexible through a raspberry pi.

Normally this input module is controlled with switches and resistors.

But the control circuit I'm designing needs to be maintained (switched) in software and there can't be any physical switches that need to be pressed. Also, since I want to control which of the current States occur with a raspberry pi, a portion of the circuit will end up running on 5 volts. The input module is supplied with 24 Volts. (range goes from 19 to 27 volts)

Originally I planned to get an Arduino Mega (which then gets its input from a raspberry pi) and with it's ~50 Digital Pins I'd be able to control a relay per State per Input. There are 8 Inputs on the module with 3 States each. That makes 24 Relays with corresponding resistor values. But this turns out to be very ineffective compared to just using resistors with switches.

See the schematics below for clarification:

^{simulate this circuit – Schematic created using CircuitLab}

Is there a different way to provide these 3 "States" with the usage of GPIO? Maybe some sort of switch that is controllable with GPIO?

Answer 1

The simplest substitution would be to replace each relay with a MOSFET operating as a switch. I'm assuming that the potential difference across the input pins of your receiver is 24V, in which case you'll want to use a low side switch, using an N-channel MOSFET (or NMOS for short), rather than a high side switch as you drew in your original schematic.

^{simulate this circuit – Schematic created using CircuitLab}

You'll need to find an NMOS transistor with a low enough threshold voltage to turn on with a 5V gate drive, as well as a low enough RDSon and capability to handle the current you need.

Since you didn't specify how much current each state is, I'll assume they are quite small (few mA or less). The BSS138 is an individual NMOS transistor that can handle up to 200mA drain current with RDSon=6Ω at 4.5V gate drive. Since you'll need quite a few of these, you could also get multiple transistors on a single chip, such as TPL7407LA-Q1.

If you want to control many switches with as few GPIO pins as possible on your microcontroller, you can use a shift register with open drain outputs (which allow the shift register to perform low side switching), such as TPIC6C596. Rather than using a GPIO pin to switch each transistor, you send serial data to the shift register using only SRCK (clock) and SER IN (data) pins, and then update the states of the switches by pulsing the RCK pin. Each shift register has 8 output switches, but you can daisy chain many shift registers together to be controlled with the same serial signals.

Answer 2

This is a good circuit for switching resistances. The gotcha is the gate level of Q1 needs to be higher than the drain so this circuit is only useful if you have a rail that is higher than the one you are switching.