I wonder if anyone can help me with this. My vehicle, a camper van, was not able to be purchased with the optional full climate control system, so I am stuck with a manual system whereby the cooling function is turned on or off by a simple button on the heating control panel. On a recent trip to sunnier climes, this led to us alternately freezing and sweltering due to having to remember to manually control this in a timely manner. To automate this I have found a suitable thermostat module which has a normally open relay output which closes to turn on the device it is controlling and opens to switch the device off. This displays actual temp and desired temp, and can work in either heating, or in my case cooling mode, and allows things like amount of hysteresis to be set, so looks ideal.

The problem is that although I haven't removed the vehicle's heating control panel yet to examine more closely, the aircon is controlled by what appears to be a non-latching button - one press to turn the aircon on, and a further press to turn it off, so it looks like it just momentarily sends a signal to a control system. It therefore looks like I will not be able to simply use the relay in parallel with the switch as might be possible with a latching switch. Instead it seems that I will need to momentarily provide a signal, be it 0V or 12V (not sure which yet) to the output of this switch to turn on the aircon and then provide another momentary signal to turn it off.

The nearest thing I have found here so far is the following, but I am unsure whether it is entirely suitable and if so how to adapt it to my requirements.

https://forum.allaboutcircuits.com/...a-relay-is-energised-and-de-energised.112984/

Thanks in advance for any help with this.

 

If the switch is actually momentary action then the problem is, it would be easy for a controller to get out of sequence.
If done, it would require some type of custom circuit.
Would you be up to building that?

One possible alternate way would be would to add a relay that controls the power to the AC unit, and control that relay with a thermostat.

 

If the switch is actually momentary action then the problem is, it would be easy for a controller to get out of sequence.
If done, it would require some type of custom circuit.
Would you be up to building that?

One possible alternate way would be would to add a relay that controls the power to the AC unit, and control that relay with a thermostat.

Thanks for the response, and I hadn't considered that things might get out of sequence - good point! One thing I didn't mention is that the ON/OFF button has a built in white LED that illuminates in the ON state so it might be possible to use this in some way to control alignment or reset a circuit.

I started my career in electronics and then spent many years in IT before retiring, but I am still handy with a soldering iron, so building a circuit, perhaps on a breadboard type panel, would not be a problem.

I would not be too keen on the brute force approach of cutting power to the AC unit for fear of causing error codes to pop up. That would also probably reset the switch to the off state so restoring power to the AC would not necessarily turn it on.

 

Certainly post #1 points directly at the probable issue. But there is another option, aside from even adding some interface. As it was mentioned in posts #2and 3,, a relay that cycles the compressor on and off, controlled by your thermostat module. The relay could simply be a standard automotive cube type mounted close to the existing AC compressor wires, so that a minimum of rewiring would be required. Then only one low current wire back to the controller will be needed. For the maximum flexibility, use the NC contact on the relay, and set the controller to operate the relay when ever the interior temperature is what you want. Compressor cycling is how OEM ac systems control the cooling.
The unintended extra benefit is that if the control device fails to operate you can simply use the manual scheme.

 

Certainly post #1 points directly at the probable issue. But there is another option, aside from even adding some interface. As it was mentioned in posts #2and 3,, a relay that cycles the compressor on and off, controlled by your thermostat module. The relay could simply be a standard automotive cube type mounted close to the existing AC compressor wires, so that a minimum of rewiring would be required. Then only one low current wire back to the controller will be needed. For the maximum flexibility, use the NC contact on the relay, and set the controller to operate the relay when ever the interior temperature is what you want. Compressor cycling is how OEM ac systems control the cooling.
The unintended extra benefit is that if the control device fails to operate you can simply use the manual scheme.

I would much prefer to make use of the existing control system by simulating the manual button presses in some way, as I am not sure that simply turning power on to the actual compressor will actually get it to run as it most probably takes its orders from some sort of control unit to which the fascia button is connected, regardless of the state of its power supply.

I also would not relish fighting my way into the bowels of the machine to find the power connection, though I guess the way to do it would be to identify the fuse feeding the AC and interrupt supply there, but as I said in an earlier edit I am not sure that would be effective and would probably just generate error codes on the dashboard.

 

the ON/OFF button has a built in white LED that illuminates in the ON state

Okay, that will allow the circuit to determine the state of the system.
But after some looking into a circuit that would incorporate the LED state to determine when a pulse is needed, it becomes somewhat complicated.

