Beer fridge to thermal chamber tweak

[kingbean]

I got it, I will disconnect up/down buttons and temperature sensor from the HT MCU and on where it originally was receiving the data from temp sensor, I will send data I want him to display from my MCU, that should work right?

[Paul Price]

You don't have to disconnect the temperature sensor or any other button inputs to the HT MCU.  You let the HT MCU still think it is controlling the temperature so you can see the temperature from the readout without any wiring for the display and extra  programming work.

You only disconnect  the HT MCU  control wires to the relays control Peltier Junction and the Fans. The HT MCU probably won't even know you've taken control of these relays.

You can also let the HT MCU  have input from the temperature Sensor and the front panel switches, but you add wires to these switches to the new MCU so you can also use them to use as switch signals with your new MCU design.

The joke is on the HT MCU. It will probably not notice it has lost control of the heating and cooling and the f ans.

 

. 

[madires]

You don't have to disconnect the temperature sensor or any other button inputs to the HT MCU.  You let the HT MCU still think it is controlling the temperature so you can see the temperature from the readout without any wiring for the display and extra  programming work.

You only disconnect  the HT MCU  control wires to the relays control Peltier Junction and the Fans. The HT MCU probably won't even know you've taken control of these relays.

You can also let the HT MCU  have input from the temperature Sensor and the front panel switches, but you add wires to these switches to the new MCU so you can also use them to use as switch signals with your new MCU design.

I wouldn't do that without reverse engineering the circuit to ensure that paralleling the inputs doesn't cause any problems. In unclear situations you would also have to reverse engineer the firmware. In some of my MCU based designs I use I/O pins to power sensors, change (voltage divider) resistors and reverse the voltage divider to increase resolution for a specific range of resistance. If you would just parallel another MCU you could create short circuits and would never get any valid measurements.

[Paul Price]

I disagree...you are making the easy complicated. You don't parallel the MCU's,  The two MCU's function together but are completely segregated except for the new MCU snooping at the switch contacts to get control input.

It is not necessary at all to spend any more time reverse engineering. You are imagining functions of the old MCU upon these switches that would be extremely uncommon and so highly unlikely to encounter in a simple cheap fridge circuit controller like this.

Besides, a simple look with a scope or a test, of course, would show any problems of the new MCU in controlling the operation using the existing switches. It would reveal any problems and a very quick fix..if necessary, just cut any wires going from the old HT MCU to the switches.

 

The new MCU would have full control of the heating/cooling and fans and the only function of the old MCU would be to display temperature..and the old MCU can do nothing to change this.

The old push button switches do no more than function as on/off switches fed only to inputs of the old HT MCU.

[madires]

It is not necessary at all to spend any more time reverse engineering. You are imagining functions of the old MCU upon these switches that would be extremely uncommon and so highly unlikely to encounter in a simple cheap fridge circuit controller like this.

The last words from the bomb squad's expert: "it's the red wire in 99.9% of all cases" 

[Paul Price]

This is a simple fridge controller, not a bomb. It can easily and safely be tested to verify correct operation.

[amyk]

You don't have to disconnect the temperature sensor or any other button inputs to the HT MCU.  You let the HT MCU still think it is controlling the temperature so you can see the temperature from the readout without any wiring for the display and extra  programming work.

You only disconnect  the HT MCU  control wires to the relays control Peltier Junction and the Fans. The HT MCU probably won't even know you've taken control of these relays.

You can also let the HT MCU  have input from the temperature Sensor and the front panel switches, but you add wires to these switches to the new MCU so you can also use them to use as switch signals with your new MCU design.

I wouldn't do that without reverse engineering the circuit to ensure that paralleling the inputs doesn't cause any problems. In unclear situations you would also have to reverse engineer the firmware. In some of my MCU based designs I use I/O pins to power sensors, change (voltage divider) resistors and reverse the voltage divider to increase resolution for a specific range of resistance. If you would just parallel another MCU you could create short circuits and would never get any valid measurements.

Also the possibility that there could be more "intelligence" in the MCU than you think; e.g. detection for temperature-out-of-range, so instead of displaying the temperature you might just get "E". There's also the issue that if the temperature sensor is only designed for the two ranges, and internally there is a calibration table for mapping values to temperature, it might not work for anything outside of the design range. Or a safety timer of some sort. You need to investigate more.

[Paul Price]

This is a simple fridge controller, not a bomb. It can easily and safely be tested to verify correct operation.

The testing tool, a thermometer.

[kingbean]

Thanks for all the replies.

Well we started some reverse engineering, measuring and programming with arduino. For now the goal is to connect the back board with
arduino and let it control those two relay drivers depending on the inside temperature. 

Good thing is that if MCU is not connected, the whole thing works in cooling mode, so no trouble there.

Now I have some problem to get proper reading from the NTC to the arduino. When I set up direct connection arduino, NTC and 10k restistor,
I was able to use the formula in here 

http://www.instructables.com/id/Build-yourself-a-clock-and-thermometer/step3/Create-the-sketch-or-arduino/

and I got results which seemed to be like 2 degrees off compared to thermometer I put inside. I realize, they are at bit different place, but
I also had the whole thing off for few hours to measure the room ambient temperature and still it was 2 degrees off.
The guy in link above gives a link for wiki where the formula should be explained, but I dont see there the same formula he
used plus for these formulas some parameters are needed. Can I somehow find out those parameters for my NTC?
I dont see any name or such on it.

Next is measuring the NTC outcome on the way do display. Here I found out there is 1k and 10k res on the way, as well as
capacitator. How do I calculate these in the formula? Reading in arduino was then 14 degrees off. If the behavior was linear
I dont mind and offset it by 14, but it will not be linear right?


One more extra question. There are two relays, each switch between ground and 12 V for peltier. To these the peltier
is connected as well as two fans. These fans are on manual switch to get 5 or 12V. But how come if polarity changes
fans  dont mind that?

[amyk]

2 degrees might be within tolerance for a thermistor, but 14 is definitely not.

There's likely a calibration table in the MCU for this.