I'm confused.
Campers often have a separate, mains powered AC, so does this AC run from mains voltage or the engine?

 

I'm confused.
Campers often have a separate, mains powered AC, so does this AC run from mains voltage or the engine?

I am talking about the vehicle aircon built into the vehicle dashboard, the equivalent of the aircon in a car, so will be 12V. Unfortunately it is not the more sophisticated climate control type, but just a basic manual system controlled with an ON/OFF button, hence why I would like to control it with a thermostat while driving.

I have installed a 240V mains powered aircon unit on the roof of the van to provide cooling when parked up at a campsite with a mains hook up. It can also provide heating which is useful on chilly evenings and saves on burning gas in the main camper heating system. This has both a remote control and a Bluetooth app for setting temp, fan speed, etc, so no mods needed for that!

 

What temperature difference will you be using to determine shutoff and startup?? That affects how often the system cycles between cooling and not cooling. Do you really want the system stopping and starting as the temperature varies one or two degrees??

The thermostat module allows the hysteresis, that is the difference between on and off temperatures, to be set over a wide range, so I will probably set it to about 3 degrees initially which should avoid excessive cycling.

 

I am talking about the vehicle aircon built into the vehicle dashboard, the equivalent of the aircon in a car,

Then it would seem just switching the current to the compressor clutch as already suggested would work, no additional circuitry required, only likely an added relay.

 

Then it would seem just switching the current to the compressor clutch as already suggested would work, no additional circuitry required, only likely an added relay.

Well that's as may be, but I would be concerned about an uncommanded deactivation of the clutch being detected as a fault by one of the various electronic control modules in the vehicle, causing a fault code to be registered with the main ECU. I always try to avoid any mod potentially detectable by the electronics when modifying my vehicles, and this policy has been successful in avoiding problems on various motorcycles and other vehicles so I will stick by it as far as possible.

 

about an uncommanded deactivation of the clutch being detected as a fault by one of the various electronic control modules in the vehicle,

I doubt anything monitors the clutch voltage/current, but it would be easy to test by just disconnecting the clutch wire when the AC is on, and see if there's an error generated.

 

Another circuit to activate the Control Button.

 

Another circuit to activate the Control Button.
View attachment 326938

Thanks for that, but can you explain how it works to produce a momentary output both when the thermostat relay closes and also when it opens?

 

Thanks for that, but can you explain how it works to produce a momentary output both when the thermostat relay closes and also when it opens?

Sure.
When the thermostat contact is closed C1 charges through R2 and the left side infrared LED inside the PC814. This activates the internal transistor which then activates Q1 and the relay momentarily as C1 charges fully.
When the contact opens C1 now discharges back through the right side infrared LED, R2 and R1 momentarily activating the relay again.
With the component values listed the activation time is appx 0.5 second verified on a breadboard.
With the right relay Q1, R3 and R4 can be eliminated.
Same circuit can use a mosfet to drive the relay eliminating R4.

 

With the right relay Q1, R3 and R4 can be eliminated.

Since the opto provides isolation you likely could do away with the relay and connect the transistor across the AC switch (observing the switch voltage polarity of course).

 

Since the opto provides isolation you likely could do away with the relay and connect the transistor across the AC switch (observing the switch voltage polarity of course).

Possibly, if we knew what current is required.

 

Possibly, if we knew what current is require.

I would expect it to be a low-current logic signal to the vehicle's computer.

 

I would expect it to be a low-current logic signal to the vehicle's computer.

I agree but would want validation on that before damaging anything. The TS will have to check that out first.

 

Possibly, if we knew what current is required.

I would not leave out the relay because with a PB there is a definite flat low resistance with contact bonce, while the opto resistance does not go that close to zero.

 

Thanks to all for your suggestions and comnents, which have been both useful and educational. I am looking forward to trying this out. First I need to get access to the wiring to the AC ON/OFF switch to see how it connects, though I am already certain that it is not a latching switch.

This because if the AC is on when the vehicle ignition is switched off it is also on when the ignition is switched back on, but pressing the button while the ignition is off does not change that. This proves to me that the state is not maintained mechanically by the button but some controller elsewhere in the vehicle's electronics.

This does introduce an example of the problem already suggested, namely that the thermostat and the AC can get out of sync. If the AC is on when the ignition is cut, next time the ignition is switched on the AC will already be on but if the temperature is such that the thermostat tries to switch the AC on it will actually switch it off. I guess I will just have to correct this manually.

 

I would not leave out the relay because with a PB there is a definite flat low resistance with contact bonce, while the opto resistance does not go that close to zero.

Yes, the opto output directly may not have a low enough resistance.
But in my post #30 my statement was to use the transistor across the switch, not the opto output.
An even lower resistance could be provided by a power MOSFET buffer if needed.

 

First I need to get access to the wiring to the AC ON/OFF switch to see how it connects, though I am already certain that it is not a latching switch.

It would also help if you can get voltage readings on both sides of that ON/OFF switch.

 

But in my post #30 my statement was to use the transistor across the switch, not the opto output.

I thought you were speaking of the opto as well.
It could be as simple as this:

 

I thought you were speaking of the opto as well.
It could be as simple as this:
View attachment 327013

We have no hint or clues as to what is connected to either side of that PB "switch" device. Thus the 12 volt connection shown totally defeats the opto-isolation.

 

We have no hint or clues as to what is connected to either side of that PB "switch" device. Thus the 12 volt connection shown totally defeats the opto-isolation.

That is correct which was my request in post #37.
The use of the opto was never meant for isolation.