The easiest would be to incorporate a push-button that would sync the thermostat with the AC if it gets out of sequence.
There would be an LED on the added circuit that shows the state of the thermostat output, and if it is out of sync with the AC LED, just push the AC button to correct that.
After that it should stay in sync.
The circuit would consist of one simple IC, a few resistors and capacitors, a small transistor, and a relay.
Does that sound feasible to you?

 

Yes, thanks for your input. It sounds like building something like that would be within my limited capabilities!

I would arrange for the thermostat module to be powered on and off with the ignition. Everything will then get reinitialised by that anyway so I'm not too concerned about sync issues. The worst that can happen is that it turns on coolng when not required or off when it is required which can be easily remedied by manually operating the ON/OFF switch to bring them back into sync.

 

I should add that the thermostat module has an LED that illuminates when it operates its relay, so it should be easy to spot if the aircon button light is not on when the thermostat is asking for cooling.

 

I should add that the thermostat module

What module?

Below is the sim of my take on a circuit to do what you need:

The CD4093 IC is configured as an XOR gate that generates a pulse when the thermostat contacts both open and close, to momentarily close the relay contacts across the momentary switch you have.

The D2 LED shows when the thermostat is asking for AC cooling.

The thermostat should be the old mechanical type (not electronic) which just closes a contact when cooling is required.

The circuit can be built on a small vector board (I recommend a socket for the 14 pin IC).

Any questions about what parts to order, let me know.

 

Consider that the control scheme I suggested would not be instead of the present system, but rather just adding temperature control. So the present button will still select cooling, the added control will only reduce that cooling effort when it reaches your preset temperature setting. That is, it would switch off the cooling portion when the temperature you set was reached. So the compressor might be switching off for a minute or two, then run again for a minute, while the blowers keep the air circulating.
If the temperature you choose is usually the same, the controller does not even need to be easily accessed.
AND, this installation does not demand " fighting my way into the bowels of the machine to find the power connection." On many vehicles the AC compressor is quite obvious, and there is a single wire connected to the clutch magnet coil. In this proposal that wire would instead connect to the relay common contact, and another wire would connect from the relay NC contact to the clutch coil connection that the wire had been removed from. Then one thin new wire will run from the relay coil connection back on the wiring harness to the new temperature control.
Of course the relay will need to be supported and that same mounting bolt will be used to tie the wire from the other side of the relay coil to the engine mounting ground. Of course good vehicle wiring practices must be followed.

 

What module?

This is the module I intend using:

https://www.amazon.co.uk/dp/B0C6J9F...=d2lkZ2V0TmFtZT1zcF9kZXRhaWxfdGhlbWF0aWM&th=1

 

What module?

Below is the sim of my take on a circuit to do what you need:

The CD4093 IC is configured as an XOR gate that generates a pulse when the thermostat contacts both open and close, to momentarily close the relay contacts across the momentary switch you have.

The D2 LED shows when the thermostat is asking for AC cooling.

The thermostat should be the old mechanical type (not electronic) which just closes a contact when cooling is required.

The circuit can be built on a small vector board (I recommend a socket for the 14 pin IC).

Any questions about what parts to order, let me know.

View attachment 326883

Many thanks for that, the waveform is exactly what I need.

The thermostat module is an electronic device but has a built-in relay which is normally open but closes when the thermostat wants to operate a connected device. The module has a light on the front panel which illuminates when the relay is operating, so I would not need an LED in the suggested circuit but I assume I could just omit that?

 

Another question - I just noticed a 5V USB supply on the diagram, but given this is for use in a vehicle can I just power it off the vehicle's ignition switched 12V supply? Would any components need to be changed for this?

 

so I would not need an LED in the suggested circuit but I assume I could just omit that?

Yes.

can I just power it off the vehicle's ignition switched 12V supply? Would any components need to be changed for this?

Yes.
The only difference would be you need a 12V relay instead of a 5V.
Modified circuit below (with added filtering for the vehicles 12V supply).

 

Brilliant, many thanks. I will report back once I get chance to take the heater control off and verify how the switch works, which I am pretty sure is as a non-latching switch. I will then build up the circuit and try it out, hopefully before a trip to Spain in September.

