Climate control for heat only

Hi, I have a heater in a large truck cab that uses a servo unit to control the heater water valve. This servo is proportional and controlled by a 10K pot which puts from 0v to 12v on the servo unit. I want to have a digital control that can be set at a temp (digital readout) and maintain that temp automatically. I figure this has already been done but all I find are relay controls.
Thanks for your input, Tom

Either You us a DAC or You use a pwm output and filters the signal.

Where\How are you going to read and display the temperature you want controlled?

How are you going to change the 0-5v Arduino output(either from a DAC or a filtered pwm) into 0-12v to drive the servo?

How do you want to interface with the controller to set and change the temperature setpoint?

With the input, outputs, and interface defined, then you need to develop a control algorithm for how to drive the servo when the temperature input differs from the setpoint. Full PID may or may not be necessary.

A 5 volt PWM can control a logic MOSFET working in the 12 volt environment and create that 12 volt signal level. Amplifying a 5 volt DAC is probaly more complicated.

What is the input impedance of the servo unit (how much current is required)?

@OP
Attach info and links for the servo system.

Since there is already a 10k pot for control - could that post be replaced by a variable resistor that is controled by the Arduino? Seems there was a tutorial using a variable resistor in one of my texts.

saildude:
Since there is already a 10k pot for control - could that post be replaced by a variable resistor that is controlled by the Arduino?

You are suggesting using a potentiometer driven by a servo?

There are programmable resistor/ladder circuits. Must be such a one that @saildude suggests.

Railroader:
There are programmable resistor/ladder circuits. Must be such a one that @saildude suggests.

Yes that is my suggestion/question - I have only ran across them a few times

Here is one at Parallax

saildude:
Here is one at Parallax
https://www.parallax.com/product/604-00010

So how are you going to connect a 5 V device to a 12 V circuit?

Hi, OK first, I misspoke, it is a 24 volt servo system.
http://www.partdeal.com/mei-electric-operated-heater-valve-universal-actuator-12-24v-with-4-pins-2391.html?utm_source=google&utm_medium=cpc&adpos=1o1&scid=scplp77287251&sc_intid=77287251&gclid=Cj0KCQiAiZPvBRDZARIsAORkq7foZF_r8GPiQ6Jk4ptEgNwlpkjHZ3n4Arm3DseBiJgshBH7eKExBjgaAhGCEALw_wcB

wiring attached as pdf. soon I will measure the current of the servo control wire. truck is away for a couple of days.

rd-6-5678rev-.pdf (70 KB)

Oops. Lots of stuff in that control unit. I see 2 pots, one for water valve and one for defrost. Are You sure how to proceed? I would suggest You break up the pot circuit measuring the current from the center tap of the pot. It might carry current, not only a controlling voltage.

Because the actuator is being controlled from a source that could be as high as 10Kohm I think it's safe to assume that the actuator has a reasonably high input impedance and could thus be driven with a low-current analog circuit.

You could probably use a simple DAC like the Microchip MCP4921 (Arduino library GitHub - michd/Arduino-MCP492X: MCP4921/MCP4922 Digital to analog converter Arduino library, using SPI.h library) to generate an analog voltage of 0-4V, then use an LM324 or similar operational amplifier with a 24V supply (sourced from the truck's power supply; overvoltage protections added as required) to give a 6x amplification to the DAC output to feed the actuator.

You'd want to bench-test anything like this before trying it in the truck.

@Blackfin
Why not PWM the 24 volt using a logic MOSFET and filtering it? The need for eventual current has to be investigated.

If you are interested in the digipot or DAC approach as opposed to low pass filtering 24v pwm (Railroader's mosfet suggestion of reply #3) the AD7386 is an SPI 128 position digital pot which can run at up to 30v as a potentiometer.

@cattledog
That's a highly interesting device substituting a 10k pot.

Well as you see in the wiring of the unit, the servo has 3 wires. They say 4 but do not use 1 as it goes nowhere. There is ground, power, and the control which is the tap of the pot so it must be just a voltage divider source for the amplifier in the servo. Also I am only concerned with the heater valve servo so disregard the rest of the circuit. I will check current by end of week. My initial question is more of who has done this before. Always easier to adapt an existing circuit that start from scratch for me. This is likely a standard system used in many vehicles.

Take a good look at the pot if You can. Most likely it is a low power type and the programmable resistor circuit suggested will work fine.

Something like this could work with proper components, 5V PWM in, 0 ~ 24V out with C1 and R5 selected for fastest response with lowest ripple.