Hi, I have a VM110 board linked to a Raspberry pi that I am running headless networked to my PC and it works fine and can control output/read inputs.
I have had a look on the forum for the answer to my questions without success but also found a few ideas for temperature sensing, but have a K2644 Frost indicator that was wondering if I could use one of the digital inputs by utilising the LED connection on the frost indicator to somehow connect the 110 board. My electronic skills are reasonable but I am no expert (however after reading many of the post on here I realise there are many whose knowledge is far in excess of mine).
I also have a spare of the thermistor used on the frost indicator and wondered if I could use it to build a temp sensor for the 110 board, only problem is I cannot identify from the device or the Frost Indicator instructions what device/model the thermistor is (only marking on the casing is “6K8”, does anyone have any information on this, and if so is it possible to use?
It is certainly possible. How simple it is depends on how the LED on the circuit is activated. Unfortunately I cannot find the schematics of the K2644. Possibly one of the Velleman staff here on the forum can upload that and post a link. There are two ways to activate the LED. One is by “switching” the supply power with a PNP style circuit (or from the OP-AMP), the other is by switching the ground NPN style.
If the LED is controlled with an NPN open collector between ground, then you can simply tap into that. I have done the same with a cheap household alert (4 wireless motion detectors). The VM110 has 20K Ohm pull up between 5V and the digital inputs, so that only puts an additional 1/4 of a mA on that open collector … negligible compared to what the LED draws.
If the LED on the other hand is controlled PNP style, then it would be best to use a generic NPN transistor (like 2N3904, 2N2222, 2N4401 or whatever you find in your average variety pack). Since that only has to “switch” the same 1/4 mA as above, it can have a base resistor in the kilo-Ohm range.
Thanks for the quick reply. I have searched for the schematics for the circuit without success and it is so long since I built it I cant remember either. What bit of instructions I still have doesn’t have a circuit diagram either.
I am still seaching to identify the type of thermistor used so I can build temp sensor for one of the analogue inputs.
I was thinking of trying to connect something like a pir motion detector to some of the other digital inputs and a ldr to the other analogue input so I can control lights etc using the vm 129 module.
Disclaimer: The following is based on looking at images on the internet. I do not have a K2644 to verify any of the below myself. If you are not sure how to verify any of my findings with a multimeter, do not attempt and wait until someone can confirm this!
That said, looking at pictures of the PCB it appears that the anode of the LED goes directly to VCC (pin 4 of the LM324) and the cathode (the pin where the LED has the flat spot on the rim) goes through a current limiting resistor to pin 14 of the LM324, which is OUT4. If that is all correct then this is the NPN configuration I was talking about. If you have a multimeter you can verify this by measuring pin 14 of the LM324 (when looking at it with the notch pointing up, pin 14 is the upper right pin, closest to the LED). It should read either the supply voltage or about 2V below the supply voltage when the LED is OFF and 0V when the LED is ON. Without the schematics it is hard to really tell what the voltage on that pin should be when the LED is OFF. But definitely it should be 0 when it is ON.
If all that is still right, you can connect pin 14 of the LM324 to a digital input of the VM110, GND (which should be the far right terminal on the K2644) to GND and be all set.
As said, this is just from looking at pictures, so please verify with a multimeter yourself before connecting the two.
Regards,
Jan
Edit: I forgot that the ULN2803 on the VM110 has some current limiters and voltage dividers at the inputs. At 12V or more supply voltage for the K2644 this will probably cause the LED to dimly light up when it should be off. To avoid that we would have to put some diode in between.
Thanks for the input to this, I found the original instructions after a lot of searching. which contains a circuit diagram. The LED is as you say connected to the output of A3 of the OpAmp via a 680 Ohm resistor. It works by A1 supplying a constant current to the sensor, A2 buffers and amplifies the sensor voltage and its output voltage decreases as the temp drops. A4 is a slow running oscillator whose output is compared by A3 with the temperature dependant voltage. When temp gets to 3 Degree C the LED will flash and below -3 Degrees the LED is permanently lit.
It does say in the instructions that a bulb or buzzer could replace the LED as long as max load doesn’t exceed 20mA. From this I assume that I can therefore take a feed from the LED to one on the VM110 digital inputs via a suitable diode (I assume a low voltage signal diode will suffice). I will check the outputs with a meter though first to check before connecting.
I think I can write something that will detect the output changes so that it indicates near freezing and then freezing temps.
The instructions also identify the Thermistor as an NTC Resistor, so I can have a look at making a further temperature monitor.
A diode like 1N400x between a VM110 digital input and pin 14 of the LM324 (cathode towards the K2644) should do the trick then. That should make the digital input activate/deactivate in sync with the LED and not interfere with the K2644 at all. The rest is software.
NTC means “negative temperature coefficient”. In other words, the higher the temperature, the lower the resistance. There is a thread about connecting one to the K8055 and using OP-AMPs to minimize interference here: viewtopic.php?f=3&t=8041
Every NTC has its own characteristic curve and they are more or less linear only over a limited temperature range. Without the datasheet for that particular NTC, you will have to do some try and error and plot the curve yourself. 6K8 may indicate its resistance at the usual 25 C.