 

Consider that the control scheme I suggested would not be instead of the present system, but rather just adding temperature control. So the present button will still select cooling, the added control will only reduce that cooling effort when it reaches your preset temperature setting. That is, it would switch off the cooling portion when the temperature you set was reached. So the compressor might be switching off for a minute or two, then run again for a minute, while the blowers keep the air circulating.
If the temperature you choose is usually the same, the controller does not even need to be easily accessed.
AND, this installation does not demand " fighting my way into the bowels of the machine to find the power connection." On many vehicles the AC compressor is quite obvious, and there is a single wire connected to the clutch magnet coil. In this proposal that wire would instead connect to the relay common contact, and another wire would connect from the relay NC contact to the clutch coil connection that the wire had been removed from. Then one thin new wire will run from the relay coil connection back on the wiring harness to the new temperature control.
Of course the relay will need to be supported and that same mounting bolt will be used to tie the wire from the other side of the relay coil to the engine mounting ground. Of course good vehicle wiring practices must be followed.

Thanks for the suggestion which could well be a simpler alternative way to achieve the same thing in many vehicles, though my concern is that everything in modern vehicles is generally tied into microcontrollers and CANBUS networks so the result of doing this sort of thing and going behind the back of the electronics is rather unpredictable.

My vehicle is fairly new so it could be that the compressor is not activated by applying or removing its 12V supply externally, but by a built-in controller which tells it when to run and switches the power on internally as required. Turning off the power extenally to any such controller could well mean that it resets and would not just resume compressor operation when the power is restored as I think you are suggesting.

 

Thanks for the suggestion which could well be a simpler alternative way to achieve the same thing in many vehicles, though my concern is that everything in modern vehicles is generally tied into microcontrollers and CANBUS networks so the result of doing this sort of thing and going behind the back of the electronics is rather unpredictable.

My vehicle is fairly new so it could be that the compressor is not activated by applying or removing its 12V supply externally, but by a built-in controller which tells it when to run and switches the power on internally as required. Turning off the power externally to any such controller could well mean that it resets and would not just resume compressor operation when the power is restored as I think you are suggesting.

It is not likely that a compressor has a built in controller, and it would certainly be an easy thing to check and verify. There is no place in a vehicle AC compressor to "build in controls". In addition, I never once mentioned switching off the compressor, but very specific description of disconnecting the clutch coil feed.

 

Use this on the compressor clutch wire.
https://www.ebay.com/itm/335127169863

 

It is not likely that a compressor has a built in controller, and it would certainly be an easy thing to check and verify. There is no place in a vehicle AC compressor to "build in controls". In addition, I never once mentioned switching off the compressor, but very specific description of disconnecting the clutch coil feed.

Well, as I said, what you are suggesting may well be a viable option. However I have a strong preference to interact with the AC system in a less invasive way via the standard mechanism provided, namely the ON/OFF button on the fascia. I will be able to do so using the interface circuit kindly provided by the forum member Crutschow, connected to a readily available electronic thermostat module. The latter also has the advantage that it will be mounted in the cabin and be easily accessible to change settings and has a useful display showing set temp v actual temp.

I have taken a similar approach to adding an automatic light controlled headlight ON switch after a couple of incidents of accidentally driving after dark without headlights, misled by the illumination from the DRLs. In this case I accessed and identified the relevant wires from the lighting control stalk and established that it was just a case of grounding one wire to turn on the lights which was easily done via a relay controlled by a light sensor circuit. There are kits available on eBay for this, but these involve tapping into the headlight power cables and providing a parallel power supply from the battery via a high power relay. This seems like using a sledgehammer to crack a nut when it is possible to tap into the existing control mechanism instead!

 

Consider that the on/off button also switches off the blower and probably shifts dampers in the AC system. and then also consider that there may be an initializing sequence each startup time.
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??
With the compressor control system scheme you will probably never notice that the cooling has changed, because it should be able to hold the temperature within whatever range the added air temperature controller is set for.
And consider that if the current system has only two options: Max cool and OFF, probably there is not a whole lot of "smart" involved. How do you think that the much more expensive system work??? It switches the compressor on and off to hold a given temperature.

One more consideration is that the single button may not be speaking a simple command to the computer system. No telling until you investigate.

The mechanical thermostat shown in the link of post #18 can only respond to that local sensor capillary tube. Plus the differential is not adjustable and also there is no calibration available. AND it is exactly the same level of control as you can get with the relay suggested, operated by the controller you described.