Forum Thread

Help with figuring out electronic/digital input value

sterlinggold 89 26 May 3, 2016 at 09:37 PM
Hi to all the electronic folks.

I got this CO2 sensor which can be forced to calibrate with a digital input to one of its pins. I'm trying to figure out what is the safe way of doing that: The document states the need for a Digital Input "when closed for minimum 4 seconds"
I'm not sure if I can just short this pin to another PWM 1kHz output pin for 4 seconds to trigger calibration.
Can anyone help me validate this? the pin is "bCAL_in/ CAL" and the info is on top of page 5 of this doc http://www.senseair.com/wp-conten...P108-1.pdf

Thanks

Sterling

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#2
Quote from sterlinggold View Post :
Hi to all the electronic folks.

I got this CO2 sensor which can be forced to calibrate with a digital input to one of its pins. I'm trying to figure out what is the safe way of doing that: The document states the need for a Digital Input "when closed for minimum 4 seconds"
I'm not sure if I can just short this pin to another PWM 1kHz output pin for 4 seconds to trigger calibration.
Can anyone help me validate this? the pin is "bCAL_in/ CAL" and the info is on top of page 5 of this doc http://www.senseair.com/wp-conten...P108-1.pdf

Thanks

Sterling
bCAL will register logic low from -0.3-0.75V
bCAL will register logic high from 2.3-3.6V)

If it's held at logic high with a constant dc voltage it will do the following:
4-8 seconds: calibrate for 400ppm
16+ seconds: calibrate for 0ppm

The spec sheet repeatedly states a lack of any protection circuity for power fluctuations, over voltage, reverse voltage, etc. Depending on what your doing a fair number of diodes, capacitors, and resistors may be required for reliable functioning.

To achieve the required signal for calibration, you wouldn't connect the bCAL pin to a PWM signal. Ideally you wouldn't just drive it high or low, it sounds like it needs a pull down resistor to keep it from floating after calibration. Whatever is interfacing with the uart should be able to generate the logic high for 6 seconds when it powers on if you program it correctly. It isn't completely clear, but it may auto calibrate on power on.
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Last edited by jkee May 3, 2016 at 11:43 PM
#3
Quote from sterlinggold View Post :
Sterling
re-reading the data sheet, it sounds like in has in internal pull up that's activated when it powers on how long it's active or if it triggers calibration is unclear. Depending on what they did this complicates my earlier suggestion that this pin may need a pull down.

It does sound like there's an auto calibration of sorts that takes about 2 weeks of continuous operation.

Without knowing what you're doing, the simplest way to trigger the calibration would be to connect a fairly large resistor eg. 100k ohm from the DVCC pin to the bCAL pin for 6 seconds.
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#4
thanks so much. will try this out and update with results. Yes I'm basically trying to bypass the two weeks self calibration process
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#5
So I tried with a 100k resistor and without to DVCC_out and PWM pins but got no calibration event. Moved the unit out to the patio for two weeks to self calibrate Smilie Thanks again.

BDW this is the unit http://www.amazon.com/upHere-Indo...ge_o01_s00

And this is a photo of the inside: http://chilp.it/1b4627e
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#6
Quote from sterlinggold View Post :
So I tried with a 100k resistor and without to DVCC_out and PWM pins but got no calibration event. Moved the unit out to the patio for two weeks to self calibrate Smilie Thanks again.

BDW this is the unit http://www.amazon.com/upHere-Indo...ge_o01_s00

And this is a photo of the inside: http://chilp.it/1b4627e
100K is probably a bit overzealous on my part. you shouldn't be doing anything with the pwm output for calibration.

I'd say solder pins to it and make the connection on a breadboard. Try a smaller resistor on the order of 1-10K ohm. It isn't clear what happens if power is lost with respect to calibration.

It might be interesting to watch the bCAL pin with a logic analyzer, scope, or volt meter when it's powered on. I wonder if the pin actually measures high and for how long, but it could all be internal and the output pin could be tri-stated.
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Last edited by jkee May 4, 2016 at 10:53 PM
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#7
Quote from jkee View Post :
100K is probably a bit overzealous on my part. you shouldn't be doing anything with the pwm output for calibration.

I'd say solder pins to it and make the connection on a breadboard. Try a smaller resistor on the order of 1-10K ohm. It isn't clear what happens if power is lost with respect to calibration.

It might be interesting to watch the bCAL pin with a logic analyzer, scope, or volt meter when it's powered on. I wonder if the pin actually measures high and for how long, but it could all be internal and the output pin could be tri-stated.
OK thanks, will try tomorrow and update.
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#8
Quote from jkee View Post :
100K is probably a bit overzealous on my part. you shouldn't be doing anything with the pwm output for calibration.

I'd say solder pins to it and make the connection on a breadboard. Try a smaller resistor on the order of 1-10K ohm. It isn't clear what happens if power is lost with respect to calibration.

It might be interesting to watch the bCAL pin with a logic analyzer, scope, or volt meter when it's powered on. I wonder if the pin actually measures high and for how long, but it could all be internal and the output pin could be tri-stated.
Got it! I used the breadboard and a soldered lead to bCAL and after some playing around I connected bCAL to UART_TxD for 5 seconds and it calibrated to 400PPM (repeatable).

This is super cool! saved me two weeks of self calibration, also makes this cheaper meter more valuable.

Thanks so much for the hand-holding, would not have been possible without it!
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